® 3 Policies to Reduce the Risk of Invasive Plant Introductions via Horticultural Trade Arianne Ransom-Hodges • Duncan Knowler* School of Resource and Environmental Management, Simon Fraser University, Burnaby, British Columbia, Canada Corresponding author: * djk@sfu.ca Keywords: black listing, economic instruments, screening, stakeholder survey, voluntary codes of conduct ABSTRACT Invasive alien plant species are known to cause both economic losses and ecological damages. However, the introduction of a non-native species can be simultaneously classified as destructive by one segment of society and desirable by another. Indeed, many potentially invasive plant species are deliberately introduced for economic benefit despite the risks associated with them. Notably, the horticulture industry is the most significant pathway for intentional introductions of invasive alien plants. Although it is increasingly recognized that the nursery trade and the invasive alien plant problem are inextricably intertwined, existing regulations do little to tackle deliberate introductions of potentially invasive plant species. Indeed, prevailing policies largely ignore the horticultural dimension of the problem. Therefore, novel policy options to address the problem have been developed and could be implemented to reduce the risk of invasive plant introductions via horticultural trade. Here, we describe the current regulatory framework, and its shortcomings, for dealing with the introduction and sale of potentially invasive plant species in Canada and the United States. The chapter also outlines new policy options for limiting the risk of invasive alien plant introductions by the green industry, including the use of economic instruments and risk assessment procedures. We discuss the merits of these options as compared to the approaches that are applied at present. Finally, preliminary results from a survey of professional horticulturists are presented. 1. INTRODUCTION Invasive plant species are defined as those species that encroach into both undisturbed and human-modified vegetation, where they subsequently become dominant, disruptive, or both (Reichard and White 2001). The majority of such species are exotic species, often referred to as alien or nonindigenous species. Therefore, these species originate in other areas and are introduced, either intentionally or unintentionally, to new and suitable ecosystems where they flourish by successfully competing with native flora (Haber 2002). The encroachment of these species has been shown to cause substantial damages in North America, both ecological and economic. From an ecological standpoint, alien species are described as the second largest source of biodiversity loss in the United States (Wilcove et al. 1998). Studies indicate that there are 2,000-3,000 naturalized non-indigenous plant species in the United States, while in Canada approximately a quarter of the estimated 5,800 plant species are alien (Kartesz 1994; Haber 2002). However, only a small percentage of the non-indigenous plant species in both countries are considered invasive (Marinelli 1996; Haber 2002). Nonetheless, alien plants are contributing to the decline of approximately twenty percent of the endangered and threatened plants in Canada (Haber 2002). In economic terms, the costs of controlling invasive plants and the residual damages caused by these species have been estimated at $35 billion per year in the United States (Pimentel et al. 2005). In Canada, approximately $7.5 billion per year is dedicated to invasive plant research and control efforts in the forestry and agriculture sectors (RNT Consulting 2002). However, economic valuations of the costs of invading species often are not able to consider the entire range of ecological impacts and, therefore, are thought to underestimate the costs associated with such species (Perrings et al. 2002; Pimentel et al. 2005). Moreover, invasive plants are difficult and expensive to control once established and, as such, most invasions are irreversible (Ewel et al. 1999; Horan et al. 2002). Consequently, damages associated with invasive alien plant species typically continue for extended periods of time beyond the initial introduction event. Since invasive plants often spread across political boundaries, regional coordination and an integrated legislative framework are required to effectively address the invasive alien plant problem (Mullin et al. 2000; Miller and Gunderson 2004). The North American horticulture industry has been identified as a key pathway for the introduction and dispersal of alien plant species (Reichard and White 2001; Haber 2002). Governments in both Canada and the United States are aware of the damages caused by invasive plants, recognize that the horticulture industry is a pathway, and are seeking solutions (National Invasive Species Council 2001; TPPWG 2004). Abbreviations: APHIS, Animal and Plant Health Inspection Service; CFIA, Canadian Food Inspection Agency; NAPRA, Not Authorized for Import Pending Risk Analysis; NRC, National Research Council; TPPWG, Terrestrial Plants and Plant Pests Working Group; USDA, United States Department of Agriculture Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 23 However, clear policies aimed at targeting the horticulture industry for its role in the introduction and dispersal of invasive alien plants in these countries have yet to be implemented. Here, we describe the Canadian and American horticulture industries. We then frame the North American horticulture industry as a pathway for plant invasions. Subsequently, using results from a survey of professional horticulturists, we quantify awareness of the invasive plant problem, characterize the horticulture industry’s role in propagating invasive plants, and assess perspectives of the problem. A description of the current policy framework is provided, followed by a review of alternative policy approaches. 1.1. The Canadian and American horticulture industry Horticultural pathways for potentially invasive plant species include the introduction of exotic plant species (or their propagative parts, such as seeds) for botanical gardens and arboreta collections, sale by nurseries and garden centers, and horticultural society exchanges (Reichard and White 2001). Here, we focus on the horticulture industry, which consists of plant growers, distributors, and sellers as well as landscape designers and installers. The industry in the United States is described as a “diverse multibillion dollar industry with importers running the gamut from small, family operations specializing in a few species to large corporations importing hundreds of taxonomically diverse species” (Mack et al. 2000, p 703). Floriculture crops, as defined by the USDA National Agricultural Statistics Service, include bedding and garden plants, cut flowers, flowering potted plants, foliage plants, and floriculture propagative material. Trees, shrubs, and ornamental grasses are classified instead as nursery production. Similarly, Statistics Canada defines nursery stock as a diverse array of inedible, living plant material grown either in containers or in fields. Nursery stock is sold with intact root systems and includes both annual and perennial plants. Thus, woody plants, such as trees and shrubs, as well as bedding plants and potted outdoor flowers are classified as nursery stock in Canada (Statistics Canada 2006). Sales data for floriculture production and nursery crop production in the United States since 2000 are presented in Table 1. In 2005, domestic floriculture production in the United States totalled US$ 5.36 billion, wholesale for growers reporting $10,000 or more in sales (National Agricultural Statistics Service 2006). Total gross sales from nursery crop production in 17 American states were US$ 3.97 billion in 2003 (National Agricultural Statistics Service 2004). The number of floriculture growers in the U.S. was 10,563 in 2005 while there were 7,742 nursery operations with sales over $10,000 in the seventeen states surveyed in 2003 (National Agricultural Statistics Service 2004, 2006). In the United States, floriculture and nursery crops are characterized as one of the fastest growing agricultural sectors in the last decade (Economic Research Service 2006). Canadian ornamental flower and plant sales, including retail, wholesale, and export sales, totalled CDN$1.5 billion in 2006 (see Table 1 for sales data from 2001 onwards). Total sales of nursery stock in Canada, including retail sales and sales to landscapers and garden centers, approached CDN$6 million in 2006 (Statistics Canada 2007). Landscapers accounted for 28% of sales reported by nursery product growers. Purchases by garden centers accounted for 23% of Canadian nursery stock sales. Table 1 Horticulture industry sales statistics from the United States and Canada, from 2000 onwards. 2000 2001 2002 2003 United States Floriculture production1 $4.57 billion $4.74 billion $4.88 billion $5.07 billion Nursery sales1 $3.32 billion data not available $1.26 billion data not available $1.42 billion $3.97 billion $1.21 billion $1.45 billion Ornamental flower and plant sales1 1 Data obtained from the National Agricultural Statistics Service Floriculture crops summary (2001-2006), US$ 2 Data obtained from the National Agricultural Statistics Service Nursery crops summary (2001 and 2004), US$ 3 Data obtained from Statistics Canada Greenhouse, Sod and Nursery Industry reports (2001-2007), CDN$ Canada 2004 $5.18 billion 2005 $5.36 billion data not available $1.44 billion data not available $1.42 billion 2006 data not available data not available $1.51 billion 1.2. Horticulture as a pathway for invasive plant introductions The horticulture industry is responsible for introducing and dispersing plant species known to be invasive as well as other exotic species that have the potential to become invasive. Horticulturists often prefer introduced ornamentals to native plants because they are aesthetically pleasing and generally easier to grow (Myers and Bazely 2003; Reichard 2004). Indeed, the majority of the annuals, perennials, and woody species sold for gardening are exotic (Harrington et al. 2003; Armitage 2004). Although few of the non-native species sold by the horticulture industry have been identified as invasive (Harrington et al. 2003), those species not yet classified as invasive have the potential to invade. Specifically, the biological traits that facilitate the cultivation of intentionally introduced plant species are congruent with those that increase the likelihood that they may become invasive (Harrington et al. 2003; Mack 2005). However, only a small percentage of the ornamental plants introduced for horticulture have escaped the garden to subsequently cause economic and ecological damage (Reichard and White 2001; Harrington et al. 2003). Nonetheless, due to the elevated rate of introduction associated with horticulture, ornamental species comprise the vast majority of invasive plants in many countries (Baskin 2002). In North America, approximately half of the 300 species of established invasive plants were introduced for ornamental horticulture (Marinelli 1996). As such, horticultural fashion and the growth of the horticultural market have been identified as important components of the contemporary humanmediated dispersal of invasive plants (Hulme 2003). Despite the scientific evidence suggesting that the horticulture industry is a major pathway for the introduction of invasive alien plant species, some plant importers deny the negative consequences of their actions (Mack et al. 2000). Indeed, large economic incentives exist to introduce and disperse novel species that may subsequently become invasive (Mack and Lonsdale 2001; Mack 2005). Certain horticulturalists refuse to cease importing new exotics since they have yet to be characterized as invasive (Armitage 2004). Moreover, empirical evidence suggests that some horticulturalists ignore laws aimed at halting the sale and transport of known invasive plant species in the United States (Maki and Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 24 Galatowitsch 2004). However, recent developments indicate that horticultural professionals are increasingly accepting their role in causing plant invasions (Baskin 2002). 2. HORTICULTURE INDUSTRY PROFESSIONALS: AWARENESS, ROLE AND ATTITUDES An online survey was designed to assess perceptions of the alien invasive plant issue held by various stakeholder groups. One such stakeholder group comprised professional horticulturists, including nursery and garden center owners and employees, landscapers, arborists, professional gardeners, and botanical garden staff. The survey consisted of four main sections: 1) general questions relating to how respondents are affected by the invasive plant problem, 2) a ranking exercise involving policy options, 3) attitudinal questions focusing on perceptions of the problem itself, and 4) standard demographic questions. Online surveys were emailed to 6996 email addresses, including addresses of horticultural association members, nursery owners and staff, and employees at botanical gardens. Email addresses were collected through web-based (Google¥) searches for individuals from five states (California, Connecticut, Florida, Montana, and Ohio) and four provinces (British Columbia, New Brunswick, Ontario, and Saskatchewan) that could be considered stakeholders in the context of the invasive plant problem. A total of n = 1801 completed survey were collected, 1300 of which were associated with the original sample. The remaining 501 survey responses originated from surveys completed by individuals who received a link to the survey from a member of the initial sample. 1005 surveys were undeliverable due to inactive email accounts, full mailboxes, etc. Thus, the response rate of the original sample (with undeliverable surveys removed) was 22%. 2.1. Professional horticulturists: Awareness and role In total, 456 respondents, 25% of the sample, identified themselves as professional horticulturists when asked how they are most affected by the invasive plant problem. Forty-seven percent (n = 215) of the professional horticulturists were nursery or garden center owners, managers, or staff, while 32% (n = 144) identified themselves professional landscapers, gardeners, or arborists, and 8% (n = 36) were botanical garden curators or staff. Thirteen percent (n = 61) of the professional horticulturists surveyed indicated that they did not belong to any of the horticultural sectors described above. Of the professional horticulturists surveyed, the vast majority were at least somewhat familiar with local invasive plant species. Just under half of all professional horticulturists classified themselves as very familiar with locally invasive plants. While 30% of professional horticulturists felt that they were very aware of government regulations dealing with the invasive plant problem, the majority stated that they were only somewhat aware of regulations and laws. Eighty-six percent of professional horticulturists agreed or strongly agreed with the statement “There is not enough public awareness of the problem”. 2.1.1. Nursery sector: Nursery and garden center owners, managers, and staff Two-thirds of the owners, managers, and employees of nurseries or garden centers surveyed stated that the nursery or garden center does not always provide customers with information about the origins of the plants being sold. However, 74% of respondents from the nursery sector stated that plants known to be invasive in the local area are not sold. Notably, survey results indicate that 21% of nursery sector respondents are employed by or operate businesses that sell plants known to be invasive in the region where the business is located (Fig. 1). Moreover, plant species known to be invasive elsewhere in North America are more likely to be sold (Fig. 1). Interestingly, of the 17 staff, managers, and owners of a nursery or garden center with mainly non-local sales (e.g., mail-order or internet sales), the majority reported selling plant species known to be invasive elsewhere in North America (Fig. 2). Almost a quarter of all nursery sector respondents stated that they do not know whether the nursery is selling plants that are invasive elsewhere in North America, indicating low levels of familiarity with plant species 90 80 70 60 50 40 30 20 10 0 No Yes Don't know Sells Plants invasive locally (n=213) Plants invasive elsewhere in North America (n=214) Fig. 1 Sales of invasive plants at nurseries or garden centers by nursery sector respondents. Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 25 90 80 70 60 50 40 30 20 10 0 No Yes Don't know Sells Fig. 2 Sales of plants that are invasive elsewhere in North America among nursery sector respondents owning or employed by firms with mainly non-local sales (n = 17). 90 80 70 60 50 40 30 20 10 0 Plants native to North America Plants native locally Non-native plants No preference Preference: Type of plant Nursery Sector: Sell at nursery/garden center (n=213) Landscaping Sector: Use for landscaping (n=144) Botanical Sector: Grow in garden (n=35) Fig. 3 Preferences for different types of plants among respondents from different horticultural sectors. known to be invasive in other regions (Fig. 1). A third of the nursery or garden center employees, managers, and owners surveyed stated that the nursery did not sell any known invasive plants, regardless of where they invade. In terms of personal preferences with respect to types of plant sold, most nursery or garden center staff, managers, and owners indicated no preference between native and exotic plant species (Fig. 3). 2.1.2. Landscape sector: Landscapers, professional gardeners, and arborists Only 9% of landscapers, professional gardeners, and arborists use locally invasive plants. Nonetheless, 21% of respondents from the landscaping sector use plants known to be invasive elsewhere in North America (Fig. 4). Notably, all members of the landscaping sector knew whether or not they used plants locally invasive while 29% stated that they do not know whether they use plants that are invasive elsewhere in North America (Fig. 4). Thus, the finding that horticulturists are less knowledgeable about invasive plant species at the continental scale holds. Preference for using plants native to the local region among landscapers, professional gardeners, and arborists was higher than within the other horticultural sectors, while a lack of preference between native and exotic plant species was lower (Fig. 3). 2.1.3. Botanical sector: Botanical garden curators and staff The majority of botanical garden curators and staff worked at a botanical garden containing locally invasive plant species and/or plant species invasive elsewhere in North America. Again, knowledge of non-local invasive plant species appeared lower than knowledge of local Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 26 120 100 80 60 40 20 0 No Yes Don't know Use Plants invasive locally Plants invasive elsewhere in North America Fig. 4 Use of invasive plants by landscape sector respondents for commercial purposes (n = 144). 90 80 70 60 50 40 30 20 10 0 No Yes Don't know Presence Plants invasive locally (n=36) Plants invasive elsewhere in North America (n=34) Fig. 5 Presence of invasive plants at gardens where botanical garden sector respondents are employed. invasive plant species (Fig. 5). Interestingly, of the curators and staff of botanical garden containing plants considered invasive (locally or elsewhere in North America), 35% stated that the garden interpreted the plants for educational purposes. Most of the botanical garden curators and staff surveyed expressed no preference with respect to the types of plants (native or non-native) grown at the garden (Fig. 3). 2.2. Professional horticulturists: Perspectives on the invasive plant problem 2.2.1. Importance of the problem The invasive plant problem was described as very important by 63% of the professional horticulturists surveyed (Fig. 6). However, a greater proportion of respondents from the landscape and botanical garden sectors, 77% and 86% respectively, thought the problem was very important as compared to the nursery sector, with 51% of respondents. Less than 2% of respondents within all three horticultural sectors, including respondents from the nursery sector, classified the problem as not at all important (Fig. 6). Hence, the majority of respondents from the horticultural sector recognize the importance of the invasive plant problem. 2.2.2. Responsibility for causing the problem When asked how important those that import, buy, or sell known invasive plants for landscaping purposes are in causing the invasive plant Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 27 100 90 80 70 60 50 40 30 20 10 0 Not at all Somewhat Very No opinion Importance All Professional Horticulturists (n=455) Nursery Sector (n=214) Landscaping Sector (n=144) Botanical Sector (n=36) Fig. 6 Importance of the invasive plant problem. 100 90 80 70 60 50 40 30 20 10 0 Not at all Somewhat Very No opinion Importance All Professional Horticulturists (n=451) Nursery Sector (n=212) Landscaping Sector (n=143) Botanical Sector (n=36) Fig. 7 Importance of those that introduce, buy, or sell known invasive plants in causing the invasive plant problem. Table 2 Importance of various actors in causing the invasive plant problem, according to the professional horticulturists surveyed. Not at all Somewhat Very No opinion Importance of those that import, buy, or sell 3% 19% 78% 1% known invasive plants for landscaping purposes Importance of those that import, buy, or sell non30% 44% 24% 2% native plants for landscaping purposes problem, 78% of the horticultural professionals surveyed thought that they were very responsible (Table 2). The three horticultural sectors answered similarly with respect to this question (Fig. 7). In contrast, only 24% of horticultural professionals survey classified those that import, buy, or sell non-native plants for landscaping purposes as very responsible for causing the invasive plant problem (Table 2). Moreover, there were noticeable differences between the sectors, with a greater proportion of respondents from the landscape sector characterizing those that import, buy, or sell non-native plants as very important (Fig. 8). Thus, members of the horticultural sector appear willing to accept some degree of responsibility in causing the invasive plant problem. Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 28 70 60 50 40 30 20 10 0 Not at all Somewhat Very No opinion Importance All Professional Horticulturists (n=451) Nursery Sector (n=212) Landscaping Sector (n=143) Botanical Sector (n=36) Fig. 8 Importance of those that introduce, buy, or sell non-native plants in causing the invasive plant problem. 80 70 60 50 40 30 20 10 0 Strongly disagree Disagree Neutral Agree Strongly agree Extent of Agreement All Professional Horticulturists (n=449) Nursery Sector (n=211) Landscaping Sector (n=143) Botanical Sector (n=35) Fig. 9 Extent of agreement with the statement "It is important to devise policies to address the invasive plant problem". However, the sale of non-native species is perceived as far less important than the sale of known invasives in causing the problem. This finding is not surprising given the widespread knowledge that most introduced non-native plants never become invasive. Hence, many professional horticulturists oppose strategies that attempt to address the invasive plant issue by promoting the sale of native plants as alternatives (Harrington et al. 2001). 2.2.3. Policies: Satisfaction and importance Survey respondents were asked to what extent they agreed with the statement “It is important to devise policies to address the invasive plant problem”. Fifty-one percent of professional horticulturists surveyed agreed and 38% strongly agreed with the statement (Fig. 9). Conversely, only 11% of respondents from the horticulture sector agreed or strongly agreed with the statement “I am satisfied with current government efforts to solve the problem” (Fig. 10). When comparing all three horticultural sectors, a greater proportion of landscape sector respondents strongly agreed that it is important to devise policies to address the invasive plant problem (Fig. 9). Congruently, respondents from the landscape sector were less satisfied with current government efforts as compared to respondents from other horticultural sectors, particularly the nursery sector (Fig. 10). Therefore, the Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 29 70 60 50 40 30 20 10 0 Strongly disagree Disagree Neutral Agree Strongly agree Extent of Agreement All Professional Horticulturists (n=450) Nursery Sector (n=211) Landscaping Sector (n=143) Botanical Sector (n=36) Fig. 10 Extent of agreement with the statement "I am satisfied with current government efforts to solve the problem". survey results suggest that professional horticulturists, especially those within the landscape sector, support the formulation of new policies to address the invasive plant issue. 3. THE EXISTING POLICY FRAMEWORK Many plant species are deliberately introduced for economic benefit such that a species can be simultaneously classified as destructive by one segment of society and desirable by another (Lodge and Shrader-Frechette 2003; Reichard 2005). The importation and subsequent sale of exotic plants by the horticulture industry is the most obvious example of the benefits derived from harmful, or potentially harmful, exotic plants species (Barbier and Knowler 2006). Indeed, “one person’s ornament is another person’s invader” (Shogren and Tschirhart 2005, p 269). In this section we review the policy framework, rooted in black listing, currently in place to prevent the importation and sale of invasive plant problem. In so doing, highlight the failure of such policies to recognize both the costs and benefits stemming from the sale of invasive and potentially invasive plant species for horticultural purposes. 3.1. Black listing Although the introduction of invasive and potentially invasive alien plants produces benefits in addition to costs (Mack 2005; Lodge et al. 2006), current North American policies frame the issue solely in terms of costs. The current approach typically involves listing known invasive species, usually on lists of noxious weed, and subsequently prohibiting their import, transport, propagation, and sale. Newly introduced plant species are deemed innocent until proven guilty, at which point they are listed and their import and sale is banned (Simberloff 2005). Thus, such policies emphasize the need to regulate the movement of plant species only once they have been listed as noxious. This approach, known as black listing, has been widely criticized. Specifically, such regulations are deemed ineffective due to fundamental design flaws. Because most invasions are irreversible, focusing exclusively on species known to be harmful is injudicious (Lodge and ShraderFrechette 2003). Thus, policies that rely on reactive adaptation strategies, such as black listing, are considered ineffective with respect to addressing the invasive plant problem. For example, banning the sale of certain exotic plant species, as is the intent of such regulations, could significantly reduce horticulture industry revenues. As such, there exists a disincentive to adhere to black list regulation, resulting in low industry compliance. Indeed, existing American laws rooted in black listing are often ignored, suggesting low industry support (Maki and Galatowitsch 2004). Furthermore, these ‘command and control’ policies have been deemed economically inefficient due to their failure to consider benefits derived from the import and sale of exotic species or to internalize invasion costs (Shine et al. 2000; Perrings et al. 2002; Jenkins 2001; Doelle 2003; Knowler and Barbier 2005; Perrings et al. 2005). 3.2. Current North American policies 3.2.1. Canada Most Canadian regulations governing invasive plant species rely on a black listing approach. The application of current legislation in the context of invasive alien plant species is compromised due to limited scope, namely a focus on agricultural concerns (White et al. 1993), and reactive measures (Sierra Legal Defense Fund 2004). Furthermore, legislation to directly regulate the deliberate introduction of exotic plant species for horticultural sale in Canada has not been enacted. The lack of specific invasive alien plant species polices in Canada is reflected in the scope of federal legislation. A review of current Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler 30 Invasive plants: Tackling the horticulture pathway legislation finds that the Canadian federal government has failed to put forth a comprehensive legislative response to tackle the issue (Sierra Legal Defence Fund 2004). For example, the species listed as noxious within the Weed Seeds Order (S.O.R./86-836), 1986 of the Seeds Act (R.S.C. 1985, c. S-8) are limited to those known to cause harm within the agriculture sector. Similarly, the regulations stemming from the Plant Protection Act (R.S.N.L. 1990, c. P-16) prevent the import, export, and proliferation of pests injurious to plants, but no invasive exotic plant species have been listed as pests despite their propensity to invade and cause harm to other plants. Thus, existing federal laws aimed at regulating weed seeds, plant pests, and noxious weeds in Canada do not explicitly target invasive plant species. In fact, very few known invasive alien plant species are regulated in Canada. As such, the Canadian horticulture industry is able to sell and import live exotic plants despite the actual or potential damages associated with them. Canadian government agency publications indicate that the federal government is aware of the invasive plant issue and has identified the horticulture industry as an important pathway (White et al. 1993; TPPWG 2004). In 2004, the Terrestrial Plants and Plant Pest Working Group (TPPWG), co-chaired by the Plant Health Division of the Canadian Food Inspection Agency (CFIA), issued a preliminary strategy, namely phase 1 of the Proposed Action Plan for Invasive Alien Terrestrial Plants and Plant Pests. Federal, provincial and territorial Ministers of forests, fisheries and aquaculture, and wildlife approved the plan, which includes the development of pathway analysis and the enhancement of risk assessments as decision support tools in invasive species prevention. However, the plan does not specify the degree to which the horticulture pathway will be prioritized. In 2005, a Proposed Implementation Plan was drafted with timelines for each area of delivery (TPPWG 2005). Nonetheless, due to the open-ended nature of many of the timelines, it is unclear when the various components of the plan will be fully implemented. 3.2.2. United States Similarly, legal frameworks for dealing with invasive plant species in the United States are, in practice, black lists. Under the Plant Protection Act (7 U.S.C. 7701-7772 et seq. Public Law 106-224, June 2000), the importation, exportation, and interstate movement of listed noxious weeds is restricted or prohibited. The noxious weed list contains 96 plant species. Thus, two-thirds of the approximately 300 alien plant invaders established in the continental United States (Marinelli 1996) are not listed as noxious weeds. In order to add or remove species from the list of regulated noxious weeds, the Animal and Plant Health Inspection Service (APHIS) must conduct a pest risk analysis. However, these risk assessments are based solely on expert opinions formulated using subjective, qualitative methods, rather than on structured protocols that ensure expert opinion corresponds to the credible application of the scientific method (NRC 2002; Maguire 2004; Lodge et al. 2006). Moreover, the screening process conducted by APHIS is only applied to a very small percentage of species being imported. Notably, many of the species found on the federal noxious weed list had established populations in the United States before they were prohibited from being imported (Lodge et al. 2006). Thus, introductions of new plant species for horticultural purposes are not routinely assessed and statutes tend to regulate the movement of species that have already invaded. Furthermore, the existing USDA regulation regarding importing plants for planting and propagation, referred to as Quarantine 37 (7 C.F.R. 319.37), is antiquated and fails to effectively protect the US environment and economy against the introduction of harmful plant species. Currently, Quarantine 37 is being reviewed and revised in order to increase its relevance. APHIS is investigating ways to address the risks, including the risk of invasion, associated with introducing alien plants. Specifically, the creation of a new category of ‘plants for planting’ consisting of those species not authorized for import pending risk analysis (NAPRA) has been suggested (APHIS 2005). However, the exact risk analysis protocol to be adopted has yet to be specified. Other federal laws in the United States that could potentially be applied to addressing the invasive plant issue include the Federal Seed Act of 1939 (7 U.S.C. 1551-1611), the National Environmental Policy Act of 1969 (42 U.S.C. 4321) and the Endangered Species Act of 1973 (16 U.S.C. 1531 et seq.). However, none of these laws can be applied to comprehensively address the issue of deliberate introductions of potentially invasive plant species by the horticulture industry. Clearly, North American policy aimed at addressing the invasive plant problem is plagued by fundamental deficiencies. The main limitation of the existing regulatory framework is the focus on reactive measures that overlook the risks associated with the intentional introduction of plant species for horticulture. Furthermore, based on the survey results outlined in the preceding section, North American horticulture industry professionals largely support the development of strategies to tackle the invasive plant problem. As such, innovative policy approaches, such as those that rely on preventative measures to limit the introduction of new invasive ornamental plants, must be considered and evaluated as an alternative to the status quo. 4. NEW POLICY APPROACHES As previously mentioned, government agencies in Canada and the United States are formulating new invasive alien plant strategies that place more of an emphasis on screening protocols. Although the benefits of a preventative approach are widely acknowledged, the feasibility of applying screening procedures in the context of the invasive plant problem is uncertain. Three possible policy options based on screening mechanisms have been formulated: white listing, industry self-regulation, and the application of economic instruments. Here, we review the viability of screening procedures in the context of intentional ornamental plant introductions and provide an overview of the three policy options. We also discuss the acceptability of these options among professional horticulturists, based on results from our invasive plant survey. 4.1. Feasibility of screening procedures Prevention has been described as the “most effective, economical, and ecologically sound approach to managing many invasive species” (Windle and Chavarria 2005). A preventative policy approach consists of the use of risk assessment frameworks to determine the capacity of a plant species to cause harm when introduced to a region beyond its original range. Using both the probability of invasion and the magnitude of the impacts anticipated in the event of an invasion, the risk associated with the introduction of the species can be calculated (Hughes and Madden 2003). Thus, expected losses, stemming from the introduction of a potentially invasive plant, can be compared to expected benefits, Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler 31 Invasive plants: Tackling the horticulture pathway such as horticulture sector profits, to determine if importing a specific species is likely to cause more harm than good. Theoretically, the probability of invasion could be obtained by assessing the biological attributes of each plant species considered for introduction (Goodwin et al. 1999; Finnoff and Tschirhart 2005). In practice, the probability of invasion can be predicted with high rates of accuracy for plants belonging to some groups using post hoc tests that compare the predicted invasive potential of previously released plant species, based on biological traits, to their actual behaviour (Reichard and Hamilton 1997). These high accuracy rates, such that a large proportion of known invasive plant species are correctly identified as invaders, have been used to promote the feasibility of screening procedures. Nonetheless, some suggest that the high accuracy rates of screening systems developed using retrospective tests of plants with known invasive tendencies may not be sustainable when applied to the import of new species. Also, the value placed on risk assessments with high accuracy rates has been questioned because, due to the low base-rate of invasions, high levels of accuracy can be achieved only in conjunction with high error rates, so many species will be misclassified as invasive (Smith et al. 1999). The high number of false positives (Type I error) results in low levels of reliability such that many harmless alien species are denied entry. The potential for such errors may underlie the failure of most countries to implement risk analysis procedures for the introduction of exotic species (Keller et al. 2007). Furthermore, substantial difficulties exist with respect to acquiring information pertaining to environmental and economic impacts (i.e., costs) needed to conduct the assessment (Parker et al. 1999; Hughes and Madden 2003). Indeed, the prediction of long-term impacts is impossible (Henderson et al. 2006). Furthermore, the inability to make generalizations about the patterns and processes that govern plant invasions impedes the completion of credible risk assessments. Risk assessments do not account for time lags, evolution, and other biological phenomena and, as such, risks are underestimated (Simberloff et al. 2005). Addressing the invasive species problem within the context of a conventional risk-management framework thus proves complicated, such that decisions must be issued despite incomplete information (Horan et al. 2002). As a result, the ability to predict invasiveness remains highly uncertain and imprecise (Mack et al. 2000; D’Antonio et al. 2004). The most reliable predictor of a plant’s ability to become invasive in the United States is information regarding the invasiveness of the species in other geographical areas (NRC 2002). In short, although “a conceptual basis exists for understanding invasions that could be developed into predictive principles”, scientific principles or dependable procedures for identifying the invasive potential of plants are lacking (NRC 2002). Even so, policies rooted in screening procedures appear to be emerging as the favored approach. Indeed, recent research involving the application of a straightforward bioeconomic model reveals that a risk assessment strategy produces positive net economic benefits over a range of plausible assumptions, including various accuracy rates (Keller et al. 2007). Nonetheless, further research with respect to quantifying the risks associated with purposeful introductions of invasive plant species would be valuable (Ewel et al. 1999; Andersen et al. 2004). 4.2. Preventative policy options Several plausible policy alternatives based on a screening approach have emerged. These include white listing, industry self-regulation, and variable taxation. Here, we describe each of these options in detail. 4.2.1. White listing The most commonly promoted application of screening procedures with respect to preventing invasions by purposefully introduced species involves excluding all species unless the risk of invasion is acceptably small (Biber 1999; Lodge et al. 2006; Keller et al. 2007). A ‘clean’ or ‘white’ list would denote species known to be noninvasive, and thus permitted, while all other species would be subject to a risk assessment or screening process by the relevant government agencies prior to introduction (Shine et al. 2000; Doelle 2003; Klein 2004). Non-native plant species shown to have a sufficiently low invasion risk would be added to the white list while the government would prohibit all species characterized by a high risk of invasion. White listing is a preventative, rather than reactive, approach. It involves the creation of approved lists such that the introduction of listed species is permitted while all other species are assumed to be risky and are banned (Andow 2005). Hence, the introduction and sale of a plant species is allowed or prohibited based on the capacity of the species to cause harm. A simplified version of white listing might be more feasible given that techniques for estimating the probability of invasion appear more refined than methods aimed at quantifying the impacts stemming from invasions. Due to the complexity inherent to assessing the impacts of a species once established (Parker et al. 1999; Hughes and Madden 2003; Henderson et al. 2006), the simplification involves ignoring the cost component of risk analysis and focusing solely on the probability of invasion (e.g., see Jefferson et al. 2004). A precautionary approach is adopted such that plant species with high probabilities of invasion are banned while those with low invasive potential are allowed. Although this approach discounts the risk associated with species having a low probability of invasion in concert with immense potential negative impacts once established, a focus on the probability of invasion appears to improve the practicality of screening as a tool to prevent the introduction of potentially invasive ornamental plants. By revealing the most harmful species, risk assessments can be used to best allocate the scarce resources dedicated to regulating invasive species. Unlike current policies, where species are presumed innocent and a burden is placed on stakeholders attempting to halt the introduction of a species, screening mechanisms shift the burden of proof onto those seeking to introduce non-native species1 (Biber 1999; Simberloff 2005). As a result, many more species would be prohibited (Biber 1999). Nonetheless, assessments should be conducted by an independent agency of scientific experts that are equipped to analyze risks objectively using a rigorous and standardized procedure (Biber 1999; NRC 2002). 4.2.2. Industry self-regulation: Codes of conduct Horticulturalists are promoting self-regulation as a suitable approach for addressing issues surrounding invasive ornamental plants (Mezitt 2005). Voluntary approaches are commonly managed by national industry associations and involve the persuasion of industry association members, including retailers, growers, and landscapers, to remove specific high-risk plant species from their inventories (Harrington et al. 2003; 1 Shifting the burden of proof in this way is consistent with the notion of a safe minimum standard (SMS). In the context of species conservation, proponents of adopting an SMS approach argue that, until it is proven otherwise, conservation of a species ought to be recognized as optimal (Tisdell 1990; Berrens 2001). Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler 32 Invasive plants: Tackling the horticulture pathway Moss and Walmsley 2005). Voluntary codes of conduct in the United States have been designed for the self-governance and self-regulation of the horticulture industry in order to limit the use and dispersal of invasive plants (Baskin 2002; Reichard 2004). Similar self-regulation schemes have also been applied in Australia and New Zealand (Moss and Walmsley 2005). Most codes of conduct recognize the importance of identifying new species likely to invade and the determination of which species currently sold should be removed from inventories and gardens (Reichard 2005). Although most voluntary schemes have focused on eliminating the sale and planting of known invasive plant species, screening procedures are also applicable to industry self-regulation schemes. However, instead of government agencies implementing and enforcing the screening process using mandatory regulations, the regulation of potentially invasive plants would be administered by the industry itself. Voluntary measures for addressing the introduction of potentially invasive plants are appealing to both government and industry. Governments often support self-regulation initiatives in order to avoid implementing potentially controversial and unpopular policies (Moss and Walmsley 2005). From an industry standpoint, self-regulation is an attractive proactive policy approach that precludes “pre-emptive government intrusion” (Mezitt 2005) by ensuring direct horticulture industry participation (Harrington et al. 2003). Thus, in addition to confirming that the industry is willing to accept some responsibility with respect to the invasive alien plant issue, such initiatives demonstrate a “strong preference for voluntary initiatives over legislation that would restrict plant introduction, propagation, use, and sale” (Baskin 2002). However, voluntary efforts are not guaranteed to produce satisfactory results. Although the rejection of voluntary codes by irresponsible horticulturists is a primary concern (Reichard 2005), the potential for horticulture industry self-regulation to fail is predominantly rooted in the characteristics of the industry itself. For example, the limited coverage of national industry associations is such that many firms are not at all compelled to adhere to regulations developed by industry representatives (Moss and Walmsley 2005). For instance, the 2,200 members of the American Nursery and Landscape Association (ANLA) represent less than 15% of the total number of floriculture growers and nursery operations in the United States. Importantly, big-box stores and supermarkets that sell garden plants are typically not members of industry associations. Limited membership in industry associations also suggests that member firms are more likely to disregard industry association policies. Specifically, some member firms may choose not to comply with the voluntary regulations in order to remain competitive with firms outside the sphere of influence of industry associations. Therefore, the shortcomings of self-regulation are not always a simple issue of negligent horticulturalists refusing to participate. The lack of incentives associated with participating in self-regulation strategies and the presence of undeniable economic disincentives are key issues (Canton 2005). However, there are instances where industry association members will choose to ignore codes of conduct in order to generate increased profits through the sale of restricted species. Codes of conduct may thus provide a perverse incentive, motivating some member firms to sell restricted invasive plants in order to capture a market niche (Moss and Walmsley 2005; Canton 2005). Recent empirical studies highlight the failure of voluntary measures in Australia, New Zealand, and the United States to significantly curb the horticultural trade of invasive plants, due to the issues outlined above (Moss and Walmsley 2005; Canton 2005). However, voluntary campaigns have successfully generated public awareness of the issue and increased knowledge of the invasive plant problem among horticultural professionals (Harrington et al. 2003; Moss and Walmsley 2005). As such, the development of more restrictive regulatory frameworks that apply to the entire industry in conjunction with voluntary initiatives, focused on education and accreditation, has been recommended (Moss and Walmsley 2005; Reichard 2005; Windle and Chavarria 2005). 4.2.3. Economic instruments Economic instruments involve the application of a monetary charge, such as a tax or environmental fee, in order to affect a change in behaviour. Charges of this nature are typically levied on firms whose activities result in unintended costs to third parties, such as when a polluting industry damages a fishery. Similarly, the dispersal of invasive species can be described as biological pollution (Elliott 2003), with nurseries, growers, and other exotic plant importers considered as the polluters. The costs stemming from the deliberate introduction of exotic species frequently are borne by parties who are not responsible for causing the problem, such as farmers and naturalists. As a result, these costs are characterized as negative externalities (Jenkins 2001; Perrings et al. 2005; Barbier and Knowler 2006). The application of a tax or, alternatively, an environmental fee to the sale of all alien plant species thus have emerged as potential policy options to address horticultural trade as a pathway for plant invasions. Recent research suggests that economic instruments could be adopted to internalize the negative externalities associated with nurseries selling exotic plant species (Knowler and Barbier 2005). Additionally, charges can act as a disincentive for those activities that increase invasion risks and can generate funds for invasive species control programs (Biber 1999; Jenkins 2001; Doelle 2003). One theoretical policy option that follows from the characterization of invasive plants as biological pollution involves the application of a variable tax to the sale of all newly imported non-native plant species. The tax rate is dependent on the likelihood of invasion of a species and its potential damages, such that species found to have higher expected damages are associated with a higher tax rate than those plants with a lower expected damages (Knowler and Barbier 2005). Consequently, through the impact of a variable tax on selling price, purchasers of potentially harmful plant species are provided incentives to change their behaviour and the likelihood of invasion associated with their activities is reduced (Touza et al. 2007). Furthermore, because it relies on screening procedures to determine the invasive potential of non-native ornamental plants, a variable tax policy can be categorized as a preventative strategy for dealing with the risks associated with the horticultural pathway. As such, variable tax policies are not designed principally to generate revenues for controlling established invasive plants. A fixed environmental fee, where a fixed monetary charge is imposed on the sale of all non-native plant species, is a policy option more likely to be implemented in order to collect funds earmarked for control efforts. The idea of applying economic instruments, such as taxes or environmental fees, to the problem of biological pollution stems from their effectiveness in addressing standard pollution problems (Shine et al. 2000). Indeed, taxes are promoted as a policy tool because “an efficient tax rate forces polluters to fully internalize the costs of their activities” (Biber 1999). Furthermore, many features of invasion events are comparable to those of standard pollution problems, including ozone layer depletion (Biber 1999) and oil spills (Jenkins 2002). The traits shared by biological pollution and standard pollution include general and diffuse damages arising from the cumulative effect of a multitude of small, disperse, actions Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler 33 Invasive plants: Tackling the horticulture pathway Table 3 Policy options, as presented to survey respondents. Import & sale Implement & enforce Option A Black list and ban all species listed Mandatory Policy options Option B Option C Screen and ban species with Screen and ban species with a high likelihood of invasion a high likelihood of invasion Mandatory Voluntary Option D Screen and variable tax Option E Fixed environmental fee Mandatory Mandatory Table 4 Definition of the terms appearing in bold in Table 2. Terms Definitions Black list Creating list of non-native plant species known to be invasive in a give region. Only species that have already invaded will be listed. Screen Assessing the likelihood that newly imported non-native plant species will become invasive. Policies will only target species that have a high likelihood of invasion. Ban Completely prohibiting the import and sale of all plant species that are considered invasive. Variable Tax Imposing a variable monetary charge on the sale of all newly imported non-native plant species. The tax rate is dependent on the likelihood of invasion of a species – i.e., the sale of species that are more likely to invade is associated with a greater charge. Fixed Environmental Fee Imposing a fixed monetary charge on the sale of all non-native plant species. Mandatory Implemented and enforced by the government. Voluntary Implemented and enforced by the horticulture industry – e.g., voluntary codes of conduct. that are associated with some degree of risk. However, despite the similarities between biological and standard pollution problems, significant differences exist. Novelty and irreversibility are characteristics of biological pollution that differ from many standard types of pollution (Horan et al. 2002). Unlike typical pollution problems, in which the level of pollution can be measured, there are no measures of how invasive species compromise biological integrity (Miller and Gunderson 2004). Moreover, invasions occur almost unnoticed and, hence, establishing obvious responsibility is rarely possible (McNeely 2001). In addition, “unlike chemicals, biological entities reproduce and spread autonomously, often over great distances, and can even evolve to adapt to changing conditions” (Simberloff 2005). Invasions are ongoing and not site-limited (Shine et al. 2000). Therefore, notable differences exist between externalities as traditionally understood in economics and biological invasions (Perrings et al. 2000). Finally, although determining the optimal level of the tax is shown to be dependent on the profitability of the exotic plant species and the level of risk associated with its introduction (Knowler and Barbier 2005), the effective use of taxes is compromised by difficulties surrounding setting the optimal, or efficient, taxation level (Biber 1999). Indeed, the optimal level at which to set a tax in the context of invasive plants is sensitive to many unknown or uncertain parameter values (Knowler and Barbier 2005). The lack of necessary data combined with the “stochastic and ex ante nature of the invasion problem” results in the need to adopt many “heroic assumptions” when designing taxes in this context (Knowler and Barbier 2005). As such, the application of polluter pays principle to biological pollution is controversial and complex. 4.2.4. Acceptability of new policy approaches The merits and limitations of the policy options outlined above have been discussed at length within the literature. However, the acceptability of the options to members of the horticultural sector has not been formally assessed. A recent survey of industry professionals in one U.S. state, namely Minnesota, revealed that preferences for government regulation and industry self-regulation were equal, with both being favoured by forty-three percent of the sample (Peters et al. 2006). However, respondents were not provided with detailed descriptions of the regulation. In our stakeholder survey, professional horticulturists were asked to rank five specific policy options based on the policy approaches discussed above. The five policy options were mandatory black listing, mandatory screening, voluntary screening, a variable tax policy, and the application of an environmental fee (Table 3). Descriptions of the five policy options were provided to respondents (Tables 3, 4). The order of the policy options presented to each respondent was randomized to avoid order biases. In the following analysis, we only discuss the most and least preferred options (i.e., the first- and fifth- ranked options). Among professional horticulturists, the voluntary screening option was the most preferred of the five options, with 42% of respondents selecting option C as their first choice (Fig. 11). Policy option B (mandatory screening) was the first choice selection of 38% of professional horticulturists surveyed. The variable tax approach (option D) and the fixed environmental fee (option E) were selected first by 3% and 2% of the sample, respectively. Moreover, options D and E were most often selected last, with 22% selecting option D and 43% selecting option E as their least preferred option (Fig. 12). Stakeholder preferences for the status quo black listing policy (option A) were less polarized, with 16% of respondents selecting it first and 19% ranking it last. However, the selection of most preferred policy option varied between horticulturists from different sectors (Fig. 11). Just over half of the owners, managers, and employees of nurseries or garden centers selected option C first whereas option B was the chosen first most often among landscapers, professional gardeners, and arborists. An equal percentage of botanical garden curators and staff selected option B or C as their first choice (Fig. 11). In sum, although voluntary measures are popular among horticulturists in general, they are most preferred by members of the nursery sector. In contrast, members of the landscaping sector tend to opt for the mandatory screening option first. Finally, professional horticulturists from Canada and the United States expressed slightly different first-choice preference for the policy options (Fig. 13). Twenty percent of American horticulturists surveyed selected option A (black listing) first, as compared to only ten percent of the professional horticulturists surveyed in Canada. These results suggest that Canadians are less satisfied with the status quo than their American counterparts. However, slightly less than two-thirds of horticulture sector respondents from both the United States and Canada expressed dissatisfaction with current government efforts to solve the problem when asked explicitly. Thus, it not surprising that the majority of horticulture sector respondents from both countries, specifically 84% of Canadians and 76% of Americans, chose options B or C first. Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 34 70 60 50 40 30 20 10 0 Option A Option B Option C Option D Option E Policy Options All Professional Horticulturists (n=456) Nursery Sector (n=215) Landscaping Sector (n=144) Botanical Sector (n=36) Fig. 11 Percentage of respondents choosing each of the policy options first. 50 40 30 20 10 0 Option A Option B Option C Option D Option E Policy Options Fig. 12 Percentage of all professional horticulturists (n = 456) choosing each of the policy options last. 5. CONCLUSIONS Clearly, empirical evidence confirms that horticultural trade is a major pathway for the introduction of invasive plant species. However, an overview of current legislation reveals that Canadian and U.S. policies do little to prevent invasions via the horticultural pathway. The development and implementation of new policies is thus required to reduce the risk of invasion stemming from the introduction of ornamental plants. Recent government initiatives in both Canada and the United States, in conjunction with contemporary academic research, highlight the potential for policies rooted in risk assessment procedures to prevent invasions resulting from the deliberate introduction of non-native plants. Furthermore, results from our stakeholder survey indicate that 83% of professional horticulturists surveyed prefer one of the three preventative policies presented to policies lacking a screening component, such as black listing and the application of an environmental fee. Only 16% of horticultural sector respondents opted for the status quo, black listing, as their first choice but this preference was twice as strong in the United States as compared to Canada. Nonetheless, the practical application of screening policies to the importation of exotic garden plants remains undeveloped. Although the variable tax option is appealing in its adherence to the polluter pays principle, it appears to be the least viable at present. The variable tax option faces challenges due in part to a lack of support from the professional horticulturists surveyed. As a result, governments may Floriculture, Ornamental and Plant Biotechnology Volume V ©2008 Global Science Books, UK Ransom-Hodges and Knowler Invasive plants: Tackling the horticulture pathway 35 60 50 40 30 20 10 0 A B C D E Policy options United States (n=276) Canada (n=178) Fig. 13 Percentage of American and Canadian professional horticulturists selecting each option (A-E) first. resist such a policy. Our results suggest that an environmental fee might raise similar concerns. Thus, the application of a screening program that prohibits the introduction of high-risk species is the approach, amongst mandatory policy approaches, most likely to be adopted by governments at this time. In addition, research from the United States, Australia, and New Zealand illustrates the disadvantages of relying on voluntary measures to restrict the sale of invasive plants for horticultural purposes. These finding suggest that regulations enforced by the government are more effective, thus highlighting the need for mandatory policies. However, voluntary initiatives serve to increase public awareness of the issue. Since our survey results indicate that public awareness of the invasive plant problem is believed to be meagre, both voluntary and mandatory policies have a role to play in curbing the introduction of invasive and potentially invasive ornamental plant species. Overall, a mandatory screening policy combined with voluntary horticulture sector initiatives appears to offer the most advantages at present for reducing the risk of invasive plant introductions stemming from horticultural trade. However, further investigation of potentially more efficient alternatives, such as economic instruments, needs to be undertaken, particularly with respect to data needs and practicality. Due to the continental scale of many invasions, coordination between government agencies from Canada and the United States is also recommended. Although coordination between nations is likely to be difficult, harmonization efforts might be facilitated by survey results demonstrating that the majority of both American and Canadian horticulturists support the development of appropriate policies. ACKNOWLEDGEMENTS We would like to thank Sarah Reichard and Edward Barbier for providing valuable information and suggestions. 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