Horticulture as a Pathway of Invasive Plant Introductions in the United States

and horticulture in North America are not native to the continent. Most of the plants that have been introduced are not invasive; they carry out their intended purpose and therefore benefit humans in multiple ways, causing no problems. A small portion of introduced plants, however, escape from cultivation and become pests of natural areas. A recent study found that invasive plants, animals, and fungi are second only to habitat loss and degradation in endangering native plant species (Wilcove et al. 1998). Fifty-seven percent of the imperiled species studied were negatively affected by nonnative invasive species. The impacts on native species include competition for resources (Melgoza et al. 1990, Hester and Hobbs 1992, Mesléard et al. 1993, Huenneke and Thomson 1994), hybridization (Thompson 1991), introduced or increased nitrogen fixation in natural areas (Vitousek et al. 1987), changed hydrologic cycles (Carman and Brotherson 1982), increased sedimentation (Blackburn et al. 1982), and increased frequency or intensity of disturbance cycles (Bock and Bock 1992, D’Antonio and Vitousek 1992). A recent estimate put the economic cost of invasive plants in natural areas, agriculture, and gardens at $35 billion per year (Pimentel et al. 1999). The majority of woody invasive plants in the United States were introduced for horticultural purposes—one study found that 82% of 235 woody plant species identified as colonizing outside of cultivation had been used in landscaping (Reichard 1997), and an additional 3% were widely distributed for soil erosion control (virtually all of the latter group were also introduced as ornamentals, however). Herbaceous invasive species are less likely to have been introduced for horticultural purposes; instead, many of these species were introduced through crop seed contaminated with weed seed (Baker 1986, Mack 1991) or through seeds in soil brought over from Europe as ship’s ballast and dumped at ports to make room for cargo (Baker 1986). The problem is certainly not limited to the United States: Between 57% (Kloot 1987) and 65% (Groves 1998) of the naturalized flora of Australia, both woody and herbaceous species, were intentionally introduced for horticulture. We define an invasive plant species as one that has or is likely to spread into native flora and managed plant systems, develop self-sustaining populations, and become dominant or disruptive (or both) to those systems. Invasive species comprise both native and nonnative species, but this article focuses primarily on those invasive plants that are not natives of the areas in which they are invasive. Because these species adversely affect land management, we refer to them as weeds, a term with more managerial than biological overtones, and one that is used in many legal contexts. Given the number of plant species that have been introduced and established outside their native ranges, it may