Introduction

English ivy (Hedera helix) is a prime example of a horticultural invasive plant. All over urban areas in the Pacific Northwest, it has been planted as a groundcover - as it has in New England. It invades natural areas in cities, such as Portland, Oregon's Forest Park. There, it climbs trees, choking them out at the top and depriving them of light. English ivy is one of many invasives that can invade in stable ecological communities - not just disturbed areas (Reichard and White 2000). Environmental and community groups, such as the Audubon Society work to remove English ivy from trees in Forest Park, but as long as people living on the edges of the park continue to plant and maintain the English ivy in their yards, fighting ivy in the park is a losing battle.

Growing up in Portland, I became familiar with the problem of this attractive, and supposedly "good" nursery plant and its ecologically devastating qualities. English ivy covers a good fifth of my parents' front yard, and they are continually battling it back, so that it does not cover the driveway. Kudzu (Pueraria lobata), an invasive in the southeast, has been known to cover abandoned cars in a single season, it grows at a rate of 100 feet per year and sometimes as much as 18 inches per day, according to Parker (1999). My parents have not removed the ivy yet because it"s an incredibly difficult plant to eradicate. My summer job at Smith and Hawken, a gardening store founded by green business guru Paul Hawken, even consisted of selling English ivy to many of the residents of Portland's West Hills, near Forest Park.

Background to the problem

At least 4500 species, including plants and animals, are non-indigenous in the United States. Approximately two to eight percent of each kingdom is non-indigenous, and about 15% of the non-indigenous species cause "severe harm" according to the Office of Technology Assessment's 1993 report. According to William Y. Brown, the science advisor to Secretary of the Interior Bruce Babbitt, invasive plants cause about $123 billion in damage every year (Raver 1999). They have invaded more than 105 million acres in the United States, and invade about 3 million more per year (Reichard and Campbell 1996, Raver 1999). Peter Kareiva, in reference to Mooney, Harold, and Drake"s Ecology of biological invasions of North America and Hawaii, concluded "that there is no such thing as a community uncontaminated by foreign species." Excluding Hawaii, which is seriously compromised by invasive plants, there are about 300 invasive plants in the United States, at least half of which were intentionally introduced as garden ornamentals or landscape plants (Randall and Marinelli 1996).

Every year the United States spends $138 billion dollars in control costs and agricultural losses and damages to invasive species (Pimentel 1999). This figure does not take into account the non-monetary losses to ecosystem functions, human health effects, habitat loss for native species, and, ultimately, biodiversity loss. Invasive plants account for 25% of the costs incurred by invasive species in the country, or an annual $35 billion. A significant portion of the nonindigenous plants that become ecological problems were introduced intentionally to the United States. Just two of these plants, the Melaleuca tree and Purple loosestrife cause $48 million every year in controls (Pimentel 1999).

Coming up with an definition of invasive plants that is acceptable to both the ecological and horticultural communities is difficult. The American Nursery and Landscapers Association and the Weed Science Society of America have settled on this definition. "Invasive plants are plants that have or are likely to (1) spread into native plant communities and cause environmental harm by developing self-sustaining populations and disrupting those systems; or, (2) spread into managed plant systems and cause economic harm."  It is important to consider that while some plants may fall under this definition, there will still be resistance to classifying them as invasive because the definition carries with it implications of restriction and regulation. Once a plant has been deemed invasive, it is subject to restricted uses and perhaps outright bans on its use and sale. Because of this, the ultimate decision of calling a plant is invasive is a political and social one (OTA 1993).

There are a few difficulties in deciding which plants can be classified as noxious or invasive, even though these definitions seem fairly cut-and-dry. It can sometimes be difficult to distinguish between native plants and non-native plants. Phragmites australis, for instance, is a reed commonly found on the Northeastern seaboard which has recently become a significant invader of disturbed areas. Phragmites crowds out native flora and fauna, so it seems to be a problem. However, deciding on a management plan for the species is difficult. Phragmites may be European in origin " though it shows up in the American pollen records as having been here for some time, so it may also be considered a "native" to the northeastern US (Clark et al 1998, Mehrhoff 1998). Then why has it recently been invading? It may be that the American strains have cross-bred with escaped European strains, or that the European strains are the aggressive invaders, or it may just be that the native Phragmites is adapting to current changes in land use and climate changes, such that it is now invading new ecosystems. Each explanation has a different management and definitional implication. The European strain may need to be controlled and eradicated, a native plant invasion may be left alone.

Other plants carry different dilemmas. Hypericum perforatum is an invasive on the west coast (Reichard personal communication, Campbell 1999), but it is also a widely used herbal medicinal plant " St. John"s Wart. Hedera helix (English Ivy) is also purported to have positive medicinal properties (interview with nursery employee in Oregon, January, 2000); it is also a common erosion control planting. Pueria lobata (Kudzu) was originally planted as a forage crop and soil stabilizer (OTA 1993). Tamarix species may provide critical habitat for native birds in the southwest (Brown 1992). And all horticultural plants that have become invasive are considered to have positive aesthetic and ornamental properties; that is why they were originally planted in the United States. NIS are used for food, landscape restoration, biological pest control, sport, pets, food processing (and gardening, etc) (Pimentel 1999) When plants that threaten natural ecosystems or are invasive weeds in agricultural and horticultural settings, but also offer medicinal, economic, aesthetic, and ecological benefits, are defined as invasive, we are potentially restricting a plant that does more good than harm. The OTA (1993) asserts that the opinion of the invasiveness of an organism ultimately depends on the point of view of the observer. This implies that there is no smooth and simple way to decide whether plants should be included on "invasives" lists; committees deciding on plant invasiveness must consider whether there are alternatives that can replace the plants when they become restricted. These committees must also be effective at promoting these alternatives such that invasives lists become acceptable.

Another consideration for creating effective invasive plant lists is whether the committee is made up of a full spectrum of interested parties. This includes the scientists, land managers, and, perhaps, environmentalists who know how and to what extent that the plant is a problem, as well as representatives nursery industry, agriculture industry, and gardeners, who may perceive the benefits of the plant. The decisions made by this committee will be more likely to be respected by the groups affected by the decisions because they will realize that the decision was made by people with their interests in mind. Committees also have their drawbacks " they engender compromise and rarely come up with solutions that please everyone.

A problem with creating lists of invasives is that plants are only able to invade on a regional scale. In some cases, as with Lythrum salicaria (Purple loosestrife), this scale has become national, but in many other cases, harmful invasive plants are only invasive within a few states. Including a plant like Schefflera actinophylla (Umbrella plant) on a federal invasive (noxious) plants list will be controversial. S. actinophylla only invades Southern Florida at this point, though it is sold widely as a houseplant throughout the United States (the Center for Environmental Studies at Brown University keeps one in its classroom space) (Hugh Gramling, in Invasives Roundtable 1999). Calling the plant invasive in Southern Florida, where it is a landscape plant, is politically difficult, but calling it invasive throughout the country will be even more difficult.

Another difficulty with deciding whether a plant is invasive is the issue of scientific proof. It is often difficult to create consensus that a plant is invasive unless an ecological study has been done to quantify the spread of the plant and its threats to natural ecosystems. With the number of potential invasives in the United States and the limited resources to do field studies, determining invasions is often done on a purely observational basis, with little quantitative data involved. Some plants have been studied to such an extent that biological control methods have been thoroughly examined, their invasions have been mapped, and the exact threats to native systems have been described. Many have not. Arguing to the nursery industry that they should stop selling a plant that has not been scientifically proven to be invasive will be difficult. The nursery industry is generally quite scientifically minded, especially due to the fact that most of the nursery professionals in US have quite a bit of plant (horticultural) knowledge. Combined with the general reverence Western society has for scientific opinion, this phenomenon cannot be ignored. I found in my survey, as I later discuss, that nurseries tend to respect science as a basis for deciding invasiveness, which points to the fact that more funding and research needs to be put into this area.

However, waiting for scientific evidence to be registered for each invasive plant before making a decision on it may be unwise. In this issue, like many other environmental issues, the precautionary principle may be the best way to prevent ecological destruction. Studies to determine invasiveness often take many years to complete, in which time the plant can become even more of a problem (Reichard personal communication March 12, 2000 - also, see Parker and Reichard 1998). If, instead, a system of "expert opinion" is used, wherein respected weed scientists decide on plants" invasiveness, we can achieve an effective prevention-oriented system, based on science. This is currently the system in place in Washington State, and it has been effective (Reichard personal communication, March 12, 2000). In addition, a combination of scientific study and expert opinion may be a good compromise. If, when an expert group decides a plant is invasive, and it proves in later study to be harmless, it could be removed from the invasives list through a petition process, or yearly review.

Closely tied to the definition of an invasive plant is that of a noxious weed. The current federal definition of an invasive plant essentially only includes the plants that are economically harmful on agricultural lands or harm human health or other economic resources (fish and wildlife). Ecological pests are not included. New legislation (Senate Bill 910, HR 1504) is currently sitting in committee that may create a more inclusive definition of noxious weeds. The noxious weed law works by creating a list of weeds that fit the definition created by the law and then regulating those weeds by prohibiting interstate commerce and importation of the listed weeds. However, the Unlisted States Department of Agriculture, who is in charge of listing weeds and policing them (through the Animal Plant Health Inspection Service - APHIS), has only listed 96 plants, while 750 plants currently fit the definition (OTA 1993, USDA APHIS 2000).