Federal agencies have identified invasive species as the second most important threat to the natural environment in the United States, behind only habitat destruction. Although members of the animal kingdom get most of the attention, invasive plants are a major threat to parks and forests, greenways and gardens, as well as endangered species.

Invasiveness - Performance is the most important characteristic of a plant in any environment. The explanation of an invasive species currently posted in Florida (www.nsis.org) is that "invasive plants are those that multiply rapidly and overwhelm an area. They crowd out and smother other plants and clog wetlands and waterways. They are usually non-native plants that have escaped cultivation and spread rapidly." Other definitions don't suggest that invasive plants necessarily ever were in cultivation.

Invasive plants are not the same as noxious weeds. Noxious is a legal definition for plants that have been determined to be major pests of agricultural ecosystems. There are both federally listed noxious weeds (USDA) and state listed noxious weeds (NCDA). There is only one NC listed Class A non-aquatic noxious weed, mile-a-minute vine, Polygonum perfoliatum. Seven Class B noxious weeds exist in NC. Three are thistles and only one is likely to show up in a landscape on purpose, any species of Lythrum not native to NC.

Not all non-native plants are considered harmful or invasive. Of the 4,000 species of exotic plants that have become established as free-living populations in the United States, roughly 10 percent have been identified as a threat to our native flora and fauna as a result of their invasive characteristics (Kastalerz, NCBG). Many non-invasive alien plants are important to the agricultural economy including apples, corn, soybeans, sweet potatoes, and tomatoes. Volunteers may reseed from these important crops but they do not "crowd out and smother" populations of native plants.

In addition, North American native plants can be considered invasive. For example, black locust, Robinia pseudoacacia, is native to the southeast but was planted for fence posts across the U.S. Outside it's native range, black locust is considered an exotic and in some places it is considered invasive because of its rapid growth and reproduction.

Where? Invasiveness of plants is a problem in two distinctly different areas:

1. Undisturbed areas such as those that are being maintained as wilderness where natural succession is allegedly being allowed to occur such as National Parks.
2. Disturbed areas such as any place where man has left a mark by clearing land, building roads or lakes, draining swamps, etc.

1. Alters ecosystem functions. Examples include species that either reduce or increase fire likelihood or intensity or that alter the water table or hydrologic regime;
2. Become established in UNDISTURBED NATURAL communities;
3. Outcompete native species after NATURAL disturbance; and
4. Prevent or depress the regeneration of native species.

Man made disturbances mimic these effects by removing trees, moving quantities of soil or changing water availablity. Plants will populate these disturbed sites through natural succession. In the mountains many of our most beautiful wildflowers, shrubs such as sumac and early succession honeybee forage trees such as black locust and sourwood fill these voids. However, in these same mountain areas we may see exotics such as princess tree or tree of heaven and vines such as kudzu or oriental bittersweet becoming dominant.

Some of the same rules Mr. Sutter suggested for undisturbed areas should be applied to disturbed areas before labeling a plant as a problem. If exotic plants can reasonably be controlled, they should not be listed as a problem.

Green Industry Impact - Most of those employed in the nursery and landscape industry are environmental heroes bringing beauty, cleaner air and water into people's lives, healing the damaged earth and providing habitat for native fauna.

To maintain a positive reputation, we must be good neighbors. Selling or installing mile-a-minute vine or purple loosestrife, even Lythrum 'Morden Pink,' will bring regulators to your door because these plants are noxious weeds. However, there is a long list of plants that are considered invaders by managers of undisturbed areas (www.nps.gov/plants/alien/). If these plants are likely to become problems in your area be aware of it and act appropriately. However, most plants like kudzu, Japanese honeysuckle, multiflora rose and oriental bittersweet are not going to be sold by nurseries.

Consider the plants you sell. If the shrub Ligustrum sinensis is a problem in your area perhaps you shouldn't sell it or support research to determine whether it is a REAL versus IMAGINED problem. If a customer asks for a recognized invasive such as Japanese wisteria, suggest some non-invasive native alternatives like purple flowering 'Amethyst Falls' American wisteria or white flowered 'Clara Mack' Kentucky wisteria.

Another impact, and one that is more likely, is that the Green Industry may be blamed as responsible for invasive plants by folks who do not understand what invasive plants are or their origins. If someone complains to you about the invasiveness of English ivy, ask him or her if they have tried to control it. Explain to them about physically removing invasives or proper timing with herbicides (Roundup sprayed in spring, not fall, eventually controls English Ivy). Be nice, don't yell and don't drive customers away.

Be involved in what is happening where plant related decisions are made. Landscape ordinances do not develop overnight. Locally developed lists of "bad" plants don't either. Well-meaning folks trying to be good citizens develop them. If you are involved, perhaps a better list can be developed or no ordinance at all.

From: 2000 Proceedings of the International Plants Propagators Society.

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North Carolina State University