One Person’s Beautiful Plant; Another’s Invasive Nightmare

Although the distinctive sweet scents of honeysuckle and wisteria have always assured me that spring is on its way, by midsummer I find myself battling them in my backyard. While some people welcome these plants, and others with similar growth habits, neither of the plants is native to the United States, and in most cases, are considered invasive pests.

Ironically, exotic, or non-native invasive plants have traditionally been on the “least wanted list” at most parks and agricultural lands, yet many are  sold by nurseries to meet homeowner and landscaper demand. Ironically, about 60 percent of plants on the exotic invasive list were intentionally brought into this country.

An exotic plant is a plant that people have introduced into an environment in which it did not evolve. In their countries of origin, predators and other conditions keep them in check. For instance, in this country, kudzu grows so fast that it has been named “the vine that ate the South” because it completely covers trees, telephone poles, houses and anything else in its way.

In China and Japan, however, kudzu grows as normally as other native plants, and predatory insects keep the vines from populating too much territory. The Chinese and Japanese use the plant in medicine, clothing, and food, and harvesting the plant for these uses also controls its growth.

The reason that kudzu grows without control in the southern U.S. is that the hot, humid weather and short winters stimulate kudzu growth of up to 7 feet per week, and without predatory insects, diseases, and frequent harvesting, the weed grows unimpeded, making it invasive.

All plants in the U.S. that are considered invasive are non-native; however, not all non-native plants are invasive. When taken out of their native habitat, many plants struggle to survive and have to be carefully tended in order to produce. Corn, wheat and oats, for instance, are non-native, grow only during a short season, and frequently succumb to insect damage and diseases.

Several species, however, found their perfect habitat when brought to this country, and without the predators and diseases that restricted their growth in their native countries, the plants multiply faster than people can control them, eventually choking out or smothering species that are natural to the area. Many of us have passed some of these aggressive intruders along the highway: mimosa, tree of heaven, wisteria, Japanese stiltgrass, privet, multiflora rose…and many others.

However, not every person shares the opinion that traditionally invasive plants are “bad.” A diligent homeowner may be able to maintain a single mimosa tree without letting the seedling grow. On the other hand, the ranger at a nearby county park may have a yearly battle with the hundreds of mimosa seedlings that sprout as a result of seeds blowing in. The invasive water plant, hydrilla, may feed large carp in one lake and provide for some great fishing, but it will snuff out the aquatic life in another.

A few weeks ago I attended an invasive species workshop at Umstead State Park in Raleigh, North Carolina. In the forest near one of the bike trails, a large clump of trees bowed under one of the thickest web of wisteria vines I’ve seen. The vines atop the trees had been killed by an herbicide treatment applied to the trunks of the vines. As part of the workshop, we searched for living wisteria vines that had not been cut and killed, and we killed them using the same cut-and-spray method used on the other vines. With all of the dead vines that surrounded us, I was amazed at how many living vines we found.

Next year, the ranger said, the staff would have to repeat the search for new vines and kill them. I was even more disheartened when I realized we had worked on only a small area of the 5,579 acre park, leaving untouched acres infested with other invasives such as tree of heaven, Japanese stiltgrass, and even more wisteria.

As I hunted for live vines to spray with my diluted solution of Garlon 3A, I thought about some of the invasive weeds in my own backyard. Because I have so little time to garden or even pay attention to my yard, the Japanese honeysuckle has spread over about a third of my backyard, and some of its tendrils have reached across my deck, tripping me when I walk outside with my dog. The Japanese stiltgrass that began in my neighbor’s yard quickly spread into mine last spring, and although I pulled up several clumps of it over the summer, it filled in a couple of weeks later. In April of this year, it grew even thicker than last year. Stiltgrass is even more of a menace in the wild. According to our workshop presenter, few park managers even bother trying to eradicate stiltgrass because it is nearly impossible to get rid of completely—unless all of it is killed before August for five years in a row.

Is there a solution? Does the park manager’s need to protect natural lands from these invaders outweigh the homeowner’s desire to plant an exotic tree in the yard? IPM’s first line of defense against a pest is prevention. Yet preventing windborne seeds from entering an area is impossible. Conversely, most homeowners who buy exotic plants are simply looking for something that is “visually appealing” and “maintenance-free” and don’t realize the plants can become invasive until they are digging seedlings out of the lawn.

To satisfy both the homeowner’s desire for a beautiful landscape and the land manager’s need to prevent the entrance of exotic seeds, many state Extension specialists have recommended gardening with native plants. But where do you find native plants? How do you know a native from a typical “bedding” plant? How hard are they to maintain? Stay tuned for next week’s blog post, where I will discuss how to use native plants in the home landscape.

For more information on exotic invasive plants in the Southeast, read Nonnative Invasive Plants of Southern Forests: A Field Guide for Identification and Control, by James H. Miller.

For information on the Web, see: Center for Invasive Species and Ecosystem Health

Southeast Exotic Pest Plant Council

USDA National Invasive Species Information Center

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