Gardening Columns

Invasive Plants: Buy Now, Pay Later

First published on garden.org on April 10 , 2008, by Suzanne DeJohn

I was shocked to see mugwort (Artemisia vulgaris) for sale at a local herb festival last spring. This pernicious perennial meets the definition of a weed in my garden -- a plant growing (and growing and growing) where it's not wanted. A poster child for good plants gone bad. And it's just one of the many invasive exotic (nonnative) plants in our region. How did the problem come about, who is responsible, and what can we gardeners do to help?

As is true of many invasives, mugwort has positive attributes that make it desirable to grow. Native to temperate Eurasia, mugwort was likely brought here as a medicinal. The plant has been used as a stimulant, for its antibacterial and anti-inflammatory effects, and to treat asthma. Its other common name, motherwort, alludes to its use in women's health. But mugwort escaped the confines of garden cultivation and subsequently spread unchecked. Now it has colonized much of eastern North America where its dense growth and allelopathic qualities (suppresses growth of other nearby plants) give it an advantage over native plants.

How Invasive Exotics Got Here
Some problem plants were likely brought here inadvertently, but many were introduced as ornamentals. These plants have the very qualities we look for in our garden plants, including adaptability, ease of propagation, and rapid growth.

Kudzu -- "the vine that ate the South" -- was brought here from Japan in 1876 so the fast-growing vine's large leaves and sweet-smelling purple flowers could decorate the Japanese Pavilion at the U.S. Centennial Exposition. Later, kudzu was widely planted to prevent soil erosion and as forage for livestock. Today, kudzu smothers some 7 million acres of land in the southeast and continues to spread. No gardener in their right mind would plant kudzu today.

But what about English ivy (Hedera helix), bugleweed (Ajuga reptans), and periwinkle (Vinca minor)? These nonnative plants were introduced to the U.S. in the 1700s as ornamentals and are common in gardens. Unfortunately, all three have escaped into woodlands and fields where they form dense evergreen mats that smother and crowd out native vegetation. English ivy, bugleweed, and periwinkle are considered invasive throughout our region and are on the Plant Conservation Alliance's "Least Wanted" plant list. Yet all are still readily available at garden centers.

A Matter of Personal Responsibility or Law?
Mugwort serves as a good example of the dilemma posed by many invasive exotics. Should this invasive medicinal plant be banned? Should it be illegal to buy, sell, or plant it? Although rated hardy to USDA Zone 3, mugwort likes heat and well-drained soils. It's much less invasive, if it is at all, in northern regions and in cold, wet soils. Should it be banned where it's invasive, and permitted where it's not? Who will draw the line?

Exotic invasives don't necessarily originate in far away lands. Black locust (Robinia pseudoacacia), native to the central Appalachian and Ozark Mountains, is prized as a long-lasting substitute for pressure-treated wood. But it is considered an invasive species in California. Mint is invasive. Should Kentucky Derby fans forego mint juleps?

What's a Gardener To Do?
You might be surprised at some of the plants listed as invasive exotics. Japanese barberry (Berberis thunbergii), burning bush (Euonymus alata), fiveleaf akebia (Akebia quinata), privet (Ligustrum spp.), and Russian olive (Elaeagnus angustifolia) are all on the North Carolina Botanical Garden's "Plants to Avoid in the Southeastern United States" list. Need help recognizing them? Take a look at your local garden center where you'll probably find some for sale.

Avoiding plants on the N.C. Botanical Garden list is one step gardeners can take. But it's not a black-or-white situation. Take the case of butterfly bush (Buddleia davidii). The species is considered invasive in several mid-Atlantic and west coast states because the plant's flowers produce abundant, readily dispersed seeds. Some of the cultivated varieties, however, produce few viable seeds. According to a study done at Longwood Gardens in Pennsylvania, the cultivated varieties 'Summer Rose' and 'Orchid Beauty' produced 20 times fewer viable seeds than 'Potter's Purple' and 'Border Beauty'. The Landscape Plant Development Center is attempting to breed sterile varieties by crossing the species with its distant cousins. Watch for news about new introductions and more information about the reseeding habits of currently available varieties. And in the meantime, if you have a butterfly bush, deadhead the plant after flowering to remove developing seeds.

A similar situation occurs with Japanese barberry. Introduced in the 1800s as a readily adaptable, deer-resistant shrub, the plant has encroached in natural areas where its deer resistance gives it an advantage over native plants. Researchers are trying to determine whether varieties in the horticulture trade, such as the popular 'Rosy Glow', are contributing to the problem, while at the same time trying to breed sterile varieties. Until more is known, it's probably best to avoid planting barberry.

What is the Horticulture Industry's Role?
The horticulture trade, like any other industry, responds to demand. Companies vie for customers in part through reliable sellers, and in part through new plant introductions -- plants often gathered from far-off lands that haven't been tested for invasiveness here. To ask the horticulture industry to stop selling plants that are in high demand is asking a lot. To ask them to refrain from introducing new plants until they've been trialed for years is also a tall order. But it's not impossible.

Last year, California's $20 billion horticulture industry began collaborating with environmentalists to address the invasive plant problem. The outcome was PlantRight, a new program aimed at eliminating invasive plants from California's horticulture trade and preventing the introduction of new ones. Voluntary partnerships like these, along with vigorous campaigns to educate the public, are probably the best chance we have of limiting damage caused by invasives and cultivating appreciation for well-behaved native alternatives.

For More Information
"Least Wanted: Alien Plant Invaders of Natural Areas" from Plant Conservation Alliance:
http://www.nps.gov/plants/alien/factmain.htm

"Invasive Plants -- A Horticultural Perspective," from Virginia Cooperative Extension: http://www.ext.vt.edu/pubs/envirohort/426-080/426-080.html

"PlantRight" from California Horticultural Invasives Prevention: http://www.plantright.org/

"Breeding a Better Butterfly Bush," from the Landscape Plant Development Center: http://www.landscapecenter.org/new/?q=node/20

"Researchers Outsmarting Popular But Invasive Barberry Shrub," from the University of Connecticut: http://advance.uconn.edu/2005/050321/05032108.htm