Cultivating weeds: is your yard a menace to parks and wild lands?

Science News, April 12, 2003 by Janet Raloff

In 1985, shortly after buying a heavily shaded home in one of Washington, D.C.'s northern suburbs, I installed 35 liriope plants (Liriope muscari), also known as turf lilies. Gardening books recommend these East Asian, shade-tolerant border plants because the 10-inch clumps of vegetation "don't creep"--that is, invade surrounding areas. And for 15 years, those plants maintained a neat border that separated my lawn from a hill stabilized with English ivy. Four years ago, something changed. A few clumps of two or three spindly liriope leaves sprouted in the lawn. By last summer, hundreds of clumps were infesting the property--in some cases, up to 50 feet from the liriope border.

This "is a classic example of invasive ecology," observes Mike Maunder, horticulture director of the Fairchild Tropical Garden in Miami. "Many species will sit absolutely blameless for decades--and then, ping!, they explode all over the place."

As big a nuisance as such episodes pose to gardeners, they risk becoming an ecological nightmare if the botanical invasion doesn't stop at a homeowner's fence line but jumps--as increasing numbers of garden plants do--into forests, parks, and wild lands. More and more people in the United States are gardening, foreign trade in plants is going up, and, as suburban areas expand, the interface between gardens and wild lands is increasing.

In many cases, as with my liriope, scientists don't understand why such invasiveness develops. However, the more domineering of these plants almost invariably have foreign origins, notes Maunder, who has studied such botanical thugs the world over.

To be sure, most immigrant species remain well behaved--which is good news, since more than a quarter of all plant species now growing in the United States evolved elsewhere. But there are an increasing number of formerly mild-mannered guests that have morphed into bullying weeds.

Of some 300 such rogues in this country, roughly half were deliberately introduced as ornamental garden plants, according to the Federal Interagency Committee for the Management of Noxious and Exotic Weeds. This committee categorizes as weeds many prized and commercially popular garden staples--from European buckthorn (Rhamnus cathartica), a Eurasian immigrant planted as specimen trees or dense hedges, to butterfly bush (Buddleja davidii), a shrub promoted for its fragrant, bright-colored flowers.

Only about 10 species out of 1,000 new introductions will prove weedy, notes Kayri Havens, conservation-science director of the Chicago Botanic Garden in Glencoe, Ill. However, her team's research shows, it takes only one aggressively invasive species to profoundly disturb the natural ecosystem of a forest, wetland, or prairie.

INNOCENCE AND NEGLIGENCE Rock Creek Park, a federally protected 2,900-acre forest cutting through Washington, D.C., now hosts 238 species of exotic plants--most of them garden escapees, notes Susan E. Salmons of the National Park Service staff there. Forty-two have proven invasive enough to ride roughshod over native neighbors. The most notorious interlopers are two vines of Asian ancestry: a bittersweet (Celastrus orbiculatus) and porcelainberry (Ampelopsis brevipedunculatus).

In forests, Salmons observes, bittersweet and porcelainberry "grow to the tops of the tallest trees, where they create a dense, smothering foliage: Within 20 years, the weight of these vines can pull down a tree. Eradicating the problem requires intense vigilance, she notes, since seeds can survive in soil for up to 18 years before germinating.

Birds probably foster most of the vines' dispersal by excreting seeds from berries they ate in someone's yard. However, Salmons cautions, even an errant cutting can start a new plant, so "people must be careful about how they dispose of any clippings."

Yet few homeowners realize that their plant choices and husbandry can lead to environmental havoc a mile or more outside their yard. Contributing to the problem: Nurseries sell some of the most aggressive invaders--usually with little or no warning.

"I don't like to vilify these plants," says Barry Rice of the Nature Conservancy in Davis, Calif., because most invaders aren't innately bad, they're just inappropriate for where they're growing. For instance, errant wind- or waterborne seeds of the red-bloomed, South American shrub Sesbania punicea have established impenetrable thickets in California's Sacramento River delta. However, Rice says, if this marshland plant were used as a garden ornamental in an arid environment, it wouldn't spread because its seeds wouldn't germinate.

Says Maunder, there has been "a sad gap between the scientific realization of the dangers posed by invasives and communicating those threats [to the public]." Several horticultural organizations are planning public outreach. But until they get the word out, Maunder says, gardeners should start scouting their greenery for signs of plants behaving badly (see http://www.sciencenews.org/20030412/fbobg.asp#Looking).