Cultivating Weeds

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 brevipedunculata).

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 “Looking for Trouble,” below).

Invasions’ costs

Invasive foliage has become a serious problem globally. Although attempts to quantify its costs have been spotty, Maunder says $4 billion annually is a fair estimate for the worldwide devaluation of natural resources and cost of combating the exotics’ aggression.

The nonprofit Nature Conservancy owns some 1,500 preserves around the nation, where combating invasive species is a top priority, notes Rice. A survey his organization conducted among its preserve managers showed that the group’s staff devotes some 36,000 hours annually in efforts to rout 271 invading species.

Volunteers contribute almost that much time again. Assembled teams sometimes just pull up the weeds. Other times they excavate them, burn them, drown them, mow them down, attack them with natural predators, or shower them with herbicides. Says Rice, “We evaluate each problem individually and then just do whatever it takes.”

Other scientists are evaluating the invaders’ ecological costs.

In woodlands invaded by European buckthorn, Pati Vitt of the Chicago Botanic Garden finds dramatic stunting and compromised reproduction in a rare native wildflower, the dog violet (Viola conspersa). Although healthy dog violets sport up to 50 leaves and lots of blooms, “under buckthorn cover, an average plant will produce maybe five or six leaves,” Vitt says, and no real flowers.

The problem, she says, is that buckthorns form a dense cover 20 feet above the ground that shrouds the forest floor in almost total darkness. Everything dies, she says, “except poison ivy, a few woodland sedges, and the occasional Viola.”

Like buckthorn, the Norway maple’s dense canopy withholds light from shorter plants. The dense, shallow roots of this European native make it hard for native forest-floor plants to find space, and slurp up more than their share of water, weakening neighboring trees. Also, the maple’s huge numbers of wind-blown seeds thrive almost anywhere. Such characteristics make the tree “too big a bully” to responsibly fit into any U.S. landscape, says Ann F. Rhoads of the University of Pennsylvania’s Morris Arboretum in Philadelphia.

Two hundred years ago, a Philadelphia gardener imported the Chinese tree of heaven (Ailanthus altissima), and Chinese immigrants later introduced it on the West Coast. This species has proven such a successful invader that it continues to establish countless new communities even though it’s unlikely many people have intentionally planted it for the better part of a century. One key to its success: It poisons the competition.

Beginning around 1995, several research teams identified the chemical ailanthone from this tree’s roots as a toxin that inhibits the germination of other plants.

In the Feb. 26 Journal of Agricultural and Food Chemistry, a European research team has not only confirmed ailanthone’s toxicity to cultivated plants, but also identified four additional poisons in the tree of heaven’s roots. Indeed, these agents may hold promise for controlling weeds in crop fields, say Vincenzo de Feo and his colleagues at the Università degli Studi di Salerno in Italy.

By creating novel tinder, even the invasion of “boring” ornamental Eurasian grasses into Arizona’s dry lands is creating big problems, Maunder notes. “Parts of the Sonoran Desert that had never burned are now beginning to,” he says. It’s devastating native cacti, which can survive drought but not fire.

Fitness or luck

Why do some plants turn invasive? The answer could help botanists anticipate which new immigrant will be the next environmental tyrant.

For years, a leading hypothesis has been that the immigrants left behind the pests and predators that had held their numbers in check. A study in the Feb. 6 Nature now supports this view.

Charles E. Mitchell and Alison G. Power of Cornell University focused on 473 European plant species that have invaded the U.S. landscape. The plants faced, on average, only 16 percent as many fungal species and 76 percent as many viruses as their kin remaining in Europe did.

Other researchers have found another means by which some nonnative plants increase their invasiveness. They hybridize with relatives they meet in their new world.

John F. Gaskin, now with the U.S. Department of Agriculture’s pest-management research unit in Sidney, Mont., and Barbara A. Schaal of Washington University in St. Louis examined desert salt cedar. In the United States, these trees—initially imported from Eurasia to shade arid plots—rank second only to purple loosestrife (Lythrum salicaria) as the most invasive nonnative plants.

Physically, many U.S. salt cedars look somewhat different from the two main species initially imported, Tamarix chinensis and Tamarix ramosissima. The U.S. plants also repel many pests that plagued one or the other of those species in Eurasia. This suggested, Gaskin says, that at least some U.S. salt cedars were novel crosses of the two imported species.

Using DNA fingerprinting techniques, he and Schaal examined 269 salt cedars—both in the United States and overseas. In the Aug. 20, 2002 Proceedings of the National Academy of Sciences, they confirmed that the most invasive U.S. specimens were hybrids. Gaskin says that this adaptive crossbreeding couldn’t have been anticipated before the trees were imported because their native ranges didn’t overlap.

Then there are some immigrant species that appear to be well behaved, but in fact are time bombs, notes Daniel Simberloff of the University of Tennessee in Knoxville. In the book Strangers in Paradise (1997, Island Press), he relates how nurseries imported 60 species of ornamental figs to Florida, where landscapers for decades employed them without a problem.

Growers propagated the plants by hand, since to reproduce naturally, each species needed a particular pollinating wasp—none of which was in the United States. About 20 years ago, however, the first fig wasp—the only pollinator of Ficus microcarpa—arrived from Asia. At once, Simberloff tells Science News, this laurel fig turned aggressively invasive.

Codes of conduct

Sixteen months ago, invasive-plant experts convened at the Missouri Botanical Garden in St. Louis and drafted a policy statement. This St. Louis Declaration asks nurseries and botanical gardens to implement a voluntary code of conduct that would have them introduce cultivars in a manner that would limit unintended harm; work toward national standards to prevent and manage plant invasions; foster research; and inform the public about risks associated with garden species.

At about the same time, several major growers and botanical gardens began testing new plants for aggressiveness before they get into garden centers, the Nature Conservancy’s John Randall observes.

For instance, Wayne Mezitt, chairman of Weston Nurseries in Hopkinton, Mass., and president of the Washington, D.C.–based American Nursery and Landscape Association, is spearheading the development of criteria for Massachusetts growers to use in assessing invasiveness. Tests of the criteria and reviews of existing data that were developed elsewhere have already led his nursery to stop selling some plants and to offer shoppers guidance on other cultivars that might prove problematic.

He sees a big role for research, not only in helping identify plants’ invasive potential, but also perhaps in genetically modifying traits that make some species so invasive.

For now, Maunder says, gardeners must begin recognizing “that what they plant can have far-reaching impacts.”

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Looking for Trouble

Plants behaving badly

Though liriope doesn’t appear on lists of prominent invasive plants, the sudden behavioral change in my yard (see “Cultivating Weeds,” above) is a classic tip-off that the cultivar may be trouble, according to Barry Rice with the Nature Conservancy’s weed-science program in Davis, Calif. The sudden spread of seedlings after years of the plant’s self containment may signal that animals have suddenly discovered the plant’s berries, that the plant has evolved new vigor, or that the climate has changed in ways that promote its propagation.

Patti Vitt of the Chicago Botanic Garden in Glencoe, Ill., notes that animals will discover most kinds of seeds, but bird-dispersed berries pose a special risk of spreading nonnative plants. Ironically, she says, many plant catalogs and garden centers promote plants with bright berries for their “winter interest” or for luring wildlife into the yard. Gardeners need to recognize the production of attractive berries as a red flag for potential long-range dispersal by birds, she says.

Another warning sign: copious seed production. Trees that produce seeds that flutter away in the wind or ride down streets in rain runoff—like those of the infamous Chinese import the tree of heaven—need no help from gardeners to establish distant colonies. A single tree of heaven can spawn 350,000 seeds a year, most of which germinate and establish taproots within 3 months. Many states already list this species as a noxious weed and ban its planting.

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Commercial species that can be invasive

Even the occasional weekend gardener may recognize many of the commercially available plants on this list. They represent a sampling of botanical bullies identified by the Federal Interagency Committee for the Management of Noxious and Exotic Weeds, as highlighted in the 1998 government report Invasive Plants: Changing the Landscape of America: Fact Book. Although most of these aggressive species hail from outside North America, at least a few natives, such as black locust, also made the cut.

• Autumn olive (Elaeagnus umbellata)
• Baby’s breath (Gypsophila paniculata)
• Batchelor’s button, or garden cornflower (Centaurea cyanus)
• Black locust (Robinia pseudoacacia)
• Buckthorn, common or European (Rhamaus cathartica)
• Burning bush or winged Euonymus (Euonymus alata)
• Butterfly bush (Buddleja davidii)
• Chinese silver grass or Eulalia (Miscanthus sinensis)
• Cotoneaster (Cotoneaster spp.)
• Crown vetch (Coronilla varia)
• English holly (Ilex aquifolium)
• European privet (Ligustrum vulgare) and its oriental cousins
• Figs, edible and various ornamental (Ficus carica, F. microcarpa, and others)
• Japanese barberry (Berberis thunbergii)
• Japanese climbing fern (Lygodium japonicum)
• Japanese honeysuckle (Lonicera japonica)
• Japanese spiraea (Spiraea japonica)
• Japanese yew (Taxus cuspidata)
• Melaleuca or cajeput tree (Melaleuca quinquenervia)
• Multiflora rose (Rosa multiflora)
• Norway maple (Acer platanoides)
• Oriental or Asiatic bittersweet (Celastrus orbiculatus)
• Pampas grass (Cortaderia spp.)
• Periwinkle (Vinca minor)
• Porcelainberry (Ampelopsis brevipedunculata)
• Purple foxglove (Digitalis purpurea)
• Scotch broom (Cytisus scoparius)
• Tall fescue (Festuca arundinacea)
• Winter creeper or climbing Euonymus (Euonymus fortunei)

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This article was reprinted in the April/May 2004 issue of National Wildlife magazine.

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