Beware the attack of plant invaders

Garden menaces come floating on air, or up from the ground

In The Garden

October 03, 2004|By Nancy Taylor Robson | Nancy Taylor Robson,SPECIAL TO THE SUN

Invasive plants are the Mongol hordes of the botanical world.

The plants - non-native vines, trees and other flora - enter the environment in a variety of ways, try to overrun the place and in the process pound into submission the native plants and well-mannered imports that are habitat and fodder for a realm of critters.

The result is destruction of a chunk of the ecosystem and its ballet of self-sustaining interrelations.

"My biggest concern is that invasives will devour the woods or will move in on the habitat of endangered species," says Jonathan McKnight, associate director of habitat conservation for the Maryland Department of Natural Resources.

Yet discerning which plants will be invasive before they actually invade, and which will be contributing members of a community, is tricky. A plant that is civilized in its original habitat - for example, Kudzu (Pueraria lobata) - can be voraciously destructive in another. Most invasive plants are exotic imports, but only a small percentage of exotic imports are invasive. For example, English ivy, without assiduous control, can be very invasive in the larger environment, while English boxwood is not. It all has to do with natural checks and balances.

"Invasives invade because they don't have other plants, pests, or diseases that inhibit their growth and keep their population in check, so they tend to proliferate unnaturally," explains Mike Galvin, supervisor for urban and community forestry for the Maryland Forestry Service in Annapolis.

There are about 300 identified invasive plant species in Maryland, Virginia, Pennsylvania and Delaware, but some on the list are worse than others. For example, Japanese Barberry (Berberis thunbergii) and Catnip (Nepeta cataria) are far less aggressive than non-native phragmites - the plumed-topped reed that is rapidly suffocating native phragmites and engulfing our wetlands and shorelines. And while many invasives advertise themselves (green-vined Kudzu again) by covering whole acres of woods, others are tougher to distinguish from their well-behaved cousins.

"You almost need a microscope to tell the native phragmites from the invasive ones," says Gaye Williams, entomologist with the National Park Service in Washington, D.C. "It's a matter of looking at leaf bases for little hairs or no little hairs."

A few of the worst offenders are: Bradford pear (Pyrus calleryana), Tree of Heaven (Ailanthus altissima), Empress tree (Paulownia tomentosa), White Mulberry (Morus alba), which chokes out native red mulberry, Norway maple (Acer platanoides), Autumn Olive and Russian Olive (Elaeagnus umbellata and E. angustifolia), Oriental Bittersweet (Celastrus orbiculatus), Multiflora Rose (Rosa multiflora), Purple Loosestrife (Lythrum salicaria), the scourge of the waterways, and Japanese stilt grass (Microstegium vimineum).

"Stilt grass is a highly invasive weed that prefers rough ground," says Lew Shell, a consultant with the University of Maryland Extension Service. "It's extremely difficult to control because its seeds stay viable for up to five years, so you have to remove it before it goes to seed in early fall."

"Mile-A-Minute-vine [Polygonum perfoliatum] is awful," says Williams. "On the bottom of the D.C. beltway up to Rockville it's even covering the kudzu."

Some invasives fall under state or federal legislation curtailing their transport, sale and cultivation. Among them are: Canada thistle (Cirsium arvense), Water hyacinth (Eichhornia azurea crassipes), common reed (Phragmites australis) and Johnsongrass (Sorghum halapense).

What you can do

"We're trying to get folks on the team," says McKnight, "to realize what these plants are and to avoid spreading them around."

Obviously, to avoid them we need to identify them. Before purchasing a plant, ask whether it is on the Maryland watch list. Sadly, there is no current comprehensive guide, though Web sites offer lists of plants to avoid. Conscientious garden centers try to inform customers about a plant's potential for thuggishness and to suggest alternatives. Some voluntarily cull known invasives from their sales.

"We don't sell Japanese honeysuckle or running bamboo [though it is legal to do so]," says Donna Shipp, garden supply manager for Homestead Gardens in Davidsonville. "But we do sell clumping bamboo."

While there is no single reference, the USDA, National Park Service, native plant societies and others offer useful sources, though more are available on the Internet than in print. One good reference is "Plant Invaders of the Mid-Atlantic Natural Areas" (National Park Service), which lists 49 regionally invasive species with photos and descriptions.

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