They’re the Mongol hordes of the botanical world. Invasive plants. They 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 ecosystem and its richly choreographed 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 Department of Natural Resources in Annapolis.
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, the infamous 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.
THE WORST OFFENDERS
There are about 300 identified invasive plant species in Maryland, Virginia, Pennsylvania, and Delaware, though some are worse than others. For example, Nandina and Buddleia, though listed as invasive, are far less aggressive than things like non-native Phragmites (Phragmite australis). 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 Maryland Department of Agriculture. “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 (Oyrus calleryana), Tree of Heaven (Ailanthus altissima), Empress tree (Paulownia tomentosa), White Mulberry (Marus alba), which chokes out native red mulberry, Norway maple, (Acer platanoides) Autumn Olive and Russian Olive (Elaeagnus angustiflora and E. umbellata), Oriental Bittersweet (Celastrus orbiculatus), which chokes out native bittersweet, Multiflora Rose (Rosa multiflora), Purple Loosestrife (Lythrum salicaria) the scourge of the waterways, and Vietnamese or Japanese stilt grass (Microstegium viminium).
“Stilt grass is a highly invasive weed that prefers rough ground,” says University of Maryland Extension Service consultant Lew Shell. “It’s extremely difficult to control because its seeds stay viable for up to 5 years, so you have to remove it before it goes to seed in early fall.”
“Mile-A-Minute-vine [Polygonum perfoliatum] is awful,” says Willliams. “On the bottom of the DC 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 (Eichhomia azura crassipes), common reed (Phragmites australis) and Johnsongrass (Sorghum halepense).
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. Web sites offer lists of plants to avoid. Conscientious garden centers try to inform customers about a particular plant’s potential for thuggishness, especially under particular circumstances, and to offer alternative suggestions. 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 net 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. Another is Sylvan Kaufman’s Invasive Plants: A Guide to Identification and the Impacts and Control of Common North American Species.
“Two of the best guides are Brown and Brown’s Guide to The Herbaceous Plants of Maryland and Guide to The Woody Plants of Maryland,” says plant ecologist Rod Simmons. “That’s the best resource we have statewide with great pictures and line drawings from coast to mountains. The index is easily accessible and helps distinguish between things like invasive white mulberry and native red mulberry.”
Sources:
MD Cooperative Extension Service
University of MD
800-342-2507CQ
410-531-1757CQ
www.hgic.umd.edu/pubs/online/misc1.pdf
Invasive Plant Control in Maryland – pub hardcopy 4 pages with line drawings
National Invasive Species Council
US Dept of Interior
1201 Fye St. NW 5th Floor Rm 79
Washington DC 20005
202-513-7245CQ
MD Invasive Species Council
MD Dept of Agriculture
MDA Plant Protection and Weed Management
50 Harry Truman Pkwy
Annapolis, MD 21491
410-841-5920CQ
free pamphlet Invasive Species of Concern in Maryland
Maryland Native Plant Society
P.O. Box 4877
Silver Spring, MD 20914
301-809-0139CQ
Plant Invaders of Mid-Atlantic Natural Areas for $3.50 postage and handling
Virginia Native Plant Society
400 Blandy Farm Lane Unit 2
Boyce VA 22620
540-837-1600
Booklet Invasive Alien Plant Species of Virginia covers most invasives in Maryland. It’s a listing without illustrations, but web site has detailed fact sheets on individual plants
Mid-Atlantic Exotic Pest Plant Council
1019 South 27th St.
Arlington, VA 22202
Alien Plant Working Group
4598 MacArthur Blvd. NW
Washington DC 20007
Alien Plant Invaders of Natural Areas