Invasive Plant Species


Phragmites, an invasive species, grows along the banks of the Anacostia River.

Krista Schlyer/iLCP

Bay Invaders: The Alien Species Among Us

Over 170 species from other areas now live and reproduce in the Chesapeake Bay watershed. Some have been brought here accidentally—through ballast water or as hitchhikers on boat bottoms—and others have been introduced to serve a specific purpose. Although some have proven beneficial, others are a nuisance, and a few threaten our ecosystem. Some thrive out-of-control with no natural predators or disease, choking out our native plants and animals. Millions of dollars are spent each year across the country trying to control the invasive species that threaten the balance of the ecosystems they invade. Which are our enemies? And which are our friends? The following are profiles of three of the most well-known, and sometimes misunderstood, aliens in our watershed.

English Ivy

Origin: It is believed that English ivy was brought to the U.S. by European settlers who prized it as a dependable ground cover. The vine is native to Europe, Western Asia, and Northern Africa.

Description: A member of the ginseng family, this evergreen vine is a virile ground cover and climber. Variable leaf forms, the most popular being three-lobed, are dark green and waxy.

Watershed Whereabouts: Sold widely in the U.S. as an ornamental plant, English ivy is found abundantly throughout the watershed.

Threats: English ivy grows quickly and densely threatening all levels of vegetation. The vines, which can reach 100 feet in length, spread by runners and by seeds. On the ground, English ivy's low canopy blocks sunlight from reaching other species' seedlings. Above ground, root-like extensions along the vine produce an adhesive substance that makes tree climbing a breeze. Host trees are threatened as the vines encase branches and twigs and block sunlight. Added weight from the vines can also make trees more likely to fall during storms.

What You Can Do:  Never introduce English ivy to your landscape; instead, try a native substitute like Virginia creeper.


Origin:  Hydrilla was likely introduced to the U.S. after being imported from Korea and India as an aquarium plant in the 1950s. It was first noted in the wild in Florida's Crystal River in 1960.

Description: This rooted underwater grass branches freely, especially near the surface. Toothed leaves, which grow to 3/4 inch, present in whorls of four to eight.

Watershed Whereabouts: Hydrilla is present in the Chesapeake Bay watershed from Pennsylvania and Delaware to Virginia. Dense beds in D.C.'s Potomac River have been especially newsworthy. 

Benefits: Where submerged aquatic vegetation was not previously present, hydrilla can provide habitat for fish and shellfish and have a positive effect on water quality.

Threats: The invasive hydrilla can spread quickly, choking out existing native underwater grasses. Its dense presence on the water's surface can make swimming, boating, and fishing almost impossible. Some methods of controlling hydrilla—like mechanical weed harvesters—can actually encourage the plant's growth. Hydrilla can propagate vegetatively through small pieces of stem or tuber.

Japanese Stilt Grass

Origin:  Once used as a packing material to ship delicate porcelain from the Far East, Japanese stilt grass made its first U.S. appearance in Tennessee in 1919.

Description:  This annual, which reaches one to three feet in height, resembles a miniature bamboo. Light-green, lance-shaped leaves are about three inches in length. Japanese stilt grass produces airy flower spikes in late summer and early fall.

Watershed Whereabouts: Now present from New York to Florida, Japanese stilt grass can be found along rivers and streams and in floodplains, wetlands, and gardens.

Threats: Japanese stilt grass spreads by rooting along stem joints and by seed. Each node can produce one new plant and each plant can produce up to 1,000 seeds. The plant is especially intrusive in areas where soil is disturbed by mowing, tilling, deer traffic, and other activities. Uncontrolled, Japanese stilt grass will destroy native understory in areas from full sun to deep shade.

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