Hydrilla

Hydrilla verticillata

Hydrilla has long, freely branching stems with:

• Whorls of four to five leaves.
• Leaves are linear or lance-shaped and usually have tiny (but visible) teeth on the edges.
• Roots form on the bottom of the stem (called the rhizome), which grows along or just below the bottom sediment.
• Fleshy, potato-shaped tubers also grow from rhizomes.

Where does hydrilla grow?

Hydrilla grows in freshwater areas of most of the Bay's tributaries, especially in the Potomac River. It can also tolerate brackish water to a limited extent. Hydrilla does not need a lot of light to grow, so it can flourish in waters with a rapid current.

How does hydrilla reproduce?

Hydrilla reproduces sexually during late summer:

• Each hydrilla plant has both male and female flowers, which are tiny and white.
• The female flowers stay attached to the plant and float on the surface of the water while the male flowers break off from the plant.
• The pollen from the male flowers must settle directly on the female flowers, which is successful in producing seeds about 50 percent of the time.

Asexual reproduction, which has a much higher success rate, takes place when buds, tubers, rootstock and fragments develop into new plants in the spring.

Other facts about hydrilla:

• Native to Asia, Africa and Australia. In the 1960s it was introduced to the U.S. through the aquarium trade. Bay scientists first found hydrilla in the upper Potomac River in 1982. Today it covers thousands of acres of shallow water just downriver from Washington, D.C.
• Hydrilla can be confused with common waterweed. You can distinguish hydrilla by its tubers and whorls of three to five leaves with visible teeth.
• Boaters consider hydrilla to be a nuisance because it chokes channels and waterways where people enjoy recreational water sports. However, hydrilla provides important habitat for fish, especially largemouth bass, and is a major food source for waterfowl.