Robert Vidéki, Doronicum Kft.

Hydrilla

Hydrilla verticillata (L. f.) Royle
Frog-bit family (Hydrocharitaceae)

Origin: Central Africa

Background
Hydrilla first appeared in the Crystal River system of Florida in 1960. Imported by the aquarium trade, its presence on the Delmarva Peninsula was confirmed in 1981. It attracted national attention when infestations were found in the Potomac River in Washington, D.C. in the early 1980s. It is a federal noxious weed.

Distribution and Habitat
Hydrilla is documented throughout the southern United States from California to Delaware. In the mid-Atlantic, it occurs in much of the Potomac River, in Virginia and Maryland freshwater tributaries of the Chesapeake Bay, in the Delaware portion of the Nanticoke River, most southern Delaware ponds, and in sites in eastern Pennsylvania. It is not salt tolerant.

Ecological Threat
Hydrilla outcompetes native submerged aquatic vegetation and can quickly fill a pond or lake, thus choking off the water body for boating, fishing, swimming and other recreational uses. Although non-native and invasive, it provides good quality habitat for fish and shellfish as well as water quality benefits.

Description and Biology

• Plant: rooted aquatic plant.
• Leaves: in whorls of 4-5; about ½ in. long; fine-toothed margins, spine at tip.
• Flowers, fruits and seeds: tiny, translucent to white flowers produced on the upper branches in late summer and fall; tubers grow from the roots; winter buds (turions) are produced in the leaf axils.
• Spreads: vegetatively through fragments of stems, stolons, or rhizomes, turions, or tubers which are carried on boat livewells, motors and trailers, bait pails and other items, and by ingestion of tubers and turions by waterfowl.
• Look-alikes: native common waterweed (Elodea canadensis) with leaves in whorls of 3 and no teeth or spines.

Prevention and Control
Physical, chemical and biological controls have been used for control of hydrilla, with varying levels of success. Water level drawdowns have generally been ineffective in our area. Mechanical aquatic weed harvesters provide temporary relief and open boating lanes, but resulting plant fragments can help spread the vegetation faster. Contact herbicides provide temporary control, but systemic herbicides provide more long-term control. Herbivorous fish such as sterile grass carp have been used for hydrilla control where allowed by law. Other biological controls are still being investigated. Each control method has its drawbacks and liabilities. On the Potomac River and other parts of the Chesapeake Bay watershed, resource managers struggle with hydrilla because submerged aquatic vegetation, including hydrilla, provides water quality benefits and habitat for fish and shellfish.

Native Alternatives
Aquatic plant species are difficult to tell apart to the untrained eye. Contact your state natural resource agency, native plant society or other resource (see References) for assistance.

David J. Moorhead, UGA