There’s No Hiding from Hydrilla

Contact: Mark Lewandowski, MD DNR, Tidewater Ecosystem Assessment |

hydrilla naylor
Hydrilla. Photo: Mike Naylor, MD DNR

ANNAPOLIS, MD (June 5, 2009) – Many of the tributaries, reservoirs and lakes in Maryland have extensive populations of the submerged aquatic plant Hydrilla(Hydrilla verticillata), sometimes called waterthyme. It is distributed all over the world, though it is native to Korea and India. It can flourish in any fresh water environment and can grow in both low and high nutrient conditions.Hydrilla can be found at depths of more than 20 feet, can withstand very low light conditions and a wide temperature range. It can grow very rapidly, and will branch profusely and spread over the water’s surface. This gives it a competitive advantage over many native species of aquatic vegetation. Hydrilla is visible now and may be present in fresh water where you swim, boat or fish, which is why Hydrilla has been chosen as the June MISC Invader of the Month.

The plant’s appearance can vary considerably between regions, depending on the growing conditions. Usually, however, Hydrilla stems are erect and freely branching with whorls of 4-8 linear leaves. The stems grow vertically without much branching, until they near the surface, where dense branching may occur. Hydrilla leaves have strongly toothed or serrated margins and are 2-4 mm wide and 6-20 mm long; they often have one or more teeth along the midrib on the underside. Although Hydrilla does produce flowers, it reproduces primarily by the regrowth of stem fragments broken off from established plants. It also reproduces from buds in the leaf axils called “turions” and small tubers, also called turions, embedded in lake bottom soil.

There is only one species of Hydrilla in the world, but Hydrilla verticillata has a dioecious type (plants having female flowers only) and monoecious type (plants having male and female flowers on the same plant). The female flowers are tiny, white and appear on elongated stalks that float on the water’s surface. The male flowers are greenish and occur at the tips of the stems, attached at the leaf axils. The monoecious type, which is found here in the Chesapeake region, allows for easier adaptation to different habitats.

US populations of Hydrilla were probably introduced to Florida as an aquarium plant in the late 1950s. By the 1970s the species was well distributed throughout Florida. The plant now occurs in fresh and brackish waters from Florida to Connecticut and all the way west to California. In the south the plants, which are perennial, grow year round. In the north, Hydrilla overwinters and regrows from the tubers each year. Maryland’s populations come from a later introduction than Florida’s and were first observed in the Potomac River in 1982. By 1992 it covered 3,000 acres. Hydrilla is also found in the Susquehanna Flats and a few upper Bay tributaries.

Hydrilla can be confused with two other submerged plants with small linear leaves. Common waterweed (Elodea canadensis) has a similar appearance; however, leaves of waterweed are in whorls of 3 and are not as markedly toothed as those of Hydrilla. Egeria (Egeria densa) has whorls of 4-5 leaves.

Hydrilla is very invasive, and can quickly form a canopy over native vegetation, reducing or eliminating light penetration and inhibiting the ability of other aquatic plants to photosynthesize. With its capacity to grow plants from overwintering turions as well as from floating plant fragments, Hydrilla can quickly take over a water body.

For Marylanders who typically boat in areas with Hydrilla, it is very important to clean your vessel, engine and trailers before using them elsewhere. Just one fragment of the plant can be enough to establish a new population if the conditions are right. The use of watercraft in areas infested with Hydrilla is severely impacted, as it can grow into mats that float on the surface. It also creates obstacles to water sports in lakes and reservoirs, can affect fishing opportunities, and thus has the potential for severe economic impacts in areas that rely on water use. Despite the fact that Hydrilla verticillata is a listed federal noxious weed, it is still sold as an aquarium plant.

Once Hydrilla is established, it is expensive to manage or remove. Techniques include herbicide applications, mechanical harvesting, and large scale habitat restoration. Mechanical harvesters must remove all of the plant material from the water, rather than chopping it up, which in Hydrilla’s case, simply makes more plants. In lake situations, a drawdown of lake water to expose and dry out the Hydrilla may work to control the plant, but drawdowns obviously affect other lake dwellers as well.

There may be some benefits from Hydrilla infestation, although these take years to appear. In the upper Chesapeake Bay and some of its tributaries, scientists have noticed native bay grasses returning to areas formerly invaded by Hydrilla. Researchers believe that Hydrilla, which can thrive in poor water quality conditions, can clean impurities from the water column enough to allow native species propagules reintroduced by waterfowl to grow. As the natives return in cleaner water, the Hydrilla often declines.

For more information, and for tips on distinguishing between Hydrilla and other aquatic plants, see:

For more information about this and other Invasive Species of Concern, visit