Species Information

Hydrilla
Hydrilla verticillata

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Similar Species

• Common and slender waterweed (Elodea canadensis and E. nuttallii)
• Brazillian waterweed (Egeria densa)

History

• There are two types of hydrilla in the United States: monoecious and dioecious
• The monoecious type is found in the northeastern states. It was first discovered in the Potomac Basin in the 1980's and was likely introduced from Korea.
• The dioecious type is primarily found in the southern states.  It was introduced in Florida in the 1950's for use in the aquarium trade.

 

Characteristics

• Submersed aquatic plant
• Stem of the monoecious type has a delicate sprawling growth that freely branches at the lake bottom with stems reaching to the surface.
• Leaves are bright green, small and pointed; 1-5 mm wide and 6-20 mm long.  Margins are toothed. Leaves grow in whorls of 3 -10 along the stem though 5 leaves per whorl is most common. 
• Roots are fibrous rhizomes and above ground stolons.
• Unique to hydrilla is the peanut-sized or smaller tubers which form along the rhizomes.  Tuber is whitish to brown.
• Flower: the monoecious type has both female and male flowers on the same plant.  The female flower has 3 small white petals, 4-8 mmwide and 1-5 m long, and is attached to the stem tip by a slender stalk.  Male flowers are produced in the leaf axils, but detach and become free-floating.  Blooms mid to late summer.
• Tuber is the primary identifier for hydrilla.

Habitat

• Lakes, ponds, rivers, streams, canals, reservoirs, drainage ditches
• Usually in shallow water (1.5-20 ft, 0.5-6 m) but as deep as 40 ft (12 m) in non-turbid water
• Acidic or alkaline waters
• Tolerates moderate salinity and high levels of raw sewage

Known Distribution in the Northeast

• Present in many areas of U.S., including Maine, Connecticut, Massachusetts, New York
• Believed to be native to Asia or Africa, now widespread  around the world

Impacts

• Competes with native plants by growing to the water surface and forming dense mats that block sunlight
• May affect fish that cannot hunt effectively in the thick mats
• Impairs recreational activities such as fishing, boating, swimming
• Clogs rivers, irrigation ditches, and flood control canals, creating stagnant water that is prime mosquito breeding habitat
• May cause flooding and alter water quality by decreasing oxygen levels and increasing pH and water temperature