Species Information

Hydrilla leaf whorl Hydrilla leaf whorl
K. Hahnel, ME DEP
Hydrilla leaf whorl
K. Hahnel, ME DEP
Hydrilla verticillata

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

  • 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.


  • 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.

  • 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

  • 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

Hydrilla (Hydrilla verticillata)
Robert Videki, Doronicum Kft.

Protecting the marine and freshwater resources of the Northeast from invasive aquatic nuisance species