florida Early Detection Network




Hydrilla verticillata (L.f.) Royle


Frogs Bit Family


Hydrilla verticillata

Whole plants out of water

Young plants





Synonyms: Serpicula verticillata L.f.


Botanical Glossary

Hydrilla verticillata is a submerged, aquatic perennial herb that can grow from depths of 6 m (20 ft.). The plants have both a monoecious and a dioecious form. The leaves of the plants are 2-4 mm (0.07-.15 in.) wide (down to 1 mm (0.04 in.) on monoecious plants) and are 0.6-2 cm (0.2-0.8 in.) long. The leaves are whorlled around the stem, with 3-8 leaves per whorl. There can be "sharp" spines of variable size along the margins of the leaves, giving them a toothed appearance. The leaves have a midrib which is reddish in color. This plant has various methods of reproduction. The monoecious form of the plant produces female flowers that have three translucent petals that may contain a few red streaks. These flowers are 1-5 cm (0.4-2 in.) long and 4-8 mm (0.15-0.3 in.) wide. There are also three sepals that are white in color. The flowers are attached to the axils of the leaves by a long hypanthium. The male flowers also have three petals that are around 2 mm (0.07 in.) long and are colored anywhere from white to red. There are three white, red or brown sepals. These flowers are short stalked, detaching from the plant and floating to the water surface. The dioecious plants in the U.S. so far are female (however the hydrilla collected in Connecticut did not have flowers present). Another reproductive structure which makes these plants a successful invaders are the stem tubers (turions). The stem tubers are bud-like structures that are produced along the stems of the plant, and can vary in color from dark green, to grey to whitish. They are 0.6 cm (0.25 in.) long and often appear spiny. These structures can break off of the plant and survive the winter in the sediment at the bottom of waterbodies. Tubers are also another way in which these plants spread. These tubers form at the end of the rhizomes of the plant and are 5-10 mm (0.2-0.4 in.) long and white or yellow in color. Page References Crow & Hellquist 31, Flora of North America 35, Gleason & Cronquist 637, Holmgren 606. See reference section below for full citations.


Egeria densa Planch. (Brazilian waterweed), Elodea canadensis Michx. (native), Elodea nuttallii (Planch.) St. John (native)


Stem tubers can break off and start new plants. Waterfowl and boats can carry pieces of the plants to new waterbodies, where these fragments can root and grow.


The origin of Hydrilla is uncertain. It is now found in parts of Asia, Australia, Europe and South America. Canada has not yet reported hydrilla, though it has been found near the boarder. In the United States, the dioecious form is found across the South from Florida to California. The monoecious form is located on the West Coast in Oregon and Washington. The monoecious form is also found in Delaware, Maryland and Washington D.C. in the Potomac River. Both forms can be found in Virginia and the Carolinas. It is also present in Pennsylvania, New Jersey, Tennessee and Iowa. In New England, it occurs infrequently in southern Connecticut, on Cape Cod in Massachusetts, and in southern Maine.


Hydrilla verticillata was first introduced into the U.S. in the 1950s in Florida for use as an aquarium plant. The first record of it outside of cultivation was in Florida in 1960. It was found in the Potomac River in 1981. The plant was collected in Mystic, Connecticut in 1989, and misidentified as Egeria densa. This error was unknown until 1995 when it was finally determined to be Hydrilla verticillata. On returning to this pond in 1996, the population was still persisting. Subsequently Hydrilla has been reported from other localities in Connecticut, a pond on Cape Cod (MA) in 2001, and a pond in southern Maine in 2002.


Aquatic, Lake or Pond, River or Stream. Hydrilla verticillata can be found in both still and running water. These habitats include lakes, rivers, reservoirs and ponds. However, in Connecticut hydrilla has only been found in ponds and lakes that range in size from a 1 acre pond to a 50 acre lake. It can tolerate acidic to basic, oligotrophic to eutrophic and fresh to brackish water.


Hydrilla verticillata forms dense, monotypic mats that can shade and crowd out the native vegetation. It can grow at lower light levels than other aquatic plants. It has the ability to store extra phosphorous, can survive a wide range of pH conditions and can tolerate a fair amount of salinity. It alters the water quality by raising the pH, decreasing the oxygen and raising the water temperature. It also causes water to stagnate, providing a good habitat for breeding mosquitoes. It is an impediment to fish as well as water recreation and navigation. This plant has spread via waterfowl and by fragments remaining attached to boats that travel between waterbodies. Its turions and tubers can overwinter and it can also reproduce via plant fragments and seeds (though no seedlings have been seen in the natural environment). It was reported as having spread along 500 ha of the shoreline of the Potomac River in eight years. Currently, it has infested over 65,000 acres in Florida alone. It can even outcompete other aggressive invasives such as Myriophyllum spicatum (Eurasian watermilfoil) and Egeria densa (Brazilian waterweed). The control and management of this plant in the South has cost millions of dollars. It is listed on the federal noxious weed list.


Documentation required: Herbarium specimen or mounted snippet of the plant with stem turions. Best time for documentation: Late spring, summer, fall.


Integrated Taxonomic Information System - Has general taxonomic information about the species.

The PLANTS Database - General information and a map

Center for Aquatic and Invasive Plants - Extensive general information

Aquaphyte Newsletter, University of Florida - Article on Hydrilla Management

Washington Department of Ecology - General Information

Washington Department of Ecology - General information and control

Maine Department of Environmental Protection - Account of Hydrilla occurance in Maine


Balciunas, J.K., D.W. Burrows, M.F. Purcell. 1996. Comparison of the physiological and realized host-ranges of a biological control agent from Australia for the control of the aquatic weed Hydrilla verticillata. Biological Control 7 (2): 148-158.

Center, T.D., M.J. Grodowitz, A.F. Cofrancesco, G. Jubinsky, E. Snoddy, J.E. Freedman. 1997. Establishment of Hydrellia pakistanae (Diptera: Ephydridae) for the biological control of the submersed aquatic plant Hydrilla verticillata (Hydrocharitaceae) in the southeastern United States. Biological Control 8 (1): 65-73.

Crow G.E. and C.B. Hellquist. 2000. Aquatic and Wetland Plants of Northeastern North America. Vol 2. University of Wisconsin Press, Madison.

De Winton, M.D. and Clayton, J.S. (1996) The impact of invasive submerged weed species on seed banks in lake sediments. Aquatic Botany 53, 31-45.

Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. Volume #. Oxford University Press.

Flora of North America Association ed. 2000. Flora of North America vol. 22. Oxford University Press, Oxford, New York.

Gleason H.A. and A.C. Cronquist. 1991. Manual of Vascular Plants of the Northeastern United States and Adjacent Canada. 2nd ed. New York Botanical Garden, Bronx, New York.

Grodowitz, M.J., T.D. Center, A.F. Cofrancesco, J.E. Freedman. 1997. Release and establishment of Hydrellia balciunasi (Diptera: Ephydridae) for the biological control of the submersed aquatic plant Hydrilla verticillata (Hydrocharitaceae) in the United States. Biological Control 9 (1): 15-23.

Hofstra, D.E., J. Clayton, J.D. Green, M. Auger. 1999. Competitive performance of Hydrilla verticillata in New Zealand. Aquatic Botany 63 (3-4): 305-324.

Holm, L.G., Pancho, J.V., Herberger, J.P., and Plucknett, D.L. (1979). A Geographical Atlas of World Weeds. John Wiley and Sons, New York, USA.

Holmgren N.H. 1998. Illustrated Companion to Gleason and Cronquist's Manual. New York Botanical Garden, Bronx, New York.

Langeland, Kenneth A. 1996. Hydrilla verticillata "The Perfect Aquatic Weed". Castanea 61:293-304.

Les, D.H. 1996. Hydrilla verticillata threatens New England. Aquatic Exotic News 3(1):1-2.

Les, D.H., L.J. Mehrhoff, M.A. Cleland and J.D. Gabel. 1997. Hydrilla verticillata (Hydrocharitaceae) in Connecticut. Journal of Aquatic Plant Management 35:10-14.

Owens, Chetta S., John D. Madsen. Jan., 1998. Phenological studies of carbohydrate allocation in hydrilla. Journal of Aquatic Plant Management 36 (JAN.) 40-44.

Schmitz, Don C., Jeffrey D. Schardt, Andrew J. Leslie, Allen F. Dray Jr., John A. Osborne, Brian V. Nelson. 1993. The ecological impact and management history of three invasive alien aquatic plant species in Florida. Biological pollution: The control and impact of invasive exotic species 173-194.

Smither, Kopperl M.L., R. Charudattan, R.D. Berger. 1999. Plectosporium tabacinum, a pathogen of the invasive aquatic weed Hydrilla verticillata in Florida. Plant Disease 83 (1) 24-28.

Spencer, David F., Gregory G. Ksander. 2001. Field evaluation of degree-day based equations for predicting sprouting of hydrilla (Hydrilla verticillata) turions and tubers. Journal of Freshwater Ecology 16 (3): 479-486.

Spencer, David F., Gregory G. Ksander. 2000. Interactions between American pondweed and monoecious hydrilla grown in mixtures. Journal of Aquatic Plant Management 38: 5-13.

Steward, K.K. 2000. Influence of photoperiod on vegetative propagule production in three turion-producing races of Hydrilla verticillata (L.f.) Royle. Hydrobiologia 432 (1-3): 1-8.

Stewart, K.K. and Van, T.K.(1987) Comparative studies of monoecious and dioecious hydrilla (Hydrilla verticillata) biotypes. Weed Science 35, 204-210.

Sutton, D.L. 1996. Depletion of turions and tubers of Hydrilla verticillata in the north New River canal, Florida. Aquatic Botany 53 (1-2): 121-130.

Van, T.K., G.S. Wheeler, T.D. Center. 1998. Competitive interactions between hydrilla (Hydrilla verticillata) and vallisneria (Vallisneria americana) as influenced by insect herbivory. Biological Control 11 (3): 185-192.

Van, T.K. and Steward, K.K. (1990) Longevity of monoecious Hydrilla prapagules. Journal of Aquatic Plant Management 28, 74-76.

Wheeler, G.S., T.D. Center. 1997. Growth and development of the biological control agent Bagous hydrillae as influenced by Hydrilla (Hydrilla verticillata) stem quality. Biological Control 8 (1): 52-57.