Last Updated: Spring 2012
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Home Range/U.S. Introduction:
Hydrilla verticillata (L.f.) Royle is an introduction from the Old World (Cook & Luond 1982) that was first discovered in the United States in 1960 and is now abundantly naturalized in many parts of the United States (Langeland 1996). Plants have attractive foliage and are planted in aquaria which are often emptied into freshwater habitats. Hydrilla is easily confused with Egeria densa Planch., Brazilian elodea or Egeria, and Elodea canadensis Michx., Canadian elodea, Waterweed.
U.S. Range Map:
Distribution was determined by a combining of the distribution information obtained from the following websites:
USDA, NRCS. 2012. The PLANTS Database (http://plants.usda.gov, 18 June 2012). National Plant Data Team, Greensboro, NC 27401-4901 USA.
U.S. Geological Survey. . Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/18/2012].
The plants grow submersed, are mostly perennial but sometimes annual, and have horizontal stems in the substrate forming tubers under certain conditions. Stems are ascending and usually are sparsely branched until the plants near the water surface and then become profusely branched. Under certain conditions, turions (actually bulbil-like structures) form in the leaf axils. Stems can be up to 8.5 m long and grow to the surface of the water where the branchlets extend horizontally. Leaves are 1-nerved, sessile, whorled, 3 to 12 at a node but mostly 5 or more, mostly shorter than 1.5 cm long, linear to lanceolate or rarely widely ovate, broadest at the base, the sides nearly paralleling to near the acute tip that terminates in a single spine cell. Leaf margins are serrate, the teeth visible to the naked eye. Fresh leaves are notably rough to the touch. The midrib on the upper surface is often tinged with red and on the lower surface, usually, has 1-celled sharp teeth or spines. Flowers are unisexual, arising from the leaf axil; plants are monoecious or dioecious. The flowers are small, less than 6 mm in diameter, translucent to white; female flowers are usually produced in the fall and are on long thread-like stalks 2 to 4 cm long from leaf axils of the upper branches that carry the flowers to the water surface. Male flowers are solitary, small, on short stalks in the leaf axil and break off as buds, opening explosively on the water surface.
Hydrilla can usually be differentiated from Canadian elodea (Elodea canadensis Michx.) and egeria (Egeria densa Planch.) by the following characters:
Leaves mostly in whorls of 4 at sterile nodes, leaves 1.4 to 2.5 cm long--------Egeria densa
Leaves of stems at growing tips at water’s surface usually in whorls of 3 or 5 or more; leaves not or mostly not exceeding 1.5 cm long, the longest sometimes to 2 cm
Leaves mostly in whorls of 5 or more; margins of the leaves with teeth perceptible to the naked eye; midribs on lower leaf surface (when fresh) with a few conical protuberances tipped by sharp 1-celled teeth; fresh leaves notably rough to the touch---------Hydrilla verticillata
Leaves mostly in whorls of 3; margins of the leaves not having teeth perceptible to the naked eye; midribs of lower leaf surface not pronounced, not bearing teeth; fresh leaves not rough to the touch---------Elodea canadensis
Plants grow in canals, springs, streams, ponds, lakes and reservoirs. Most populations of hydrilla in the United States are dioecious. However, populations of monoecious hydrilla occur in North Carolina and northward into the mid-Atlantic states (Langeland 1996). Hydrilla can reproduce by four methods: fragmentation, tubers, turions, and seed. Tubers in the hydrosoil can remain viable for several years (Langeland 1996) and allow the plant to survive cold temperatures and periods of drought (Tarver et al. 1986). Although the importance of seed production in the spread of hydrilla has not been researched extensively, it is probably of minor importance compared to vegetative reproduction (Langeland 1996). Hydrilla has a high growth rate and lower light requirement for photosynthesis than most other submersed plants (Langeland 1996) which allows it to grow at greater depths and outcompete most other species. It also forms a dense canopy at the surface of the water and "shades out" other submersed plants (Tarver et al. 1986).
This species is probably the worst submersed aquatic weed in the United States. Plants form large, dense populations which displace native species, restrict flow, and impair small boat navigation and other recreational uses (Tarver et al. 1986, Langeland 1996). In addition to being spread by natural fragmentation, plants are sometimes spread from lake to lake by fragments attached to boat motors and trailers. Hydrilla also is thought to be intentionally introduced into "new" water bodies in an effort to enhance sport fishing for black bass.
Cook, C. D. K. and R. Luond. 1982. A revision of the genus Hydrilla (Hydrocharitaceae). Aquatic Botany 13:485-504.Langeland, K. A. 1996. Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae), the perfect aquatic weed. Castanea 61(3):293-304.
Tarver, D. P., J. A. Rogers, M. J. Mahler, and R. L. Lazor. 1986. Aquatic and Wetland Plants of Florida. Third Edition. Florida Department of Natural Resources, Tallahassee, Florida.