DESCRIPTION: Hydrilla plants grow submersed, are mostly perennial but sometimes annual, and have horizontal stems in the substrate forming tubers under certain conditions. The stems are ascending and usually are widely branching with ultimate numerous branchlets that, under certain conditions, form turions (actually bulbil-like structures). The stems can be 8.5 m long and grow to the surface of the water where the branchlets extend horizontally. The leaves are 1-nerved, sessile, whorled, 3-12 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 terminating in a single spine cell. The 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, on upper branches, and usually produced in the fall. The male flowers are solitary, small, short-petioled, breaking off the stem as buds and opening explosively on the water surface. Female flowers are solitary, subsessile but with a long threadlike structure that carries the flower to the surface..

PATHWAYS/HISTORY: Hydrilla grows in canals, constructed lagoons, channelized streams, ponds, lakes, and impoundments. So far, in a given locality only one type of plant has been found, either monoecious or dioecious with male flowers. The range of monoecious hydrilla includes the mid-Atlantic states south to South Carolina; dioecious plants with male flowers are found elsewhere. Hydrilla was introduced from the Old World, now abundantly naturalized in many parts of the U.S. Plants have attractive foliage and are planted in aquaria from which they may escape. Hydrilla is easily confused with Egeria densa (Brazilian elodea or egeria) and Elodea canadensis (Canada elodea, waterweed). Comparisons of whorls of leaves of these three and the key given in the description of Egeria densa emphasize their differences.

RISKS/IMPACTS: Hydrilla is probably the worst naturalized aquatic weed in many places. Plants form large, dense populations that displace native species and impair water use. Undoubtedly, plants are spread from lake to lake by fragments attached to boat motors. Presumably the capsules or seeds, or specialized buds and tubers (turions), are dispersed by water. The capsules are sometimes spiny and probably dispersed by animals.

MANAGEMENT: Hydrilla is all but impossible to erradicate by manual removal techniques. Chemical control is possible, but harms other aquatic life. Prevention of further spread is an importnat approach to reduce possible impacts.

WHAT YOU CAN DO: Transportation can occur via contaminated equipment of recreational boaters. Always inspect your watercraft and equipment before leaving a water body. Remove all vegetation and thoroughly clean everything before transporting to other locations. Never move aquatic vegetation. Never release aquarium plants or animals into the wild.

PROFILE CREDIT: David Webb, USACE - IMAGE CREDIT: Raghavan Charudattan, University of Florida