Europe, Africa to Australia
Species commonly cultivated
Hydrilla verticillata (L.f.) Royle (cosmopolitan)
U.S. Federal Noxious Weed: Hydrilla verticillata
Identification: Hydrilla can be distinguished from Egeria, Elodea, and Lagarosiphon by its whorled leaves, which bear small spines on the midveins on the underside of each leaf. (See below for further details.)
almost cosmopolitan, apparently absent from South America
Annual or perennial, attached. Monoecious or sometimes dioecious. Stems elongate, branched regularly. Leaves caulescent, arranged in whorls of 3-10, relatively evenly distributed along stem, crowded at apex, sessile; leaf blade linear to lanceolate, midvein distinct; margin finely serrate. Turions develop either on stolons below substrate or are axillary on stems. Inflorescence axillary; spathe of two united bracts. Flowers unisexual; male flowers sessile or subsessile, subtending spathe opens violently, liberating flowers which then float on water surface; female flowers sessile but appearing long-pedicellate (hypanthium), opening at water surface. Dispersal by seed, turion or stem fragments.
still waters of lakes, ponds, swamps, and rivers
Hydrilla verticillata is the only species in this genus. Hydrilla is a very fast growing plant which quickly outcompetes other plant species. Extensive growth of this plant quickly chokes waterways, limiting their use by people. Variation in leaf number, stem elongation, and leaf shape can result in misidentification with other genera similar in appearance (check comparison illustrations at right).