|Species:||Hydrilla verticillata (L.) Caspary, hydrilla|
The exotic invader hydrilla closely resembles common waterweed (Elodea canadensis) and Brazilian elodea (Egeria densa), another noxious weed. All have bright green leaves arranged in whorls around the stem, although hydrilla leaves have small spines on the edges and at the tips. The most reliable way to identify hydrilla is to look for small potato-like tubers attached to the roots.
Leaf: Bright green leaves are 1-5 mm wide and 6-20 mm long with sharply toothed margins (visible without magnification). The reddish midrib often has small spines. Leaves grow in whorls of 3-10 along the stem, although 5 leaves per whorl is most common. Whorls can be closely spaced and bushy, or spaced widely apart along the stem.
Stem: The monoecious variety of hydrilla found in Washington in 1995 has a delicate sprawling growth form that freely branches at the lake bottom. The dioecious variety branches more at the waters surface.
Flower: Monoecious hydrilla has male and female flowers on the same plant. The female flower has 3 small, translucent, white petals, 4-8 mm wide and 1-5 mm 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.
Fruit: Small spindle-shaped fruits, 5-6 mm long. Rarely seen.
Root: Fibrous rhizomes and above ground stolons. Peanut-sized or smaller tubers on the roots.
Propagation: Fragments, tubers, seeds (rarely), and scaly overwintering buds called turions.
Importance of plant: Because of its dense and profuse growth habit and the ability to grow in most environments, hydrilla is considered the most problematic aquatic plant in the U.S. If you think you have found hydrilla, report it immediately to the State Noxious Weed Control Board.
Distribution: Native to Africa, Australia, and parts of Asia. Introduced to some east coast locations and the southern United States, California, and Washington.
Habitat: Lakes, rivers, ponds, and ditches.
May be confused with: Common waterweed (Elodea canadensis) and Brazilian elodea (Egeria densa) which have similar form and leaf arrangement. With hydrilla, look for small tubers on the roots, spiny leaf edges, and a reddish midrib on leaves.
Line Drawings: Hydrilla verticillata
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