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Glossary

Last Updated: Spring 2012
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Synonyms: Anacharis densa (Planch.) Victorin
Elodea densa (Planch.) Caspary


Family: Hydrocharitaceae

Home Range/U.S. Introduction:
This species is native to southeastern Brazil but now naturalized in the eastern United States, from New Hampshire and Vermont southward to central Florida, westward to Nebraska, Kansas, Oklahoma, Texas, Oregon, California, Arizona, New Mexico, and Utah (Cook & Urmi-Konig 1984). The earliest record in the United States was in 1893 from Long Island. It was praised in horticultural literature as an attractive aquarium plant and, for this reason was offered for sale by 1915. It was reported cultivated in Hawaii in 1932 and had become established in the wild by 1937. It is also a popular object for experimental investigation in the laboratory which may have aided its wide distribution.

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. [2012]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/18/2012].


Species Description:
Plants grow submersed, rooted in the substrate or, commonly, free in the water. Stems are elongate, slender, 2 to 3 mm thick, single or sparingly branched. Lowermost leaves are opposite, otherwise in whorls, the number per whorl variable, usually 3 to 6, mostly 4 on the upper portion of the stem and the growing tips at the water’s surface. The leaves are sessile, nearly linear, their apices subobtuse or abruptly very short acuminate, very finely toothed on the margins and sometimes on the midrib below, 1.4 to 2.5 cm long, 1.6 to 5.0 mm wide. The flowers are 1.2 to 1.8 cm wide, unisexual (only male plants known to us in our area), produced in spathes which are sessile in the axils of the leaves, their stalks bringing them to or raising them just above the water surface. The three sepals are green; the three petals are white.

Egeria can usually be differentiated from hydrilla, Hydrilla verticillata (L.f.) Royle, and elodea (Elodea canadensis Michx.) 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


Habitat/Growth Characteristics:
Plants grow submersed, rooted in the substrate excepting when pieces of plants are found free in the water. Populations are found in streams, ponds, lakes, reservoirs, and constructed lagoons. Plants grow in both still and flowing water. The roots are adventitious, developing at nodes bearing lateral branches. Egeria produces "double nodes" (each branching or budding node actually consists of two nodes close together) on the stems which may readily root and develop into new shoots.

Problems:
In the state of Florida, E. densa is said to occupy a total area of 53.4 ha; in South Carolina, it is reported by Getsinger & Dillion (1984) to have colonized over 10,000 ha in the Santee-Cooper River System in the late 1970’s and early 1980’s. Populations of plants dominate the environment by vigorous growth and are often found as pure stands. The spread of this plant is probably directly the result of man's activity since it is perhaps the most universally available aquarium plant.

References:
Cook, C. D. K. and K. Urmi-Konig. 1984. A revision of the genus Egeria (Hydrocharitaceae). Aquatic Botany 19:73-96.Getsinger, K. D. and C. R. Dillon. 1984. Quiescence, growth, and senescence of Egeria densa in Lake Marion. Aquatic Botany 20:329-338.