Hydrilla
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| Hydrilla | ||||||||||||||
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 Hydrilla verticillata
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| Hydrilla verticillata (L.f.) Royle |
Hydrilla (Esthwaite Waterweed or Hydrilla) is an aquatic plant genus, usually treated as containing just one species, Hydrilla verticillata, though some botanists divide it into several species. Synonyms include H. asiatica, H. japonica, H. lithuanica, and H. ovalifolica. It is native to the cool and warm waters of the Old World in Asia, Europe, Africa and Australia, with a sparse, scattered distribution; in Europe, it is reported from Ireland, Great Britain, Germany, and the Baltic States, and in Australia from Northern Territory, Queensland, and New South Wales.[1][2][3]
It has off-white to yellowish rhizomes growing in sediments at the water bottom at up to 2 m depth. The stems grow up to 1–2 m long. The leaves are arranged in whorls of two to eight around the stem, each leaf 5–20 mm long and 0.7–2 mm broad, with serrations or small spines along the leaf margins; the leaf midrib is often reddish when fresh. It is monoecious (sometimes dioecious), with male and female flowers produced separately on a single plant; the flowers are small, with three sepals and three petals, the petals 3–5 mm long, transparent with red streaks. It reproduces primarily vegetatively by fragmentation and by rhizomes and turions (overwintering buds), and flowers are rarely seen.[2][4][5][6]
Hydrilla has a high resistance to salinity (>9-10ppt) compared to many other freshwater associated aquatic plants.
The name Esthwaite Waterweed derives from its occurrence in Esthwaite Water in northwestern England, the only English site where it is native, but now presumed extinct, having not been seen since 1941.[7] In the British Isles it is now only Hydrilla closely resembles some other related aquatic plants, including Egeria and Elodea.
[edit] Status as an invasive plant
Hydrilla is naturalised and invasive in the United States following release in the 1960s from aquariums into waterways in Florida. It is now established in the southeast from Connecticut to Texas, and also in California.[8] By the 1990s control and management were costing millions of dollars each year.
Hydrilla can be controlled by the application of aquatic herbicides and it is also eaten by grass carp. Insects used as biological pest control for this plant include weevils of genus Bagous and the Asian hydrilla leaf-mining fly (Hydrellia pakistanae). Tubers pose a problem to control as they can lay dormant for a number of years. This has made it even more difficult to remove from waterways and estuaries.
[edit] References
- ^ Flora Europaea: Hydrilla
- ^ a b Flora of Taiwan: Hydrilla
- ^ Australian Plant Name Index: Hydrilla
- ^ Flora of NW Europe: Hydrilla verticillata
- ^ Blamey, M. & Grey-Wilson, C. (1989). Flora of Britain and Northern Europe. ISBN 0-340-40170-2
- ^ Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan ISBN 0-333-47494-5.
- ^ Environmental Change Network: Esthwaite Water
- ^ Flora of North America: Hydrilla verticillata
 US Army Corps of Engineers collect herbicide-resistant Hydrilla from Lake Seminole in northern Florida |
