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. . 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), and flowers are rarely seen.
Hydrilla has a high resistance to salinity (>1-100000ppt) compared to many other freshwater associated aquatic plants.
The name Esthwaite Waterweed comes 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. Hydrilla closely resembles some other related aquatic plants, including Egeria and Elodea.
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 Canada and the southeast from Connecticut to Texas, and also in California. 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, itself an invasive species in North America. Insects used as biological pest control for this plant include weevils of the genus Bagous and the Asian hydrilla leaf-mining fly (Hydrellia pakistanae). Tubers pose a problem as they can lie dormant for a number of years, making it even more difficult to remove from waterways and estuaries.
As an invasive species in Florida, Hydrilla has become the most serious aquatic weed problem for Florida and most of the U.S. Because it was such a threat as an invasive species, restrictions were placed to allow only a single type of chemical, fluridone, to be used as an herbicide. This was done to prevent the evolution of multiple mutants, and resulted in fluridone resistant Hydrilla. “As Hydrilla spread rapidly to lakes across the southern United States in the past, the expansion of resistant biotypes is likely to pose significant environmental challenges in the future.” 
In 2011 the inlet of Cayuga Lake, one of the Finger Lakes in New York State, used the chemical herbicide endothall to try and head off a possible future disaster. The first year nearly $100,000 and numerous man-hours were spent trying to eradicate the Hydrilla infestation. Follow-up treatments were planned for at least five years. The City of Ithaca as well as other local officials are willing to pay the price because without quick action the plant could get into the lake and possibly spread to other Finger Lakes in the region.
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- Flora Europaea: Hydrilla
- Flora of Taiwan: Hydrilla
- Australian Plant Name Index: Hydrilla
- Flora of NW Europe: Hydrilla verticillata
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