Hydrilla
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 Hydrilla verticillata
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Hydrilla is an aquatic plant, native to the cool and warm waters of Asia, Europe, Africa and Australia.
Plants may be monecious or dioecious and can reproduce vegetatively by fragmentation and by tubers and turions (overwintering buds).
Stems branch near water surface. Dioecious species may be up to 30 feet in length and form surface mats. Leaves are toothed, about 3/4 inch long and 1/4 inch wide. Female flowers are small, white and have 6 petals on long stalks. Male flowers are of inverted bell shape, green.
Hydrilla verticillata became an aquatic weed in the United States in the 1960's following release from aquariums into Florida's waterways. As an invasive species, by the 1990s hydrilla was well-established in the southern states where control and management costs millions of dollars each year. Infestations of Hydrilla verticillata have also been detected in California and Washington. Hydrilla has been spotted as far North as Maryland and Delaware.
Hydrilla can be controlled by the application of aquatic herbicides and it is also preferred by grass carp. Tubers pose a problem to control as they can lay dormant for a number of years. In 2006 Bishop-Taylor found that Hydrilla has a high resistance to salinity (>9-10ppt) compared to many other freshwater associated aquatic plants. This has made it even more difficult to remove from waterways and estuaries.
Hydrilla verticillata closely resembles two aquatic plants: Brazilian elodea (Egeria densa) and native American waterweed (Elodea canadensis).
- Hydrilla has tubers that are 0.2 to 0.4 inch long, off-white to yellowish, that are buried in sediments.
- Leaves are arranged in whorls around the stem (generally five leaves per whorl).
- There are serrations or small spines along the leaf edges.
- The midrib of the leaf is often reddish when fresh.
Also called Esthwaite Waterweed because of its former occurrence in Esthwaite Water in England.
