Photographer: Leslie J. Mehrhoff Affiliation: University of Connecticut Source: Bugwood.org Copyright: CC BY 3.0 US
Hydrilla is an aquatic plant that grows in freshwater, but can tolerate some brackish waters, and even those with limited water clarity. It is usually submerged with roots at the bottom, long branching stems that reach the surface, and form thick mats. The stems are covered in small leaves in 4-8 whorls. Small white flowers appear above the water line.
This plant forms dense mats at the surface of the water. It changes pH, removes oxygen, restricts native plant growth, block out nutrients for aquatic animals, hinders irrigation, recreation and water flow. It is extremely competitive and resilient, surviving freezing or drought because it can break off into portions and sink down to safer waters. It is easily spread through boating and fishing activities and by waterfowl.
In North Carolina, hydrilla is a suitable habitat to grow a deadly algae Aetokthonos hydrillicola. This algae produces a neuro-toxin, that has been linked to the death of several bald eagles and coots. So little is known about this new algae, researchers aren't sure what the chemical makeup of the neuro-toxin is. However, it has been found on Eurasian milfoil and Brazilian elodea; emphasizing the necessity to remove these invasive aquatic plants.
Hydrilla reproduces both asexually and sexually. Asexual reproduction occurs when the plant, tubers or roots fragment and develop into new plants. Sexual reproduction occurs in late summer when male plants pollinate female flowers. However, the seeds that form only have about a 50% chance of becoming new plants, showing that asexual reproduction is more successful for this plant.
It was originally sold and imported as an aquarium plant in the 1950s. Some sources state that, after importation in Florida the plants were deemed unsuitable and were dumped into canals near Tampa Bay. By 1976 it was reported in Maryland and the Potomac River near D.C. in 1980. Since then it has spread as far west as California and Washington.
Asia or Africa
U.S. Habitat: Freshwater habitats, such as lakes. It grows in depths of 5 inches to 20 feet. It can also tolerate turbid and brackish waters.
U.S. Present: AL, AR, AZ, CA, CT, DC, DE, FL, GA, IA, ID, IN, KY, LA, MA, MD, ME, MO, MS, NC, NJ, NY, OH, PA, SC, TN, TX, VA, WA, WI and WV.
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Hydrilla is one of the most invasive weeds in the world, and it is capable of clogging waterways, and even public water supplies. It is now illegal to possess or transport this species in Texas, and is also listed as a Federal Noxious Weeds. Despite 30 years of research and intensive management efforts, hydrilla is still a major problem where it is successfully established.
What has allowed this plant to spread is not only its durability, but also human interaction. When boating some tend not to clean off their boats or drain ballast water, if collected; with this plant being able to spread from a single fragment it is easily transported by boating, fishing, and other aquatic activities. BE SURE TO FULLY CLEAN YOUR BOATS AND HULLS TO PREVENT FURTHER SPREAD!
Since hydrilla is able to spread from single fragments, mechanical management alone will not work. There are some companies that are producing cutters and rakes that facilitate this but the work better in conjunction with other management strategies such as herbicides, and biological agents. Fertilization to produce an “algal bloom” can help prevent the establishment of rooted aquatic plants like hydrilla, along with physical barriers that shade the bottom.
Godfrey R.K. and J.W. Wooten. 1979. Aquatic and wetland plants of the southeastern United States, the Monocotyledons. University of Georgia, Athens, GA. 933p.
Langeland K. A. 1996. Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The perfect aquatic weed." Castanea 61: 293-304.
Madeira P.T., Jacono C.C, and T.K. Van. 2000. Monitoring hyrilla using two RAPD procedures and the Nonindigenous Aquatic Species Database. Journal of Aquatic Plant Management 38:33-40.
McCann J.A., Arkin L.N., and J.D. Williams. 1996. Nonindigenous aquatic and selected terrestrial species of Florida.