Hydrilla: Invader that Chokes Waters


© Raghavan Charudattan, University of Florida, Bugwood.org

Scientific name: Hydrilla verticillata.

Aliases: Water thyme, Florida elodea, Indian starvine, oxygen plant.

Origin: The hydrilla in the southern United States came from India. The type found north of South Carolina probably came from Korea. (The two types are “biotypes” of the same species, differing chiefly in their flowers. Southern plants produce only female flowers. In the north, each plant typically has both male and female flowers.)

Introduction to North America: Hydrilla was introduced into the US at least twice. The southern, Indian strain was introduced during the 1950s and the northern, Korean strain was later introduced in the early 1980s.

Introductions become invasive: The southern, Indian strain was first identified as an invasive in Tampa and Miami, Florida.  The northern, Korean type was identified in the Potomac River Basin.

Latest distribution: Hydrilla can now be found along the southeast United States and up the eastern seaboard, as far north as Cape Cod in Massachusetts. The west coast, California and Washington have also been infested. For more on hydrilla’s distribution, see the US Geological Survey’s hydrilla web page.

Hydrilla’s ability to grow in more temperate waters means it will probably spread to additional states unless boaters and others are vigilant. Experts in New England believe hydrilla represents the most serious threat to natural aquatic communities in their region.1

Responsible for US introduction: A tropical plant and fish farmer in St. Louis, Missouri imported hydrilla and shared it with a Tampa Bay farmer in Florida. The samples, stored inside a wire cage in a canal, flourished and spread throughout the canal. The Tampa Bay farmer and other growers then started deliberately planting the southern, Indian strain and marketing it for aquarium use.2 Separate introductions by aquarium plant growers in the Washington, DC area resulted in the establishment of the northern, Korean strain in that region. In California, hydrilla hitchhiked along with a shipment of water lilies that were deliberately planted in a lake. Washington’s plants probably arrived the same way.3

Could it have been avoided?: Yes. If the US government required that aquatic plants intentionally brought into the country be evaluated for their potential invasiveness before being allowed in the country (a process called screening), hydrilla would not be as big of a problem as it is today. At the time of its introduction, hydrilla was known to be invasive in Southeast Asia4—it should have been kept out.

© Raghavan Charudattan, University of Florida, Bugwood.org

Impacts: Hydrilla chokes waterways. It grows into a thick layer at or near the surface, shutting out light underneath. “It’s like putting a blanket over a body of water,” says biologist Don C. Schmitz of the Florida Department of Environmental Protection. Water temperature and pH increase, while oxygen below the mat decreases.

Hydrilla grows aggressively, replacing native plants and decreasing open water, which crowds out ducks and other waterfowl (though some birds do feed on the plant). There have been documented cases of sportfish declining in size and even fish kills. Mosquitoes, however, often benefit from hydrilla’s presence, as they can breed in the stagnant water created by an infestation.

The social and economic costs of hydrilla are frequently extreme. Extensive mats of hydrilla physically impede water traffic, both commercial and recreational. Reduced water flow due to infestations can lead to flooding or hamper irrigation. Like the infamous zebra mussel, mats of hydrilla clog water intake pipes at power plants and other facilities.

Once hydrilla infests a waterway, it is all but impossible to eliminate. A square meter of the plant produces thousands of small, long-lived tubers that regenerate, as do small fragments of stem. “It’s like herpes,” says Schmitz—”once you’ve got it, you’ve got it forever.” Consequently, lost tourism and commerce and reduced real estate values are permanent. States, municipalities, utilities, and industries are forced to spend money on controls—mainly herbicide or mechanical removal—to contain infestations. Herbicide applications cost $500 per acre, while mechanical cutting can cost more than $1,000 per acre. Both methods require repeated use. Since 1982, Florida alone has spent $150 million to control the plant.5 South Carolina spends about $2.5 million annually on hydrilla control.6

Federal Policy: Hydrilla was named a federal “noxious weed” in 1983, which limited further imports under the Federal Noxious Weed Act of 1974. Now, after passage of the Plant Protection Act of 2000, interstate movements of noxious weeds are also constrained. At least eleven states restrict hydrilla within their borders: Arizona, Arkansas, California, Colorado, Florida, New Hampshire, Nevada, North Carolina, Oregon, South Carolina, and Washington, as well as Puerto Rico. State law may limit the introduction, cultivation, distribution, sale, or possession of hydrilla or it may lay out how state officials are to inspect, monitor, quarantine, treat, or dispose of infestations. The specifics vary from state to state.

What YOU can do7:


  1. Educate yourself and others to distinguish hydrilla from look-alikes. For example, compare photos under “Might be confused with…” at this hydrilla information page from the University of Florida Sea Grant office.  Then avoid using waters infested with hydrilla.
  2. Report new sightings of hydrilla and other invasive plants in natural waterways to your local or state agricultural extension agent, university Sea Grant office, or natural resource management agency.
  3. If hydrilla is not yet in your state, see if your state Department of Agriculture designates “noxious weeds” and, if so, if hydrilla is included. If not, ask your governor to request that agriculture officials add it.


  1. When deciding where to locate a backyard pond, be sure that if it floods, it will not be close to any waterways where plants or plant fragments could escape.
  2. Never dump plants or plant fragments from aquariums or ponds into waterways. Instead, completely freeze or dry out unwanted plants and dispose of them in the trash (not with compost).
  3. When purchasing plants for your aquarium or pond, choose reputable nurseries and aquarium stores that abide by state and federal restrictions on invasive plants.  Identify plants yourself because several tend to be mislabeled and confused by sellers.
  4. Buy non-invasive plants or plants native to your region. Ask your local native plant society to suggest plants.
  5. Buy only aquatic plants free of hitchhikers.

BOATERS AND OTHER RECREATIONAL USERS OF FRESH WATER: Clean your boat and other recreational equipment before leaving the ramp or water’s edge. Be sure to inspect your boat’s hull, motor, livewell, bait containers, and trailer for any fragments so that you do not spread hydrilla to other waterways.

For details and more guidelines on what YOU can do to prevent and control invasive species, go to:
What you can do to Prevent Species Invasion.


  1. Les D.H. and Mehrhoff L.J. 1999. Introduction of nonindigenous aquatic vascular plants in southern New England: a historical perspective. Biological Invasions, vol. 1, pp. 281-300.
  2. Schmitz D. 2002. Personal communication. March 27, 2002. Don Schmitz is with the Florida Department of Environmental Protection.
  3. Washington State Department of Ecology. Technical information about Hydrilla verticillata. Accessed May 6, 2007.
  4. Schmitz D. 2002. Personal communication. March 27, 2002. As above.
  5. Ibid.
  6. Washington State Department of Ecology. As above.
  7. Modified from Charlebois P.M. and TePas K. 2001. “Invasive Aquatic Plants—What Every Plant Enthusiast Needs to Know” Accessed May 6, 2007.