Hydrilla is an aquatic plant that has spread rapidly through much of the United States. We don’t know for sure where hydrilla originally came from. Some scientists say the plant is native to Asia; others say Africa or Australia. Hydrilla was introduced to North America in the early 1950s when it was brought to the southern United States for use as an aquarium plant. It spread into waterways when people emptied their aquariums into lakes or rivers.

Hydrilla is highly adaptable, and it thrives in still and flowing waters, including rivers, lakes, ponds, wetlands, streams and wet ditches, as well as in a range of nutrient and light conditions. The plant grows up to 2.5 centimetres a day. It has a competitive advantage over many native plants because it begins converting sunlight to energy that helps it grow – the process known as photosynthesis – earlier in the day than most plants. Because tiny plant pieces can develop into new plants, hydrilla is easily spread when water currents, boat propellers, trailers, fishing gear or people carry plants or plant fragments to new areas.

photo of hydrilla attached to boat motor.

Hydrilla attached to boat motor. Photo: Wilfredo Robles, Mississippi State University, Bugwood.org


Hydrilla has not been detected in Canada, but it has been found in neighboring American states. In the United States it has spread across the southern states from Florida to California, along the west coast in California and Washington, along the entire Atlantic seaboard, and inland through Pennsylvania, Indiana, Wisconsin, New York and several other states. Hydrilla populations can now be found on every continent except Antartica.

photo of hydrilla.

Photo: David J. Moorhead, University of Georgia, Bugwood.org

Impacts of hydrilla

  • Once established, hydrilla is able to grow aggressively, outcompeting native plants.
  • It forms dense mats that block sunlight from reaching other submerged plants, including native species.
  • The plant degrades water quality by raising pH levels, decreasing oxygen and increasing water temperature.
  • It can hinder the flow of water, as well as recreational activities such as swimming, fishing and boating.
  • By causing stagnant water, hydrilla may provide habitat that allows mosquitoes to breed.

How to identify hydrilla

  • The plant is a perennial that grows underwater.
  • Stems are rooted, erect, either branched or unbranched, and grow up to 7.5 metres long.
  • Leaves are green, attached to the stem and arranged in whorls of three to eight. They have visibly saw-toothed edges, and sometimes have prickles on the underside.
  • Flowers are very small, with petals two to four millimetres wide, and are white to reddish or white to light green with red stripes. When open they float on the surface of the water.
Hydrilla line drawing.

Illustration courtesy of: IFAS Centre for Aquatic Plants, University of Florida, Gainsville, 1990

photo of hydrilla.
Hydrilla (Hydrilla verticillata)

Photo: Robert Videki, Doronicum, Kft., Bugwood.org

photo of Brazillian Elodea.
Brazillian Elodea (Egeria densa)

Photo: Robert Videki, Doronicum, Kft., Bugwood.org

photo of Canada Waterweed.
Canada Waterweed (Elodea canadensis)

Photo: Robert Videki, Doronicum, Kft., Bugwood.org

SpeciesHydrilla (Hydrilla verticillata)Brazillian Elodea (Egeria densa)Canada Waterweed (Elodea canadensis)
  1. Three to eight leaves in a whorl.
  2. Prominent sharp teeth on leaf edges.
  3. Prickly hairs on underside of leaf.
  4. One to two centimetres long.
  5. Small white potato-like tubers can form on stem ends.
  1. Four to six leaves in a whorl.
  2. Minutely toothed leaf edges are only visible if magnified.
  3. No prickly hairs on underside of leaf.
  4. Up to four centimetres long.
  5. No tubers.
  1. Four to five leaves in a whorl.
  2. Minutely toothed leaf edges are only visible if magnified.
  3. No prickly hairs on underside of leaf.
  4. Up to 1.5 centimetres long.
  5. No tubers.

What you can do

  • Learn how to identify hydrilla and how to prevent accidentally spreading this plant with your watercraft.
  • Avoid infested areas or reduce your speed when travelling near hydrilla infestations. Your propeller can break off fragments and spread the pieces to new areas. New plants can grow from small fragments of the plant.
  • Inspect your boat, trailer and equipment after each use. Remove all plants, animals and mud before moving to a new waterbody.
  • Avoid planting hydrilla in your aquarium or water garden. Aquarium hobbyists and water gardeners are advised to use only native or non-invasive plants and are encouraged to ask retailers for plants that are not invasive.
  • Never release unwanted aquarium plants or pets. Return or donate unwanted plants to a garden centre or pet store, or put them in the garbage.
  • If you’ve seen hydrilla or other invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, or visit Ontario’s invading species awareness program to report a sighting.

Other resources:

For more information:

Please contact the Invading Species Hotline at 1-800-563-7711 .
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© 2012
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