What Ontario is Doing

To prevent this unwanted invader from coming into the province, Ontario has regulated Hydrilla as prohibited under the Invasive Species Act. For more information on the Invasive Species Act and Regulations visit www.ontario.ca/invasionON.


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.


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 Antarctica.

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.

Similar Species


Hydrilla - J. Chris Greene, Alabama Wildlife and Freshwater Fisheries, Bugwood.org
(Hydrilla verticillata)
  • Three to eight leaves in a whorl.
  • Prominent sharp teeth on leaf edges.
  • Prickly hairs on underside of leaf.
  • One to two centimetres long.
  • Small white potato-like tubers can form on stem ends.


Brazillian Waterweed-Graves Lovell, Alabama Department of Conservation and Natural Resources, Bugwood.org
Brazillian Elodea
(Egeria densa)
  • Four to six leaves in a whorl.
  • Minutely toothed leaf edges are only visible if magnified.
  • No prickly hairs on underside of leaf.
  • Up to four centimetres long.
  • No tubers.


Canada Waterweed-Rob Routledge, Sault College, Bugwood.org
Canadian Waterweed
(Elodea canadensis)
  • Three smooth leaves in a whorl.
  • Minutely toothed leaf edges are only visible if magnified.
  • No prickly hairs on underside of leaf.
  • Up to 1.5 centimetres long.
  • No tubers.

What You Can Do

  • Learn how to identify Hydrilla and how to prevent the introduction or spread of this plant with your watercraft or fishing equipment.
  • Never buy, plant or keep Hydrilla in your aquarium or water garden. It is against the law to buy, sell, trade, possess or transport Hydrilla.
  • Never deposit unwanted aquarium or water garden plants into Ontario lakes or rivers. Dispose of them in the garbage or away from any body of water.
  • Avoid infested areas and reduce your speed if 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, be sure to remove all plants, animals and mud before moving to a new water body.
  • If you have any information about the illegal importing, distribution or sale of Hydrilla, report it immediately to the MNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).
  • 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 EDDMapS Ontario to report a sighting.


OFAH/OMNRF Invading Species Awareness Program. (2012). Hydrilla. Retrieved from: www.invadingspecies.com.
This factsheet may be reproduced for non-commercial purposes.

Header photo by Chris Evans, River to River CWMA, bugwood.org