From: http://www.cdfa.ca.gov/phpps/ipc

Hydrilla are submerged aquatic perennials. Stems typically grow rooted , but fragment easily into free-floating pieces that root. Fragments may start new colonies when carried elsewhere. Brazilian elodea and hydrilla can aggressively invade new aquatic environments, displace native aquatic vegetation by forming dense stands or large sub-surface mats, and alter the dynamics of aquatic ecosystems. Other detrimental impacts from heavy infestations can include water flow impediment in waterways, increased flooding, clogged pumps and boat propellers, diminished water clarity, reduced use of lakes and waterways for recreational activities, and economic loss.

  • Hydrilla: Noxious. Many biotypes exist, including monoecious and dioecious types (male and female flowers on the same plant; male and female flowers on separate plants, respectively). Both monoecious and female dioecious populations occur in California.

  • Brazilian elodea: Dioecious. At publication time, populations in the U.S. consist only of male plants. Brazilian elodea is more common than hydrilla in California, but less threatening from a management perspective. Commonly sold as aquarium décor, plants can naturalize in warm temperate to cool sub-tropical regions when unwanted aquarium contents are released into lakes, ponds, or waterways. Introduced from eastern South America.

  • Common elodea: Dioecious. Common elodea is native to North America, where it is an important component of natural aquatic ecosystems. Populations are rarely troublesome in natural habitats, but plants can become dominant in altered or created aquatic systems, especially when bicarbonate, reduced iron, and phosphorus are plentiful. It is widely distributed throughout the world and considered a serious weed in Europe and Australia.