Weed of the Month: Brazilian Elodea

Crossposted from Noxious Weeds Blog

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A patch of Brazilian elodea (Egeria densa) starts to top out at Fishermen’s Terminal, Seattle.

Have you visited a lake or river to swim on one of these hot summer days, only to find the water thick with submerged plants? Some of those aquatic plants are natives, but others are nasty invasive species that can quickly dominate a waterbody. One of King County’s peskiest submerged invasives is Brazilian elodea (Egeria densa), a regulated Class B noxious weed. Originally from parts of South America (including Brazil), Brazilian elodea was introduced to the U.S. as an aquarium plant. When people dumped their aquariums into natural waterbodies, Brazilian elodea took over. You can now find it in most U.S. states, as well as in Europe, Asia, Australia, New Zealand, and southern Africa. Washington State has banned the sale and transport of Brazilian elodea, but it still occurs throughout the western part of the state, with the highest concentrations in King County.

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Brazilian elodea is a perennial plant that occurs in still or slow-moving water up to 20 feet deep. In addition to growing underwater, it can also form dense floating mats at the water’s surface.

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On the lower parts of the stem, densely packed leaves, smooth to the naked eye, appear in whorls of 3. Higher up, leaves grow in whorls of 4-8.

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Brazilian elodea has two major growth periods, in the spring and fall. Some individuals often persist through the winter.

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Mature plants produce flowers with three white petals and yellow centers that float on the water. Only male plants have been found in North America, so the species seems to spread here exclusively through plant fragments.

Hitching a ride on boats and trailers is one of the easiest ways for Brazilian elodea fragments to spread among King County’s waterbodies. After using a boat in a waterbody that might have Brazilian elodea, be sure to carefully remove all plant material from the boat, motor, and trailer, and check bilge water for plant fragments. And of course, never dump unwanted aquarium or water garden plants or animals into a natural waterbody.

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Boats and trailers are some of the major vectors for Brazilian elodea fragments to spread among King County’s waterbodies. Always clean your boat, motor, and trailer after entering a waterbody that might have Brazilian elodea.

Where it shows up, Brazilian elodea causes big problems. Its dense stands can reduce biodiversity and change predator-prey relationships. Recreationally, it can ruin swimming areas, harm boat motors, and snag fishing lines. Mats on the water’s surface can serve as breeding grounds for mosquitoes, deplete water oxygen, and increase water temperature. When plants die back in the fall, they not only use up the water’s dissolved oxygen but also pump it with nutrients, potentially increasing algae growth in the area.

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Dense stands of Brazilian elodea can reduce biodiversity and change predator-prey relationships in a waterbody.

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It’s not fun to swimor divearound Brazilian elodea.

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When Brazilian elodea plants die back, they can use up the water’s dissolved oxygen and increase water nutrients, potentially increasing algae growth in the area. Photo by the Thurston County Noxious Weed Control Board.

That said, not all submerged aquatic plants are Brazilian elodea, or even invasive. Brazilian elodea is easily confused with its native look-alike, American waterweed (Elodea canadensis). American waterweed resembles and grows much like Brazilian elodea, but it’s smaller and less robust, with smooth-edged leaves usually in whorls of 3.

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American waterweed (Elodea canadensis) (left) resembles and grows much like Brazilian elodea (right), but it’s smaller and less robust, with smooth-edged leaves usually in whorls of 3.

Brazilian elodea also looks somewhat like hydrilla (Hydrilla verticillata), a Class A noxious weed. However, hydrilla has visibly toothed leaves that grow in whorls of 3-10. It reproduces not only through stem and rhizome fragments, but also via seeds, tubers, and turions. Hydrilla has only been found once in King County, in 1995, when it occurred in two small lakes. It has since been eradicated from the county.

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Hydrilla looks somewhat similar to Brazilian elodea, but it has noticeably toothed leaves that grow in whorls of 3-10. Photo by the Washington State Noxious Weed Control Board.

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Hydrilla reproduces not only through stem and rhizome fragments, but also via seeds, tubers, and turions. Photo by the Washington State Noxious Weed Control Board.

For information on Brazilian elodea control, visit the following pages:

As always, if you have any questions about Brazilian elodea or another noxious weed, feel free to contact us at 206-477-WEED (206-477-9333) or noxious.weeds@kingcounty.gov.