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South American waterweed (Egeria densa (Elodea)
ODA rating: B
 
USDA Symbol: EGDE
Oregon South American waterweed distribution
 
Other common names
Brazillian elodea, Brazilian waterweed, leafy elodea, dense waterweed
 
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South American waterweed
Image courtesy of T. Pennington,
Center for Lakes & Reservoirs, Portland State University.
 
South American waterweed
Image courtesy of T. Pennington,
Center for Lakes & Reservoirs, Portland State University.
South American waterweed
Image courtesy of Vanessa Howard Morgan, Center for Lakes & Reservoirs, Portland State University.
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
 
Description
Perennial aquatic herb that grows under water. Leaves one to three cm long, up to 5 mm broad, and in whorls of four to eight. Leaves linear and short internodes give it a very leafy appearance. Leaves and stems generally bright green. Stems erect, simple or branched and grow until they reach water's surface where they form dense mats. Flowers small, white, have three petals, and float on or rise above the water's surface on thread-like appendages. This plant reproduces by spread of plant fragments and has double nodes on the stem.
 
Impacts  
South American waterweed aggressively invades new aquatic environments, displacing native vegetation by forming dense stands or large subsurface mats and altering the dynamics of the aquatic ecosystem. It also impedes flows in waterways, increases flooding, clogs pumps and boat propellers, and limits use of waterways for recreation.
 
Introduction
This plant is originally from South American and was introduced to North American for aquarium use.  The first documented United States introduction was in 1893.
 
Distribution in Oregon
The first record in Oregon is 1934 in Marion County.

 
Biological controls
No approved biological control agent is available at this time.

 
Informational links
Printable trifold S. American waterweed brochure (pdf 430 KB