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Brazilian waterweed

Egeria densa

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Background

Identification

  • Grows rooted in mud, submerged or floating, with stems up to fifteen feet long, 1/8 inch thick.
  • Small smooth spear-shaped leaves 3/4 to 1-1/2 in. long, 1/16 to 1/8 in. wide, arranged in a whorls of 3 to 6 leaves, many whorls along stem.
  • Prominent white flowers floating on water surface (or emerging just above surface), with thread-like attachment to stems.
  • Compare to Hydrilla, p. 19.

Identification key in: Hickman, J. ed. 1993. The Jepson Manual: Higher Plants of California. University of California Press.

Growth and spread

  • Can reproduce from fragments of above- and below-ground stems.
  • All plants in California are male (Hickman 1993).

Habitat and local distribution

  • Streams, ponds, and sloughs (Hickman 1993).
  • Native to South America, introduced to California more than 30 years ago and is now found in the Delta and other Bay Area freshwater. A popular plant in aquariums, it may have been introduced to Delta from an aquarium (CDBW 2003).

Impacts

  • Displaces native plants and associated wildlife; shades submerged habitat.
  • Dense growth impedes water flow, blocks irrigation pipes, and interferes with boating, swimming and other water recreation.

Prevention and Control

Prevention

  • Invasive plant awareness and regular monitoring is critical to identify and stop a new invasion before it takes off.
  • Inform the public and any boat launch area staff of the need to remove all plant debris from boats and equipment at the ramp area after each use. The California Department of Boating and Waterways (CDBW) has developed educational resources for boaters (CDBW 2003a; CDBW 2003b).
  • Inform the public of the importance of refraining from dumping aquarium contents into natural water bodies.

General control notes

  • Control efforts are usually most successful in areas where there is minimal movement of water, e.g., ponds and lakes (Anderson and Hoshovsky 2000).

Manual or mechanical control

  • Mechanical harvesting, cutting, and rotovation—expensive and may promote spread of plant due to the sprouting of plant fragments left behind (Anderson and Hoshovsky 2000; WAPMS 2003).
  • Estimated costs: harvesting costs range from $500-800 per acre, with additional costs for mobilization and equipment ($35,000-110,000) (WSDE 2001); cutting costs range from $400-3,000 for portable boat-mounted equipment to $11,000 for mechanized underwater cutters (WSDE 2001); and rotovation costs range from $1,000-1,700 per acre, depending on the size of the treatment area, plant density, equipment needed, and scale of removal (Gibbons et al. 1999). There may be additional fees for disposal of plant material.
  • Manual removal—can be used for small infestations, but care must be taken to also remove fragments and roots, which can resprout (CDBW 2000).
  • Estimated costs: vary depending on if volunteers conduct removal and on the plant density; if divers and dive tenders need to be contracted, costs may range from $500-2,400 per day (Gibbons et al. 1999). There may be additional fees for disposal of plant material.
  • Dredging—may be used to remove plant material; costly and intrusive, but may be appropriate for small, seasonally dry water bodies.

Biological control

  • Sterile (triploid) grass carp—stocking permit for aquatic plant management required by California Department of Fish and Game (Anderson and Hoshovsky 2000); however, not permitted in many San Francisco Bay-Delta waterways and not suitable for water bodies with inlets and outlets.
  • Estimated costs: costs per fish range from $7.50-15.00 (Gibbons et al. 1999); quantity dependent on plant species, density of plant, and water temperature (WSDE 2001).

Chemical control

  • Application of herbicides—diquat, copper-based product, acrolein, and fluridone have been applied in California in the control of Brazilian elodea; special care must be taken when applying herbicides in aquatic systems (Anderson and Hoshovsky 2000).
  • Estimated costs: vary depending on size of treatment area, scale of treatment, and herbicide dosage; costs per acre for materials and application by a contractor may range from $900-1,400 for fluridone. It is recommended to contract a licensed professional for herbicide applications (Gibbons et al. 1999).

References and more information

Anderson, L. and Marc C. Hoshovsky. 2000. Egeria densa. In Invasive Plants of California Wildlands. Carla C. Bossard, John M. Randall, Marc C. Hoshovsky, Editors. University of California Press. Available at http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands .

CDBW (California Department of Boating and Waterways). 2003a. Aquatic Pest Control: Water Hyacinth and Egeria Densa. Available at http://www.dbw.ca.gov/aquatic.htm.

CDBW (California Department of Boating and Waterways). 2003b. Clean Boating Habits. Available at http://dbw.ca.gov/Pubs/CleanBoatingHabits/index.htm .

CDBW (California Department of Boating and Waterways). 2000. Draft Environmental Impact Report for the Egeria densa Control Program. Available at http://dbw.ca.gov/.

Gibbons, M.V., M.G. Rosenkranz, H.L. Gibbons, Jr., and M.D. Sytsma. 1999. Guide for Developing Integrated Aquatic Vegetation Management in Oregon. Center for Lakes and Reservoirs, Portland State University, Portland, OR.

Hickman, J. ed. 1993. The Jepson Manual: Higher Plants of California. University of California Press.

WAPMS (Western Aquatic Plant Management Society). 2003. Egeria densa (web page). http://www.wapms.org/plants/egeria.html.

WSDE (Washington State Department of Ecology). 2001. Final Supplemental Environmental Impact Statement for Freshwater Aquatic Plant Management. Publication Number 00-10-040.

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