News From the Aquatic Weed Front
A slimy green alien weed is clogging waterways and boat propellers. A
nuisance to boating enthusiasts, it's a real threat to California's irrigation
water delivery systems. If the weed continues growing unchecked, it could
substantially impede waterflow to farms and cities.
"Egeria densa is an aquatic weed from Brazil that was probably
dumped unthinkingly into the Sacramento River by a fish tank owner who tired of
the hobby," says Lars W. J. Anderson. He is an ARS plant physiologist who
has studied Egeria for the past 2 years.
"The weed has been in the Sacramento Delta for perhaps 30 years, but it
only recently became a problem."
Trouble started when the prolonged drought of 1987-94 reduced flow in the
The water moved more slowly and carried less silt, compared to more normal
years when larger amounts of swiftly flowing water kept sediment suspended.
Shallower, more slowly moving water heats up more quickly in the spring and
stays warmer later in the fall. And with less suspended sediment, aquatic weeds
get additional sunlight that penetrates to the deeper areas.
Egeria can double in size every 4 or 5 weeks during hot summer
months. It roots in channel bottoms, and its long stems collect sediment,
further slowing waterflow.
Because Egeria grows from roots attached to channel bottoms,
herbicides have to be placed in the water. Other aquatic weeds, such as water
hyacinth, live on the surface and can be controlled by foliar herbicide sprays.
The weed has so far established itself in several sloughs southwest of
Sacramento and is spreading into other waterways.
"Some waterways are so clogged with Egeria that they look more
like golf courses and are ruined for boating, swimming, or fishing," says
Anderson, who is in the ARS Aquatic Weed Control Research Unit at Davis,
California. "Waterfront homeowners especially worry about their property
But cutting, harvesting, or dredging are futile attempts at control.
Now Anderson has successfully subdued Egeria on a 5-acre area in a
delta slough with a commercially registered aquatic herbicide known as Komeen.
This contact herbicide defoliates the weed, usually down to its roots. It
contains copper bound to an organic chelate that makes the copper more
accessible to the weed and less toxic to algae.
Approved by both federal and state environmental regulators, Komeen doesn't
harm fish and other native aquatic life, but it does suppress two or three
alien weed species, along with Egeria.
Unfortunately, underground rhizomes are unaffected by only one Komeen
application, and the plant can grow back within a few weeks. So the herbicide
would have to be applied at 3- to 5-week intervals throughout Egeria's
growing season, from mid to late April until the end of September.
Anderson estimates it would cost several hundred thousand dollars to control
current infestations by applying the herbicide with power boats. He hopes to
cut costs in half by using tides to spread the herbicide at amounts required by
Ocean tides at the Golden Gate near San Francisco affect waterflow 50 miles
inland. If scientists can figure out when and where to place the herbicide in
the complex web of waterways that make up the delta, it might be possible to
harness the tides and use themrather than boatsto distribute the
"At some sites, we might run injector lines across a slough and release
the chemical at the end of low tide. The following high tide would push water
back into the weed-infested arealike filling a troughand hold it
there for several hours," says Anderson.
He also hopes to test Sonar, a systemic organic herbicide that has proven
successful in other states. It is absorbed by aquatic weeds and prevents growth
by interfering with the synthesis of pigments that protect chlorophyll.
Sonar is usually applied at lower rates10 to 60 parts per billion,
compared to about 500 for Komeen. And Sonar might control Egeria with
only a treatment or two per year. By Dennis Senft, ARS.
Lars W. J.
Anderson is in the USDA-ARS
and Invasive Weeds Research Unit, University of California, Botany Dept.,
Davis, CA 95616; phone (530) 752-7870, fax (530) 752-4604.
"News From the Aquatic Weed Front" was
published in the April
1995 issue of Agricultural Research magazine.