Pesky aquatic Ludwigia weed is back

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Five years after the end of a $2.1 million campaign to attack it, the invasive aquatic weed Ludwigia once more is clogging Sonoma County waterways.

"It's back and it's as bad as ever," said Erik Hawk, assistant general manager of the Marin/Sonoma Mosquito and Vector Control District, which funded much of the original cleanup effort because the plant provides prime habitat for mosquitoes to breed.

The problem, according to agencies involved in that first campaign, was the unexpected difficulty of killing the plants and a lack of funding to do follow-up study and cleanup. There is some disagreement over why the funding dried up; whether it was the economic downturn, an easing in the public panic over mosquito-borne West Nile Virus, or bureaucratic inertia depends on who you ask, but it clear that the affected waterways, particularly the Laguna de Santa Rosa, are once more clogged with a weed that is almost unbelievably tenacious.

"It's a big hairy monster," said David Bannister, executive director of the Laguna de Santa Rosa Foundation, which coordinated the earlier eradication effort and now is looking for new sources of funding to control the plant.

The willowy flowing plant is a native of tropical sections of South America, but has gained a foothold in recent decades across North American and Europe, possibly after escaping from aquariums used to house exotic fish. It roots on the bottom in shallow water and grows into a dense, leafy canopy that pokes above the surface.

"It is so invasive that once it gets established, it makes it hard for anything else to get a toe-hold," said Keenan Foster, principal environmental specialist for the Sonoma County Water Agency, which is experimenting with ways to limit the range of Ludwigia in the Laguna and parts of the Russian River system.

The plant grows in the the same shallow, slow moving waters where mosquitoes like to breed. It grows in such thick, leafy patches that it is difficult to spread the pellets used to kill mosquito larvae, Hawk said. It has grown back so thickly in recent years that the control district has to turn to a helicopter to drop the pellets, relying on the intense wind from the rotors to shake the plants and drop the chemicals in the water under the leafy canopy.

The plant is not in itself dangerous, biologists say, and can provide beneficial habitat for certain smaller fish. But when it finds ideal conditions — warm, shallow, slow moving water — it grows into an underwater jungle that traps sediment, interferes with waterfowl and blocks the movement of fish and boats. The plant particularly likes the water of the Laguna, which is polluted with more than a century of organic runoff from nearby cities and farms.

The dead stems and old leaves, meanwhile, create a decaying mass covered in bacteria that sucks the oxygen out of the water, killing other nearby plants and fish.

While there is little money for another major cleanup, the water agency is working on ways to control the plant in some areas as part of ongoing sediment removal projects. Since Ludwigia doesn't like cool, fast moving water, Foster said, the agency is working on terracing the river banks near the Stony Point Road bridge over the Laguna near Rohnert Park. That would force more water into the bottom of the channel, creating a faster flow that makes it harder for the plant to take root in the stream bottom.

The agency also is working on planting shade trees along slow-moving waterways, since Ludwigia prefers full sun and warm water.

Given the tenacity of the plant, however, it seems impossible to eradicate it, Foster said. The goal is to trim it back periodically and, where possible, make the environment less favorable so it can't grow in such destructive abundance.

"Probably the best way to deal with these invaders is to figure out how to live with them," he said.

(You can reach Staff Writer Sean Scully at 521-5313 or

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