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Pesky aquatic Ludwigia weed is back

  • Laguna de Santa Rosa Foundation's executive director David Bannister spots birds in the knee-high Ludwigia-choked Laguna de Santa Rosa north of Occidental Road, Thursday June 6, 2013 in Santa Rosa. ((Kent Porter / Press Democrat))

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.

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