Three years ago, Sonoma County declared war on ludwigia, a fast-spreading plant that was clogging west county waterways and providing safe harbor for West Nile-carrying mosquitoes to breed.
But this summer the warriors charged with leading the fight, the Laguna de Santa Rosa Foundation, will temporarily put aside their herbicides and mechanical harvesting equipment to investigate more effective ways to battle the plant.
What they've learned is that ludwigia is the Frankenstein of invasive weeds, refusing to die despite a $2.1 million, three-year campaign to kill it off. The fight was funded by the Sonoma County Water Agency, the city of Santa Rosa, the Marin/Sonoma Mosquito & Vector Control District and the California Wildlife Conservation Board.
"The results have been mixed," said Dan Carlson, Santa Rosa's deputy utilities director. "Some techniques worked, some didn't."
Dan Schurman, the foundation's executive director, agreed.
"We accomplished some short-term relief from the worst and heaviest infestation and the elevated population of mosquitoes that threatened public health," he said. "But did we get rid of the plant? No way."
In terms of how much of the weed has disappeared, the program has been effective, said Mike Thompson, deputy chief engineer for the Sonoma County Water Agency.
"What we see this year is nothing like years past," he said, noting in many areas the weed's thick growth beneath a creek's waterline proved nearly impassable for migrating and spawning fish.
Ron Keith, an ecologist with the mosquito and vector control district, said he now can see free-flowing water in most of the waterways once clogged and thickly carpeted with the weed.
"The ludwigia has been knocked back severely," he said, enough so that the temporary break in herbicide spraying and mechanical harvesting will not be long enough to allow the plant to reestablish itself.
"It will take a long time for it to come back to where it was three years ago, maybe five to 10 years," Keith said.
Some 30,000 cubic yards of the weed were removed from the most heavily infested waterways, including 5.5 miles of drainage creeks west of Rohnert Park and along Occidental Road and 125 acres of flood plain in the Sebastopol area, according to statistics developed by the Laguna Foundation.
Santa Rosa Deputy City Manager Greg Scoles said the most profound discovery of the foundation's work was that it is easier to control the weed in deeper, cooler waterways than shallower, warmer areas where ludwigia "grows back quickly."
"We knew going in it would only be a stopgap measure," Schurman said of his organization's efforts.
The focus of the past two years has been to develop a long-range plan to control the amount and spread of ludwigia, which has gained a foothold along portions of the 14-mile-long Laguna de Santa Rosa and hundreds of miles of tributaries that feed into it.
Scoles said the idea now is to develop, based on three years of work and analysis, a more effective attack and to seek potential state and federal funding for the fight.
Containment, restoring riparian habitat to cool the waters and addressing problems that fuel its rapid spread -- nutrient-rich run-off from lawns, golf courses and dairies -- are among longer-term strategies.
The new focus will be to develop non-chemical ways to control the weed in the future.