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The morning serenity and the egrets and herons lifting off the still water at the approach of the air boat gave Laguna de Santa Rosa a faraway, safari-like feel.

The tranquility, however, masked the failing health of the laguna, a summer waterway and winter catch-basin being choked to death by the return of the virulent weed ludwigia.

Hundreds of acres of the laguna's wetlands are carpeted with the green weed, and shallower channels are either totally blocked or will be within a month, the plants reaching out with finger-like tentacles from bank to bank.

And despite decades of work to bring back the wetlands and more direct efforts to remove the weed, ludwigia has emerged as the most significant threat to the laguna, a 16-mile waterway from the Russian River to Cotati recently designated as a wetlands of international importance by the Ramsar Convention, a group represented by 160 countries, including the U.S.

And there is no plan for permanently dealing with it.

"Nasty stuff," said Erik Hawk, assistant manager and biologist for the Marin-Sonoma Mosquito and Vector Control District.

Hawk last week worked his way by air boat through the swamp-like setting, periodically stopping in shallow water to wade into the ludwigia looking for mosquito larvae.

But the mosquito habitat it creates is just one of the problems brought by this weed, which started gaining a foothold about 20 years ago.

It also removes oxygen vital to a variety of fish that includes Pacific and river lamprey, prickly sculpin, California roach, minnows, hardhead and hitch and the coho, chinook and steelhead that pass through it on the way to spawning streams.

"I think you could have areas that have some good habitat and other areas that will become really sterile," said Bill Cox, a retired state Department of Fish and Game biologist, referring to the weed's effect on fish.

Wildlife, such as otters, cannot swim through it and ludwigia clogs flood control channels.

From a purely recreational and aesthetic standpoint, you can't paddle a boat through ludwigia, which will grow three to five feet above the water surface. It also yellows quickly, turns the water a tea brown and gives off a sulphur smell as it decomposes.

The first work to eradicate ludwigia was in 2005, a two-year program that cost $1.7 million to poison it and mechanically harvest it in the wetlands areas and from the channels.

However, it was a short-lived effort, driven by the need to remove mosquito habitat in the face of the West Nile virus threat and to clear flood control channels.

"If you pay $1.7 million, do you expect it to be gone for more than five years? You would hope so," said David Bannister, executive director of the Laguna Foundation.

Luis Rivera, assistant executive director of the North Coast Regional Water Quality Control Board, said the agency is trying to identify the conditions that ludwigia finds favorable and then enact regulations to eliminate the sources.

"The efforts at eradication have been tried, and I will defer to the experts on them, but so far it has not worked," Rivera said.

The water quality board is part of the task force that is attempting to find a solution. It includes the nonprofit Laguna Foundation, the city of Santa Rosa, the Sonoma County Water Agency, Fish and Game and the mosquito district.

In the 2005 attempt, ludwigia was uprooted and poisoned, and 30,000 cubic yards of the noxious plant was hauled away.

Santa Rosa wants to remove about 40,000 cubic yards of the plant this summer from several miles of channel in the Stony Point Road area. But it has to meet new regional water quality rules requiring Santa Rosa to offset the nutrients from Santa Rosa's winter discharge of treated wastewater into the laguna.

The county Water Agency is planning to deepen some of the laguna's feeder creeks as flood control measures and to plant trees to shade the creeks and cool the water, both of which could slow ludwigia's growth.

"We are looking for sustainable solutions rather than just going after it with a sledgehammer, by addressing the conditions that sustain it," said Keenan Foster, a Water Agency environmental specialist.

Ludwigia has thrived since it first showed up in the laguna. It's a South American native plant that may have been dumped in from an aquarium or ornamental planting.

It is admittedly only one of the laguna's many problems, which include sediment washed in from the hills, nutrients from runoff of the ranches that border it and the Santa Rosa wastewater treatment plant that discharges into it during the winter.

"There are other things that have impact on the health — 150 years of changes to land use, cutting down the riparian forest that used to exist along the banks of the laguna, clearing the land for agriculture and using the laguna as recipient of human and agriculture waste," Bannister said.

Ludwigia remains the most immediate threat and a priority, and Bannister is putting his hopes in the task force to combat it.

"Our vision is let's get the smartest people we can all in one room, and let us try to figure out a way to get it done," Bannister said.