The county's massive$5 million counter-attack on arundo and other invasive weeds destroying native habitat in Matilija Canyon will be launched Sept. 4.

With the Zaca fire lapping at the borders of the Matilija wilderness as recently as Wednesday, project managers had feared the project would be delayed. But fire officials have given the go-ahead.

"We're ready to begin," said Tom Lagier, project manager with the Ventura County Watershed Protection District.

The yearlong arundo-removal project will be the county's largest effort yet to eradicate the giant cane that clogs riverbeds, poses a significant fire risk and chokes native plant growth in local habitats.

Crews will spray herbicide on dense thickets of the giant reed infesting 208 acres of flood plain along the Matilija Creek and Ventura River.

The herbicide glyphosate, which breaks down quickly and is approved for use in water, will be used to kill the invasive plants, said Pam Lindsay, a watershed ecologist with the district. Glyphosate, which is sold under brand names Roundup and Aquamaster, blocks the plant's ability to grow by disrupting protein production.

Plants close to water or homes will be cut by hand, then daubed with concentrated herbicide, Lindsey said.

A county survey of 72 properties in the canyon found 24 had arundo or other invasive species. Residents have agreed to allow crews onto their land for the project.

Other invasive species — including tamarisk, the yellow-flowering Scotch broom, Peruvian pepper trees and castor bean — also will be sprayed and removed.

Six sampling sites will be set up to monitor the water for contamination along the entire project area — 1,274 acres from the Highway 150-Baldwin Road bridge north to 2,000 feet upstream of the Matilija Creek falls.

Spraying will not be conducted on windy days or before a forecasted rainstorm, Lindsay said.

Work will begin in an area above Matilija Dam where thickets of arundo are most dense. As required by state and federal regulations, the area was surveyed this week and determined to be clear of endangered red-legged frogs. Monitoring is also ongoing for steelhead trout.

Under terms of the state grant that is funding the project, work must be completed by August 2008, Lagier said. Nature's Image of Forest Lake was awarded a $3.5 million contract to spray and remove the weeds.

The project is part of the larger Matilija Dam Ecosystem Restoration that will eventually remove the 60-year-old dam and replenish the habitat of native plants and animals in the watershed.

The dam's removal also will allow sediment to flow once more to the beach, which has eroded since the structure was built.