Marin County approved federal funding last week for what officials described as a critical year in the battle against an invasive plant threatening the San Geronimo Valley. Japanese knotweed, a deciduous plant related to buckwheat that is considered one of the world’s 10 worst invasive plants, was first sighted in the county in 2011, in Lagunitas.
Now, Japanese knotweed can be found on state, federal and private lands in and along Lagunitas and San Geronimo Creeks. Last year, a coalition of state and federal land management agencies began surveying and treating the plant, which typically infests waterways. A year later they noticed an over 96 percent reduction in stem counts within treated patches.
The grant approved by supervisors last week, worth $29,011, will support the removal of knotweed from private lands in the watershed.
“Unless we can all work together, we will fail,” Bobbi Simpson, a liaison for the National Park Service’s California exotic plant management team, told the Light. “We are working so hard on it on this end. If our neighbors upstream don’t also work on it, we’ll be like gerbils running around on a wheel.”
Japanese knotweed can sprout through pavement from a root fragment the size of a fingernail. With its fast-moving growth and broad leaves, it chokes native ecosystems, creating a nearly impenetrable canopy above native plants and tree seedlings.
“It out-competes native vegetation by emerging really early in the season, before native plants, then grows really fast and prevents seedlings from germinating,” Stefan Parnay, the county’s deputy agricultural commissioner, said. “It is known as one of the top 10 worst invasive plants in the world, and you can’t say that about a lot of plants.”
Knotweed also threatens to make creek banks more vulnerable to erosion, destroying habitat for the endangered coho salmon and threatened steelhead trout, officials said. If nothing is done about the issue within the next year, Japanese knotweed will “be with us permanently, which will have lasting negative consequences,” according to the county’s proposal to extend funding to the eradication program.
Under the proposal, the Marin Knotweed Action Team will continue to reach out to landowners, asking to survey their property and treat any knotweed patches. Of the 170 streamside parcels in the impacted area, the team has already received permission from nearly 50 homeowners to have their land surveyed.
Once patches are identified, the team’s will apply glyphosate, imazapyr and the surfactant Competitor over multiple years.
The use of glyphosate, a controversial chemical that was banned by the county parks department last year, has thus far raised no public complaints, officials said. Some homeowners have inquired about its use, but none have refused the treatment.
The team discourages residents from trying to remove or dispose of the plants themselves.
“Knotweed can grow 10 to 20 feet horizontally and six to 10 feet deep, so pulling it doesn’t do anything except get the plant excited and stimulate it to grow more. And it leaves a lot in the ground,” David Lewis, director of the University of California’s Cooperative Extension of Marin, said.
He went on, “The San Geronimo and Lagunitas Creeks are really key watersheds for steelhead trout and coho salmon on the California coast—and wonderful places people live—and we think we can reduce the impact on both of those at the same time through homeowner participation. We’re feeling confidant and optimistic that we’re at a stage that we can eradicate it and get out ahead of it getting widespread in large patches. This next year is going to be really important.”