Japanese Knotweed Eradication Efforts Continue Along Lagunitas Creek

Japanese knotweed growing near Lagunitas Creek. It can be identified by its shield-shaped leaves with flat bases and smooth margins and by its zig-zagging, red-speckled stems with regular nodes like bamboo. It should never be pulled up or cut, as these will only spur more growth.

© deekyn / Photo 3568929 / 2016 / iNaturalist.org / CC BY-NC

July 2019 - Homeowners, land managers, and coho salmon and all share a common concern along Lagunitas Creek in Marin County: Japanese knotweed. The plant is one of the world’s most invasive species, and was first reported in the area in 2011. Homeowners fear its tendency to damage concrete foundations and asphalt driveways. Land managers fear its tendency to spread fast, ruin roads, crowd out native plants, and destroy habitat for wildlife. Endangered coho salmon are threatened by knotweed’s potential to alter overhanging vegetation along the banks of the creek. Juveniles in particular depend on the shade of trees to keep creek water temperatures cool. They also depend on insect deadfall from the leaves of creekside plants for food. Other aquatic species like threatened California red-legged frogs and endangered California freshwater shrimp may face similar perils in the case of a serious knotweed invasion.

Luckily, Japanese knotweed was found early along Lagunitas Creek. National Park Service staff have been able to make a lot of progress finding and treating patches on Golden Gate National Recreation Area lands. Other land managers have made progress as well. Still, with so many stakeholders, more coordination was needed. Even the smallest upstream knotweed population can continue to re-infest downstream areas if everyone doesn't work together. Thus in 2018, county, state, and federal agencies joined forces with NGO's to form the Marin Knotweed Action Team (MKAT). An MKAT community outreach coordinator is currently working to bring private landowners in to the effort as well. With everyone's participation, the team aims to completely eradicate knotweed along the creek.

This year, an early round of Japanese knotweed treatments is focusing on an area along Lagunitas Creek where a coho salmon habitat enhancement project will be taking place later in the summer. These treatments will help ensure that knotweed isn't spread during the project’s construction phase. Then, the One Tam Conservation Management Team will spend the next several weeks surveying roughly eight miles of creek for new knotweed infestations. In the fall, contractors will return to treat any infestations that turn up in the surveys. The National Park Service Invasive Species Early Detection and California Exotic Plant Management Teams are also continuing to map and treat knotweed downstream. Their recent treatments have reduced the number of knotweed stems per patch by an average of 92%. All of the teams tackling knotweed on Lagunitas Creek are looking forward to this year's efforts being similarly successful!

Explore the links above, or contact Marin Knotweed Action Team Community Outreach Coordinator Anna Dirkse to learn more.

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