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2020 Update on Japanese Knotweed

Thank you for your support of the Marin Knotweed Action Team. Even with the roadblocks that 2020 has presented, from the coronavirus to poor air quality from fires, surveying and treatment of Japanese knotweed still occurred on both private and public lands along San Geronimo Creek and Lagunitas Creek.

Of the 70 sites on private lands that were treated in 2019:

  • 24 sites had no detectable aboveground stems in 2020
  • 46 sites did have stems still present, though the height and number of stems were significantly reduced

In addition to these 70 sites, six new sites (ranging from one to ten stems) were found in 2020 during the streambank survey from Roy’s Pools in San Geronimo to the Inkwells in Lagunitas. A third of the 76 sites required no treatment as there were no visible stems present. All but three of the remaining 52 knotweed sites were treated in July and September.

MKAT is continuing to educate and work with property owners to manage Japanese knotweed on their property. Monitoring of sites and treatment, as needed, will happen in summer of 2021. Continued community support of this effort is key to its success. By working in a coordinated manner across the watershed, we can protect the accomplishments being made in eradicating Japanese knotweed on both private and public lands.

For more information regarding MKAT and Japanese knotweed, please read the 2019 letter to the community below:


First of all, I would like to thank all streamside property owners and residents who have responded to the Marin Knotweed Action Team’s request to survey their property for Japanese knotweed and have allowed us to treat it if found. It has been a pleasure working in the Valley this past year – getting to know fellow residents (and your pets) and hearing both your concerns for and what makes the San Geronimo Valley a special place for you to live. As a more recent arrival to the Valley, this work has incidentally given me a richer knowledge of the community, from both an ecological and social perspective. The support from the San Geronimo Valley community towards the eradication efforts being undertaken by the Marin Knotweed Action Team (MKAT) has been heartening and without it, our work would not be possible.

MKAT is a coalition of various land managers (comprised of local, state and federal agencies and non-profit organizations) who are dedicated to sustaining the vibrant and resilient Lagunitas Creek and San Geronimo Creek watersheds, specifically the control and long-term eradication of Japanese knotweed from these areas. UC Cooperative Extension’s role is to foster a connection between the university and the public by helping to solve problems related to the environment and human and community wellbeing. We strive to be a resource to educate community members about Japanese knotweed and to help landowners identify and manage Japanese knotweed on their land.

Japanese knotweed (Fallopia japonica) is a noxious, invasive plant that lives along streams, spreads easily through flood events, and disrupts the environment in the process. It is known to be here in San Geronimo Valley since at least 2011, posing a serious threat to environmental and built systems with the potential to cause significant damage to the San Geronimo Creek and Lagunitas Creek watersheds. While we do not know when it officially arrived in the Valley, it probably arrived as an ornamental planting and we are only now beginning to see the impacts of its spread. It is believed that the Japanese knotweed population is small enough to be successfully eradicated though, unlike other non-native plants that are already firmly established. While it won’t be easy, eradication of Japanese knotweed on both public and private land is possible with your help.

It is my pleasure to report that all known in-stream populations of Japanese knotweed on private land were treated in 2019, a total of 69 sites. These sites ranged in location from Creamery Road in San Geronimo to the confluence of San Geronimo Creek and Lagunitas Creek in Lagunitas. The National Park Service and California State Parks are also managing sites on public lands as part of a larger watershed eradication effort. Additionally, there was one Japanese knotweed site in Bolinas that was treated as well. This was an intentional planting and not in a stream setting. By far, the most effective treatments are a foliar spray of an aquatically-approved formulation of imazapyr or glyphosate, depending on the landowner’s preference. MKAT continues to try to work with homeowners who are hesitant to use herbicides and those homeowners who haven’t responded to our request to survey their property.

The size of these sites ranged from newly deposited, single stem plants to mature stands of Japanese knotweed larger than half a tennis court. Over half of the sites were less than the size of a parking space. Based off the experiences of other land managers in northern California and Washington State, we anticipate needing to treat larger stands for three to five years with less and less herbicide being used in each subsequent year as the populations are reduced in size and number.

Characteristics of Japanese Knotweed

Haven’t ever heard about Japanese knotweed? Although its stems resemble bamboo, Japanese knotweed is actually a member of the buckwheat family. It is a shrub-like perennial with hollow stems (it is not a vine nor is it woody). It can grow up to eight feet tall in late summer before dying back in the winter, though some plants may remain only a foot or two feet tall during the growing season. Distinctive characteristics include spade-shaped leaves with a flat base, zig-zag stems and plumes of small cream-colored flowers appearing in mid to late summer.

It is known for its vigorous growth and its extensive rhizome (underground stem) network. It outcompetes other vegetation, establishing monoculture stands and reducing native species, and prevents tree seedlings that are important to nutrient cycling from regenerating. As a result, salmon are impacted by the loss of leaf litter and shade from new trees. In addition, Japanese knotweed can grow through asphalt, septic systems and cracks in home foundations. While it has an extensive root system, its roots fail to keep soil in place on streambanks, leading to increased erosion.

Mechanical or physical means of removal and control of Japanese knotweed are either not effective or not feasible; in addition, mechanical removal stimulates the plant. Japanese knotweed relies on its extensive rhizome system to come back each spring. Not only does this underground stem system store an incredible amount of energy that the plant can utilize, tiny fragments of rhizome can create new infestations. Japanese knotweed can also spread via cut stems, which is why we do not recommend cutting or mowing it. The rhizomes can stretch 10 feet down and up to 23 feet away from a patch, making it highly unlikely that you will be able to dig it up completely without either stimulating it or spreading it in the process. Any plant material needs to be carefully bagged and sent to a landfill facility because of its propensity to regrow and spread.

You can see more photos of Japanese knotweed and learn how to identify it by visiting: https://ucanr.edu/mkat.

What Can You Do?

If you are a creekside landowner, you received a letter from the Marin Knotweed Action Team in 2019 asking for permission to survey your property for knotweed. If your property is not adjacent to San Geronimo Creek, you would not have received a letter. We are asking that landowners allow us to survey their property for Japanese knotweed and use a licensed vegetation management business to treat infestations with aquatically approved herbicides when needed. Continued monitoring and surveying is an essential part of eradicating Japanese knotweed; even if your property was surveyed this year and found to have no knotweed on it, we will need to survey it again on an annual basis as flood events can disperse Japanese knotweed throughout the watershed.

There is no cost to the landowner for the survey or treatment. If you did not initially receive a letter, but think you might have seen Japanese knotweed or would like a property survey, please contact me at acdirkse@ucanr.edu or by calling (415) 473-6070. You can also fill out a permission to survey form at https://ucanr.edu/marinknotweedparticipationsurvey. We do not recommend mechanical removal due to the chance of inadvertent spread. If you have any questions, please contact me.

As Eric Freyfogle writes in his book, Our Oldest Task: Making Sense of Our Place in Nature:

“For people to live well on land they must instinctively if not consciously understand their roles as members of what Aldo Leopold termed the land community. They must see themselves as community members with responsibilities as such. For that to happen, for people to act responsibly in and toward nature, they must strive to understand the natural world. And they must be willing to act on their best understandings, even as they remain cognizant of the limits on what they know and can know and thus of the virtue of acting humbly. In the end, stances must be taken, moral choices must be made, and decisions must arise based on best-available evidence.”

The Marin Knotweed Action Team is here to help the San Geronimo Valley community act in eradicating Japanese knotweed by being a source of information and support. While this effort will be an ongoing and difficult process, its success is essential to protecting the health of the watershed of which we are all a part.



Anna Dirkse
Japanese Knotweed Outreach Coordinator
Marin Knotweed Action Team
University of California Cooperative Extension, Marin County