Reviled invasive taking root in San Geronimo Valley

Eric Wrubel, NPS
INVASIVES: The canes of the deciduous Japanese knotweed can punch through concrete.  

Invasive plants are not uncommon in Marin, but a relatively new encroacher in the San Geronimo Valley has a particularly fearsome international reputation.

Fallopia japonica, or Japanese knotweed, is considered one of the world’s 100 worst invasive species. The plant can survive on volcanic slopes and punches through asphalt and concrete; thriving stands grow so tall that people can find shade beneath the foliage. In England, where knotweed has long persisted, securing a mortgage for a home with an infestation typically necessitates a mandatory treatment plan. Once established, the plant is incredibly difficult to eradicate: knotweed can re-sprout from root fragments the size of a fingernail. Experts say it can create monocultures and imperil native plants and fish. In some places in the United States, it is illegal to plant it.

“It’s what’s known as an ecosystem engineer,” Eric Wrubel, who works for the National Park Service’s invasive plant monitoring program in the Bay Area, said at a meeting at Lagunitas School last week. “It can transform the natural processes in an ecosystem.”

The informational evening, organized by the Marin Resource Conservation District to alert people to the presence of Japanese knotweed in the valley, drew just a dozen or so people. That small number was troubling to the district’s urban steams coordinator who helped organize the meeting, given that an effective community response grows more difficult as time passes. 

Sarah Phillips said that despite outreach that exceeded her typical efforts—for this event, she posted fliers and reached out to the San Geronimo Valley Stewards, the San Geronimo Valley Planning Group, the Salmon Protection and Watershed Network, the Lagunitas Creek Technical Advisory Council, and any valley residents she had contact information for—only about four residents showed up, the smallest turnout she has had for an event.

“This is the first species that I have come across that has me absolutely terrified. I worry not only about the threats this species poses on riparian habitat and watershed health, but I also worry about homeowners having to deal with this plant taking over their properties, growing through the foundation of their homes and costing more money that I can comprehend to remove it from their property,” she wrote in a follow-up email. 

Japanese knotweed is native to Japan, China and Korea. It came to Western Europe in the mid-19th century as an ornamental plant and is now present in most U.S. states. In Washington, the state’s department of agriculture has spent millions of dollars coordinating with thousands of landowners on knotweed control, allocating $1.2 million to treat 640 acres between 2013 and 2017.

Knotweed is not particularly prevalent in California, but has appeared in the north and the Bay Area, and a few places inland. A local man discovered it in San Geronimo in 2011, and the park service found more along Lagunitas and San Geronimo Creeks in 2013, according to Mr. Wrubel. 

The next year, the park began surveying and mapping its presence on federal parkland. As of 2016, Mr. Wrubel said there is “less than a hectare,” mostly in Jewell and Tocaloma.

The populations in the park have so far been controlled by hand-pulling, but knotweed also exists in Samuel P. Taylor State Park and on private land, including near the Two Bird Café, around the Meadow Lane bridge in San Geronimo and in Forest Knolls. Without a concerted effort, Mr. Wrubel said, it will probably spread. 

“Our chances of success are going down quickly, but if we act fast, we have a chance of keeping it out of the watershed,” he said.

Japanese knotweed, a deciduous plant related to buckwheat, is considered part of a complex of four knotweeds, including Himalayan, giant and bohemian. Its canes pop up from an underground rhizome. Figures very, but rhizomes can be found as far as 10 feet underground and can spread over 20 feet horizontally, according to John Summers, a project coordinator with a nonprofit called the Mattole Restoration Council, in Humboldt. He said he’s heard of one rhizome that was as thick as a person’s thigh.

The canes, with bamboo-like hollow stems, sprout vaguely heart-shaped leaves starting in March, and white flower sprays bloom in summer. By late autumn, the leaves turn brown and fall off. Canes are often abundant and tall, reaching up to 13 feet high. (Mr. Wrubel cautioned people not to confuse the weed with native elk clover or Oregon ash.)

Knotweed establishes monocultures in a few key ways. A combination of rapid growth, dense stands and broad leaves block the sun for native plants below. Even trees can suffer, since knotweed can shade out seedlings; that’s a problem, since woody debris is a key part of stream and watershed ecology. A 2008 study from Massachusetts found that areas with knotweed had less species diversity than areas without the invader, with knotweed-free areas having anywhere from 1.6 to 10 times as many species.

Completely eradicating an invasive plant becomes more difficult the longer it is established. But one group in Humboldt that has fought Japanese knotweed for over a decade believes eradication in its watershed is in sight—though the success resulted from breaking one of the group’s cardinal rules by resorting to the use of chemicals.

The Mattole Restoration Council, a 34-year-old nonprofit, conducts ecosystem conservation and restoration in the Mattole River watershed. About 70 percent of land in the watershed is privately owned, so the group works with many landowners. Like Marin, many people in Humboldt prefer nonchemical weed treatment. “Our county is very anti-herbicide, as is our organization,” Mr. Summers said.

The organization discovered knotweed along a road and down a slope in 2005. For two years, the group attempted to eradicate it without using chemicals. They pulled the canes and laid down tarps in hopes that, without leaves to photosynthesize sunlight, the rhizomes would starve and die. Mechanical remocal was not feasible, as removing the rhizome, which can stretch far underground, would have required removing the road. Removing knotweed mechanically close to a stream would require permits and, on a steep bank, could cause serious erosion, Mr. Summers said.

But the manual removal efforts were unsuccessful. Given that knotweed can withstand volcanic conditions, perhaps that’s not surprising. “It evolved to be inundated with ash and poisonous gases for years at a time… It’s fast-growing, tenacious and incredibly hard to get rid of,” Mr. Summers said.

In 2007, the group deployed pesticides, sponging leaves and injecting canes with Aquamaster, which contains glyphosate. It also posted “no mow” signs on roadsides, to stop Caltrans from inadvertently spreading the plant. 

Those efforts worked to some degree, but did not eradicate the knotweed. In 2013, the group switched to Polaris, an herbicide containing imazapyr, which is said to be more effective. Mr. Summers said the nonprofit only used about a half-gallon of pesticides in the last decade, and only two teaspoons last year.

In 2016, the group counted about 40 canes—just 2 percent of the original infestation. “I think eradication [in the Mattole watershed] is in the near future,” he said.

Is a similar success story possible in Marin? The invasion may be relatively new enough to stop, but doing so would require a coordinated effort between public agencies and private landowners. 

Education is particularly key to ensuring that well-intentioned people don’t make the problem worse. People may want to pull the weed if they see it, but even small fragments of rhizome can lead to the plant’s regeneration. People may also drop pieces elsewhere, spreading the weed. 

Mr. Summers said one person in the Mattole “wanted to do the right thing,” but after he dug up the weed, he disposed of it in the creek, which could have led to its spread. (The pulled plant was discovered downstream and removed.)

Disposal is also a problem. Sarah Philips, the urban streams coordinator with the Marin Resource Conservation District, warned against dropping knotweed in green waste bins for fear that it could survive the composting process and potentially spread. Burning is a good bet, Ms. Phillips said.

A multi-year grant to an organization like the R.C.D. could fund the work necessary to coordinate with landowners for an eradication program. But such a program would require community organizing and buy-in, and could be difficult for an area like the valley, where many oppose pesticide use.

The small audience at Tuesday’s meeting prompted concern about whether enough people would back an effort. Waiting until the plant spreads further could make it much more expensive to wipe out, if that is even possible. “I’m afraid that people don’t feel the weight of the implication,” Ms. Phillips said. 

She said she is willing to convene another community meeting, “but if we don’t have a whole lot of people interested, then getting a grant to do the work—that’s a lot [of work],” she said.

One San Geronimo resident, Greg Reis, a hydrologist, said he has been pulling and tarping a patch of knotweed on the property he rents. After covering it all last summer, he removed the tarp last fall, to see if it was still alive. In recent days, it has started growing once again.