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Japanese knotweed, the Godzilla of weeds

September 22, 2018
Nanette Londeree

There are weeds, and then there are serious weeds. A common description of a weed is any plant growing where you don’t want it. But what about invasive weeds and the worst — noxious weeds? What’s the difference, and why be concerned?

The Weed Science Society of America defines a weed as “a plant that causes economic losses or ecological damage, creates health problems for humans or animals, or is undesirable where it is growing.” Invasive weeds take it up a notch — plants that “establish, persist and spread widely in natural ecosystems outside the plant’s native range.” The Godzilla of weeds, noxious weeds, are “any plant designated by federal, state or local government officials as injurious to public health, agriculture, recreation, wildlife or property.” These terrorists of the plant world, often lacking natural enemies to limit their growth, muscle out native and other desirable plants or crops, hog water, nutrients, sunlight and space, enabling them to completely overwhelm existing vegetation.

Some non-native invasive plants end up in new environments unintentionally, hitching a ride in nursery plants, soils, even on car tires. Others are purposefully introduced. Japanese knotweed, Fallopia japonica, considered one of the top 10 most aggressive, destructive and invasive plants in the world, was introduced into Europe in the 1850s as a beautiful, hardy ornamental plant. Also known as Mexican bamboo or fleeceflower, this extremely vigorous, clumping perennial has large, heart- or spade-shaped green leaves arranged in a zig-zag pattern along a hollow stem. Lushly growing to 12 feet tall and bedecked in summer with clusters of creamy-white flowers, you’d never guess the near indestructibility of the plant. Even hot asphalt can’t kill it! The plants underground stems (rhizomes), can extend more than 20 feet horizontally and nearly 10 feet down. Rhizomes break easily, and a tiny piece, the size of a fingernail, can result in a new plant.

Found in most of the U.S. and across Europe, this rogue plant has proven to be nearly impossible to manage once established. A tiny patch can quickly overtake acres of land, squeezing out native vegetation and altering habitats. It grows through pavement, sidewalks, foundations and even into septic systems, impacting the environment and property values.

Huge impact

“I’ve worked on the identification and removal of non-native invasive species for a decade now and I’ve never seen anything like Fallopia japonica,” says Sarah Phillips, Marin Resource Conservation District’s urban streams program manager. “The impact to a watershed’s ecological integrity, not to mention to infrastructure and property value, is unnerving.”

Japanese knotweed has been found in public and private locations in Marin, including the San Geronimo Valley. To eradicate this noxious weed before it takes over, a coalition of land managers, including local, state and federal agencies and nonprofit organizations, have formed the Marin Knotweed Action Team (MKAT), dedicated to sustaining a vibrant and resilient Lagunitas Creek watershed.

“This is a rare opportunity to solve a problem that will eventually become quite serious, and ultimately cost us more if we don’t act now,” says Steven Swain, Marin County Environmental Horticulture Advisor. “The integrity of the stream systems in San Geronimo and connecting waterways will never be the same.”

Know your plants

What can homeowner’s do to help combat this pest?

“Get to know the plants on your property,” recommends Katherine Knecht, Marin County IPM specialist, and member of MKAT. “Take advantage of the many local experts like MKAT, the UC Cooperative Extension and Marin Resource Conservation District, to learn how to take action on knotweed.”

MKAT also recommends to be on the lookout for knotweed; if you think you’ve found it, leave it alone! Don’t attempt removal on your own. Provide any observations or questions about the plant to MKAT at ucanr.edu/sites/MarinKnotweedActionTeam.

Phillips adds, “We all must work together to get a handle on this invader in order to remove it from Marin County.”

To learn more about invasive species in Marin, go to the California Invasive Plant Council’s website at cal-ipc.org.

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