Japanese knotweed

What Have We Learned About Knotweed in Two Years of Battling It at Our Bikeway Meadow?

Ask anyone knowledgeable for a list of the top five invasive plant menaces in the northeastern U.S., and chances are Japanese knotweed (Polygonum cuspidatum, syn. Reynoutria japonica) will be on the list. First brought to this country in the late 1800’s, possibly as a landscaping plant, it has spread to become a major nuisance and threat to our native ecosystems. How to control it is a hotly debated topic among invasive plant experts.


Japanese knotweed, like many invasives, has few natural enemies in this country, helping it to grow unrestrained by the normal processes (herbivory, predation, disease, etc.) that tend to keep native species in check. In good conditions it grows aggressively; though it dies back to the ground each winter, by June a vigorous stand can be 7-10 feet tall. Its dense canopy shades out other plants, creating a monoculture that crowds out our native flora.

But what makes knotweed the bane of gardeners and land managers everywhere is how hard it is to kill and how quickly it spreads. Credit its root system, the below-ground crown and rhizomes that overwinter and send up new shoots each spring. They are incredibly hardy, and the rhizomes can grow up to 10’ deep and 50-60’ laterally, making it very resistant to control efforts.


In England they’ve been grappling with knotweed even longer than we have, and they take it even more seriously. Concern that knotweed can damage building foundations has made houses with knotweed in the yard difficult to mortgage or insure. The financial cost of knotweed is just now being reckoned with in the real estate market here; in the first known Massachusetts case, a new homeowner just won a $300,000 judgment against the developer for spreading knotweed-contaminated soil on the property they purchased.

Can One Eradicate a Knotweed Stand?


Knotweed is very difficult to eradicate entirely – its deep and extensive root system, from which it can easily regenerate, gives it great staying power. In many cases a more realistic goal is to control and limit it, rather than eradicate it entirely.


There are two primary control methods: mechanical and chemical. A young stand of knotweed can sometimes be eliminated by pulling and digging it up, with diligent follow-up for any missed bits of rhizome. Some have had success with a cut-and-smother approach; the Lexington Field and Garden Club has used this at the Hancock-Clarke House.


Established stands present a far greater challenge because of the depth and vigor of their underground roots. They are almost impossible to dig up; and if you miss even a small piece of rhizome, the stand will re-establish. Cutting repeatedly, thereby robbing the underground crowns and rhizomes of their photosynthetic engines, will weaken it, and one may eventually succeed in eradicating the stand altogether, but success will take many years (maybe decades). Mike Bald, proprietor of the Vermont firm Got Weeds?, who specializes in non-chemical knotweed control, suggests landowners approach the task with a “12, 11, 10...etc.” plan: remove the above-ground stems 12 times the first year, 11 the second, 10 the third, and so forth. But beware the edges; cutting tends to stimulate growth, and knotweed has been known to send rhizomes out 50’ or more in search of more hospitable ground to colonize.

Knotweed in May 2021 at the Pollinator Meadow site. You'll want to work on mechanical control by this height, not letting it grow bigger.

Knotweed in bloom

Many land managers attempting to control knotweed have come to rely on chemical means as their preferred approach, generally combined with cutting. Often what’s recommended is an early season cut so the resulting stand is not too high, and then application of glyphosate (Roundup) or triclopyr either to cut stems or as a foliar spray. For the latter, treatment in late summer is recommended, after the plant blooms (to avoid harming pollinators, which love the flowers) and before the plant begins to translocate nutrients from the canes to the root system in anticipation of winter.


If you cut and pull, as part of either approach, what to do with the canes? All parties agree that any crowns and rhizomes need to be treated with respect, as they will readily resprout; disposing of them in the trash or drying them for extended periods on an impervious surface is recommended. Experts differ on how much of a threat the cut canes are; they can resprout from the nodes, but some feel if properly dried until brown and brittle they become harmless and can be composted on site, while others recommend they be treated as waste and bagged and disposed of with trash. (Russ Cohen, Cambridge resident and long-time proselytizer of wild plant foraging, offers a third alternative in his wonderful little book, Wild Plants I Have Known...and Eaten: “If you can’t beat it, eat it!” Knotweed is a close relative of rhubarb and can be used in similar ways, and he offers a great recipe for Strawberry-Knotweed Pie.)

Is there any prospect of biological control? Realistically this may be our only hope of limiting its spread across the landscape. Scientists have been examining a psyllid, a small insect that sucks plant fluids for nutrition, as a possible biological control agent. Most psyllids are host-specific, and this one, Aphalara itadori, found in Japan, appears specific to knotweed. It has been tried in Britain with mixed success, and scientists have recently released an environmental assessment paving the way for release of individuals from a strain that may be more suited to the U.S. climate and environment.

So what’s a homeowner to do?


Start by reviewing some of the many sources of information out there, including advice from state experts in New Hampshire, Pennsylvania, and Wisconsin, and closer to home, the talented professionals who collaborate as part of the Cooperative Invasive Species Management Area in the Concord River basin.


If you have a small infestation, try pulling and digging it up. A large, established colony needs a more thoughtful approach. If the core of your knotweed stand is on a neighboring property and only extends onto yours, you could just cut or pull where it appears on your property, keeping it in check. If its core is on your property, be mindful that cutting may stimulate the colony’s growth into your neighbors’ yards, and approach them about a collaborative approach. If you’re willing to go the chemical path, bring in an experienced, licensed professional.


What we’ve done at our pollinator meadow along the Minuteman Bikeway, which was an 8-foot high monoculture when we started two years ago, is to eschew chemicals in favor of the starve-the-roots approach, cutting and pulling regrowth on a regular basis. We’ve kept an eye on the edges to minimize its spread to new ground, but there’s little harm to be done here – knotweed was already established all around us before we started.


Our ambition is limited: eradication is a pipe dream, but with diligent effort we are reclaiming this spot of ground for a native plant meadow that is more ecologically productive. An integral part of our strategy has been to provide competition for the knotweed by planting the perennial meadow plants we are trying to establish, seeding annual cover crops (buckwheat and Partridge Pea) while the perennials are settling in, and giving “volunteers” less invasive than the knotweed some latitude to grow and compete while we try to steer the transition.

Dense stand of knotweed, just prior to the first cutting in June 2021

Our first cutting effort in June 2021. For detail on how we've managed this project, see our pollinator meadow page.

At the end of the day, be realistic. The most you might hope for is to keep it sufficiently in check that you can use the ground for other purposes. In the home garden, that is absolutely a realistic goal. Eternal vigilance is key; ignoring the knotweed for even a short while will only open the door for the stand to regain its former vigor.


Good luck! Visit our bikeway meadow to judge our progress, and share your own experience with us by writing us at lexlivingland@gmail.com