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.

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.

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.)