Targeting knotweed in Winsted

WINSTED — North Canaan nature photographer and tree preservationist Tom Zetterstrom brought out a souvenir May 6 for the packed Winchester/Winsted Town Hall meeting room to see — a stalk of the plant itself, plucked that day from beside the “Welcome to Winsted” sign.

The presentation, which was hosted by the Winchester Conservation Commission and titled “Eradicating Japanese Knotweed Along Roadsides and Riparian Borders,” was loaded with similar warnings about allowing the tenacious plant to establish itself in Northwest Corner towns.

Zetterstrom said his intention for the presentation was to “bring the Housatonic model” to Winsted.

The “Housatonic model” refers to a knotweed survey, removal effort and shoreline restoration project that identified a large population of the plant just north of the West Cornwall covered bridge. Zetterstrom dubs that specific growth the “Wild and Scenic Housatonic Presentation Patch,” as it became a demonstration of the effectiveness of the herbicide glyphosate in dealing such robust growths of the plant.

HRC members and licensed herbicide applicators Robert Gambino and Christian Allyn treated the knotweed with the chemical in 2018 and 2019. After a few years, a few spot treatments and several more plantings, the area was restored to native plant habitat with minimal knotweed reemergence.

Zetterstrom said glyphosate works well for knotweed because it breaks down quickly and binds to soil, reducing the chance that it runs off into the river. When handled carefully, he said, it is easily controlled: “I guarantee not one droplet of glyphosate touched the river.”

While this population was a riparian (riverside) growth, Zetterstrom said that it had been introduced to the area by contaminated fill from the town of Canaan. This is a common occurrence, he said: “These stem fragments are being cultivated in a sense in every town gravel pit and land fill.”

He explained that roadside and riparian knotweed populations are deeply connected. Roads follow rivers, and as such, a seemingly inconsequential patch on the side of a highway could lead to a shoreline-choking infestation.

Japanese knotweed at its “full biological potential” becomes a “monoculture,” he explained — “it basically means it owns the land.”

This is a major issue for riparian ecosystems in which the aquatic life below the surface is deeply intertwined with what’s happening onshore. To illustrate the point, Zetterstrom referenced a study which found that rivers with knotweed-lined shores lost about 96% of their insect biomass, which could have dire consequences for the river’s ecology, such as for the trout that eat those insects.

“In terms of salmonids, that’s a bad story,” he said, noting that the renowned Housatonic trout fishery could suffer gravely.

The best way to avoid those situations is to locate the plant early on before it has the chance to deeply establish itself. While herbicides are his preferred method for treating monoculture growths, he said that other methods have also proven effective to varying degrees, especially with younger populations.

An intensive cutting regime — three times a summer for three years in a row — has yielded good results, however, careless mowing will only disperse the stalks elsewhere and spread the growth laterally.

A potential treatment involving the introduction of a Japanese insect that feeds on knotweed — and only knotweed — has also shown promise, though Zetterstrom said it lacks urgency: “It won’t solve the problem in my lifetime.”

Foraging goats have also been used, but Zetterstrom noted that 60% of the plant is underground and remains unaffected by their feeding. “There’s a certain romance to bringing in goats,” he conceded, though.

“Can you eat it?” shouted out an audience member during the Q+A portion of the evening, shifting the conversation to human diets. “Absolutely,” Zetterstrom replied — “I was even served it in Canaan,” though he compared it unfavorably to rhubarb.

“We’re not going to eat our way out of this,” he said.

For detailed information on knotweed management, a full recording of the presentation is available on the Town of Winchester’s YouTube page.