09/13/2025
Invasive Knotweed: A Group Effort
by Pomfret's Conservation Commission
In case you missed last month’s article, this is Part Two of managing invasive knotweed in Pomfret. We’re seeking volunteers to help treat knotweed at priority project sites, and to report knotweed sightings. Please email [email protected] to get involved.
Also, mark your calendars! Join us for a knotweed management presentation with Jane Seymour of CT DEEP on September 24th at 6:30 pm at the Pomfret Community Center. This Town Event is posted on FB.
There are three critical parts to managing knotweed: (1) timing of treatments, (2) appropriate product applied, and (3) correct tools used. Missing one of these parts can have a negative impact. Following best practices for all three parts is safe, effective, and highly targeted. I’ve asked several conservation experts about how to manage knotweed with the most positive environmental outcomes. In all cases, treating it with glyphosate was recommended because knotweed is so resilient. They shared about aquatic-safe products and methods we detail in our new Knotweed Guide.
Glyphosate is a systemic herbicide. This means it remains in the plant tissue when applied to the leaves or cut stem, and is not active in the soil. This allows the invasive plant to be targeted without affecting the existing seedbank or nearby desirable trees and plants. Glyphosate typically takes about two weeks to be effective. This is ideal because the plant needs time to draw the herbicide down to the underground rhizomes. The above-ground growth won’t turn brown immediately – and that’s a good thing. Some herbicides damage plant tissue too quickly, before the plant can move it to the roots.
Up to two-thirds of knotweed biomass exists underground! This is where timing of treatment is everything. Spring treatment isn’t effective. That’s when it’s sending nutrients up to new shoots above ground. It won’t draw an herbicide down at that time of year. A late summer or early fall treatment is best, when it prepares for winter dormancy and carries nutrients back down to the root system (along with the herbicide). For foliar spray, it’s best to wait until after the flowers have passed, and until 2-4 weeks before the first frost. Our new guide shares details on trusted treatment methods and timing windows. You can find it posted here: https://www.pomfretct.gov/conservation-commission/pages/invasive-knotweed-management-guide
Some invasive plants do have successful outcomes with non-herbicidal control. Invasive garlic mustard can be controlled in just a few seasons by pulling the root or cutting before seed is produced. This strategy is feasible because it’s a short-lived biennial plant. The garlic mustard seed bank will be greatly reduced and exhausted over time. Invasive knotweeds are long-lived perennial plants. If no action is taken, it will persist and overtake an area.
For more information, please email [email protected]. Stop by our table at the upcoming Fall Festival to say hello to commission members and ask us any questions!
By Beth Manteuffel, Pomfret Conservation Commission