DURHAM — Town officials are considering using herbicide to combat a Japanese knotweed infestation.
Earlier in May, the town Conservation Commission asked the Board of Selectmen to lift its moratorium on using herbicides for town property and roadside maintenance. The commission requested an exception in order to eradicate spreading Japanese knotweed since the invasive species doesn’t respond well to organic treatment.
The commission reviewed instances of Japanese knotweed throughout the town before concluding it poses a significant problem.
The knotweed is spreading near a storm drain on Parmelee Hill Road near Wildwood Lane; along the guardrail on the east side of Tri-Mountain Road, and along Seward Road to the east of Saw Mill Brook. Knotweed is also spreading on the east side of Howd Road and extends into the town-owned parcel there.
Conservation Commission member Lorrie Martin noted that there are towns that have been taken over by knotweed.
During the Board of Selectmen meeting on Monday, Luke Johnson, owner at Johnson Environmental LLC, advised the town officials to use thin invert emulsion herbicides to treat an infestation of Japanese knotweed.
The herbicide breaks down the waxy cuticles of the plants faster than just water-based formulations. That allows using a much lower percentage of the herbicide over a larger area.
“Using that thin invert emulsion, you are using at most five gallons an acre,” said Johnson. “You can go as low as one gallon an acre, depending on the mixture that works best for that particular site.”
After breaking down the waxy cuticles, the herbicide translocates into the roots of the plant and stunts its growth, preventing it from flowering, Johnson explained. Managing knotweed prior to flowering avoids bees and other pollinators from consuming the herbicide.
“Then, mowing it down during the dormant season like winter, or fall or spring, and getting ready for any new growth that comes up the next year, allows you to continually decrease that herbicide usage as the [knotweed] population decreases,” said Johnson.
He added that Johnson Environmental strives to work in an “environmentally friendly way.”
“The end goal is to do what’s best for the environment and not just go in there and kill everything or harm the public or the people doing the applications, or the environment and wildlife,” said Johnson.
Although Johnson said it’s hard to determine how many years it will take to completely eradicate the knotweed, he gave an estimate of two to five years.
“I’ve seen it where it’s one season and it’s gone, because it's a small enough infestation, and I’ve seen it take the full five years and even longer,” said Johnson. “You have to stay on top of it. I would say plan for up to five years, hope for less than that.”
The Board of Selectman has not yet voted whether to approve a request from the Conservation Commission and use the herbicide to eradicate the knotweed, although Selectman George Eames was inclined towards that solution.
“It really is an invasive species,” said Eames. “You can’t take a little bottle of vinegar and spray this and maybe it will go away. It has to be a little more stronger approach.”