Invasive Knotweed Treatments
Invasive knotweeds, such as Japanese knotweed, giant knotweed, and Bohemian knotweed, are extremely difficult to control. Because the plants can reproduce by fragmentation (a piece as small as an inch long or, in extreme cases, a few millimeters can start a new plant!), cutting and digging only make the problem worse. Because of their volcanic origins, burning these plants is also ineffective.
- Herbicide: ISN and our contractors treat with 1.3% aminopyralid (trade name Milestone) OR 5% imazamox (trade name Clearcast) with 1-2% glyphosate (trade name AquaNeat), depending on the best choice for the site. We work with landowners to help them choose the best herbicide for their site.
- Milestone treatments usually include 0.2 oz/gal of an aquatic-approved, non-ionic surfactant (such as CygnetPlus). Clearcast usually uses an aquatic-approved crop oil concentrate (e.g. Agri-dex) at a rate of 4%.
- Timing: Treatments can happen twice per year, with the first treatment when the plants are three to four feet high (early June, in our region), and again at least 60 days before the first frost (early August).
- Methods: Most often, we use backpack or hand-sprayers to apply the herbicide to the plants’ leaves. In a few cases, we use an injector to put herbicide directly into the plant.
- Other: Treatments may take as little as one year to be successful, but more often two to four. In some cases, as many as seven years of active treatment are needed. In all cases, because of its immense underground root and stem mass, monitoring for new sprouts is needed for at least 5 years past the last treatment.