WHO KNEW Japanese Knotweed Can Be Killed and Will Be Replaced by Non-invasive Plants?

For more than ten years, The Adirondack Park Invasive Plant Program (APIPP) has had funding to coordinate control of the two most difficult invasive plants in the Adirondack Park: Japanese Knotweed (Raynutria japonica) and Common Reed Grass (Phragmites australis). Chemical treatments have been used to control both these plants; in very small infestations manual removal was possible. If growing near a water body, small fragments of these plants can easily break away by flood water or ice scour and get carried downstream, further harming wildlife habitat.

What makes these plants so difficult to control? Basically, both have massive root systems. In the case of knotweed, the roots can go down more than 4’ into the ground. Plus, it can send out lateral roots that can extend for 20’ or more and thus start up a new colony. So, one year of treatment for an established colony might set the plant colony back a good bit, but won’t quite destroy it. While some folks are reluctant to use herbicide, in many situations this treatment is the most effective treatment option available today.

There is research on biocontrols for these plants, and several insects that attack knotweed have been tested, but have not yet proved effective. To evaluate its progress, in 2021 the Program hired an independent firm to visit a sample of the treated sites and report on the health of the vegetation where there had once been one of these weeds. After three years of treatment, the APIPP had eradicated 91 Japanese knotweed and 322 common reed grass sites. For this study, 71 of these sites were chosen for study in 2021, roughly an equal number of knotweed and reed grass sites. Each site was matched with a nearby reference (control) site similar in size, geography, drainage, overstory, slope, aspect and distance from roads or man-made structures.

At each site species richness (most common non-invasive plants), percent bare ground in the sites, and the average density (i.e. stem count) of the 5 most common non-invasive plants were observed. The sites chosen for study had experienced varying numbers of years of recovery from treatment. The treated and reference sites were very similar. Knotweed sites took about two years of recovery to catch up to the species richness in reference sites. For common reed sites, the recovery time after treatment was also slower, with more bare ground for the first 2 years after treatment.

In summary, it appears that knotweed can be controlled with roughly 3 years of treatment, and non-invasive plants will return to fill in the gaps within another three years. For more information or contact sender for a knotweed fact sheet.