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  • Tovin G.H.

Unlikely Japanese Knotweed Killers?

Updated: Aug 1, 2022

Invasive species can be a serious problem for biodiversity. Unfortunately, they seem to be EVERYWHERE. The Intervale is no exception: the forest has a plethora of invasive species, while native populations have grown smaller. If you don’t believe me, look at this map of invasive species spotted in the Intervale; each color is a different invasive species.

One solution to this problem is applying herbicides. However, herbicides can have many unintended consequences, including killing native species. Mechanical control (digging, cutting, pulling, etc.) takes a huge amount of labor and degrades the soil. Clearly there has to be a better way. To solve this problem, we experimented with using native plants that can compete with invasives to supplement mechanical control methods. Our experiment focused on controlling japanese knotweed–one of the most infamous invasives in New England. Japanese knotweed spreads primarily through rhizomes. Knotweed grows back quickly: these two photos were taken a month apart. As you can see, despite pulling up all of the knotweed, it quickly grew back vigorously.

Worst of all, even small pieces of its roots or stalks can become new plants. This means that to manually remove it you would have to dig every root and shoot fragment out of the soil: a truly sisyphean task. Of course, if you are persistent in pulling knotweed for many years, the plant will eventually weaken and die. The goal of this experiment is to test a technique that limits the amount of time needed to manually remove japanese knotweed. This method of control also works with forest ecology to promote growth of native species.

We selected two species to plant in a plot where the knotweed has been manually controlled for a couple of years but is far from being eradicated. We are hoping that these species will suppress the knotweed. This would remove the need for more years of pulling knotweed.

The first native species that we are using is white snakeroot.

White snakeroot is very aggressive in its introduced range (Eurasia) and exudes natural herbicide (allelopathy) that will hopefully poison the japanese knotweed. Fortunately, white snakeroot is known to coexist with the native plants that grow in the Intervale. We propagated this plant from locally-sourced cuttings.

The next species that we planted was jewelweed, both yellow and orange.

We are using plant biology to our advantage by outcompeting the knotweed when it is still small and vulnerable in the spring. Jewelweed is not only an aggressive plant that we have observed compete with other invasives, but it starts growing early in the spring giving it a head start against the knotweed. This head start will deny nutrients and light to the knotweed, ideally killing the knotweed.

We divided the experiment area into three five-foot squares. One for each species and one that would be left alone to act as a control.

After the plants are planted, the squares will not be touched for at least a year, to properly evaluate whether the native species can control knotweed (in its weakened state). The knotweed surrounding the plot will continue to be cut or pulled, to prevent it from interfering with the experiment.

Stay tuned for updates on the progress and results of our experiment!

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