Treating Japanese and Giant Knotweed

Wednesday, April 19, 2023

Japanese Knotweed, Giant Knotweed, and the hybrid Bohemian Knotweed can grow to 10 feet tall and forms dense stands, crowding out all other plants except for large shrubs and trees, because it shades out other plants and it releases toxins to suppress the growth of other species. Its thick roots and rhizomes can extend up to 60 feet horizontally and six feet deep. So in large patches, the plants cannot be dug out. Once knotweed is fully established over wide areas, such as some stretches along the Conemaugh and Allegheny Rivers, it is laborious to eliminate.

In Indiana County there are public spaces were knotweed is just getting established and the patches are fortunately small. These are prime targets for treatment. With a manageable effort, we can have a huge impact on the future of native plants at these locations. This summer, we are targeting a section of Ghost Town Trail and an area in Pine Ridge Park. Both locations have an abundance of native wildflowers and other native plants that can fill in after we have eliminated the knotweed.

Our protocol is a two-step process. We cut the knotweed to the ground in June, so that later in the season we can use less spray on the smaller plants. At least six weeks later we return and spray the resprouted knotweed, which will only be one or two feet tall, with glyphosate herbicide. That late in the growing season, the plants' nutrients will be flowing back into its roots. The herbicide will be drawn into the knotweed roots and kill the plants. If you are available, please join us in this effort.

For a more detailed explanation, see Penn State Extension's publication on treating Japanese Knotweed along rights-of-way. https://plantscience.psu.edu/research/projects/vegetation-management/publications/roadside-vegetative-mangement-factsheets/5managing-knotweed-on-roadsides.

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