There are three species of invasive knotweeds in Minnesota: Giant knotweed (Polygonum sachalinense), Japanese knotweed (Polygonum cuspidatum) and Bohemian knotweed (Polygonum bohemicum), which is a hybrid of the two species. These three species look relatively similar, but with a few minor distinctions. They all have large, oval leaves and green stems that resemble bamboo. However, Giant Knotweed is heart-shaped at the base, while Japanese Knotweed is not, and Bohemian knotweed can be anywhere in between (see image below). Knotweeds form dense thickets, growing up to 12 feet tall, often crowding out native plants and reducing native habitat. All knotweed species have been added to Minnesota’s Prohibited Control Noxious Weed List. Knotweeds spread by expanding root systems as well as by seed, and are aggressive growers. Infestations of Bohemian Knotweed have been found in Mendota Heights near Roger’s Lake.
Control methods for Knotweeds:
- Hand-Pulling
- Cutting and herbicide treatment: Best used in the fall. Cut plants close to the ground, and immediately treat with herbicide.
- Girdling: Wire mesh can be placed over dense areas once they are cut. The mesh then eventually girdles and kills plants as it attempts to grow through it.