Invasive Species Profile: Bohemian Knotweed (Polygonum bohemicum)

The most common invasive knotweed in western Washington, the bohemian knotweed (Polygonum bohemicum) is an aggressive perennial plant commonly found in disturbed habitats, roadsides, and along stream banks. This knotweed species entered the U.S. from Asia in the late 1890s to be used for erosion control as well as a hedge plant and ornamental screen. Shortly after its arrival in the U.S., the bohemian knotweed’s destruction of ecosystems and infrastructure drove communities to remove and mitigate the spread of this invasive species.

In 2004, the State of Washington listed bohemian knotweed as a class B noxious weed; a classification given to non-native species located in limited areas of the state that require control in regions they are not found. This invasive species has also been added to the Washington quarantine list, prohibiting the sale, purchase, and transportation of plant parts, seeds, and blends of this knotweed.

How to Identify: A hybrid between Japanese (P. cuspidatum) knotweed and giant (P. sachalinense) knotweed, Bohemian knotweed is commonly misidentified for other invasive flora. Bohemian knotweed shares characteristics with both parent plants, taking on a shrub-like appearance and reaching 6.5 to 16.5 feet in height. This perennial’s alternate leaves are oval shaped and have a leathery appearance, with small hairs residing along underside leaf veins. Leaf tips range from a blunt to tapered point, with indented or heart shaped leaf bases. Stems are thick with grooves and appear reddish-brown. Some may describe these stems as bamboo-like due to their texture and hollow nature. Flower clusters are located at the tips of stems and branches, with male and female flowers located on separate plants. Male plants that lack seeds make up the majority of Bohemian knotweed though seed-bearing hybrids have begun to appear, indicating a backcross with a parent species.

Spread: Bohemian knotweed are extremely adaptable, growing in dry, moist, partially shaded, and sunny environments. This species spreads by seed, vegetatively from long rhizomes, and can re-sprout from stem and root fragments. When located near water, fragments can carry infestations downstream. Once established, bohemian knotweed is difficult to control, as its thick rhizome formations create a dense mat. The species’ lack of natural predators also attributes to its rapid spread.

Impact: Bohemian knotweed is extremely difficult to eradicate, threatening riparian areas and altering natural ecosystems. It’s deep extensive roots and rapid growth allow knotweed to outcompete native species for space and resources. Dense stands formed by knotweed monoculture reduces stream access to wildlife, increases bank erosion, destroys anadromous fish habitat, and crowds out native plant species. Bohemian knotweed also causes damage to infrastructure, growing through cracking pavement, building foundations, and brickwork.

Prevention: There are several approaches to prevent the spread of bohemian knotweed. When working in areas containing bohemian knotweed: cleaning, draining, and deconning gear after every use minimizes the risk of fragments spreading and infesting new areas. Other methods such as mowing, up-rooting, covering, and herbicide injections/spraying all prevent the spread of bohemian knotweed, but could take years to completely eradicate the species from the area. There are many resources available to understand the risks, laws, and prevention methods surrounding Washington’s invasive species. Organizations such as the Washington State Noxious Weed Control Board  and the King County Noxious Weed Control Board can help identify a control strategy and suggest actions to protect the environment.