Polygonum x bohemicum
Why Is It a Noxious Weed?
It forms dense colonies that exclude native vegetation and greatly alter natural ecosystems. Established populations are extremely persistent and colonies are extremely difficult to eradicate. It poses a significant threat to riparian areas.
How would I identify it?
Clumping and spreading perennial with large leaves, hollow stems and long creeping rhizomes.
Flower clusters commonly made up of male flowers and located at stem and branch tips. Clusters (panicles) are around 8 to 13 ¾ inches long. Sepals are white to pinkish white in color.
Leaves are alternate, leathery, and oval. The leaf tip ranges from being blunt to a tapered point. The base is slightly indented to deeply heart-shaped. Leaf midveins have hairs. Leaf size is intermediate between Japanese and giant knotweed.
Main stems upright, often arched near top, simple to minimally branched, grooved, thick, hollow, weakly woody, swollen at nodes, usually reddish-brown at maturity.
May Be Confused With
Invasive knotweed species look very similar. Bohemian knotweed is a hybrid between Japanese knotweed and giant knotweed. If you need help with plant identification, please contact your county noxious weed coordinator.
Where does it grow?
An escaped ornamental, knotweed is often found in waste places, neglected gardens, roadsides, streambanks and riverbanks. Please click here to see a county level distribution map of Bohemian knotweed in Washington.
How Does it Reproduce?
Bohemian knotweed reproduces vegetatively from rhizomes and by seed. Each node on the plant stock is able to produce roots and new plants. New plants can sprout from small fragments.
How Do I Control It?
GENERAL CONTROL STRATEGY
Controlling invasive knotweed species typically takes a number of years and monitoring the site for regrowth is critical. Care must be taken not to produce new plants. All plant material should be removed and properly disposed of as new plants can sprout from very small fragments. Check with your county noxious weed control board about how to best dispose of plant parts. Plants can be dried and burned but make sure to check with local burning restrictions and if any permits are needed.
Grubbing out small clumps when discovered can prevent new colonies from establishing but is time-consuming. Rhizomes and fragments left in the ground, or nearby, can regenerate and spread infestations. The entire root system must be removed since re-sprouting can occur from rhizomes. Frequent, repeated cutting of plants over many years can eventually kill the plant. Covering plants with a thick landscape fabric for up to 5 years may provide control. Fabric should be installed after cutting plants back to the ground, provide some give in the fabric so new stem growth won't puncture through, monitor fabric and flatten knotweed growth under it, keep fabric cleared of debris, and repair any holes.
There are currently no registered biological control agents for use on any of of invasive knotweed species. Grazing may provide some reduction in plants but additional control methods are typically needed. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.
For More Information
See our Written Findings for more information about Bohemian knotweed (Polygonum xbohemicum).