Bohemian Knotweed
Polygonum x bohemicum
Washington State Classification
Class B — Control Required (Designated)
It forms dense colonies that exclude native vegetation and greatly alter natural ecosystems.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Clumping and spreading perennial with large leaves, hollow stems and long creeping rhizomes.
Leaves
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.
Flowers
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.
Impact
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.
Ecology & Spread
Habitat
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.
Spread Mechanisms
Reproduction
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.
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Control Methods
Mechanical
- 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.
Biological Control
- 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.