Knotweed

Within North America the invasive knotweeds can be found throughout the northern midwest and coastal regions. Originally from Asia, knotweed has been planted as ornamentals since the 1800s. These large plants have since moved into riparian areas and are forming huge thickets. The roots are rhizomatous and change the physical structure of creeks and rivers, which can result in severe erosion and collapse. In areas like the states of Oregon and Washington, this has severely impacted some native fish spawning regions. The plants can also grow through asphalt and building foundations. Knotweeds are also known to be invasive and control efforts are undertaken in Europe, England, New Zealand and Australia.

In Colorado, Japanese and Bohemian knotweeds are A-List noxious weeds. Giant knotweed is not well known in Colorado. Previous reports of Giant knotweed may have been misidentified and are probably the hybrid.

Some features you can use to distinguish the different types include plant height, leaf size, leaf hairs, and size of flower clusters. (See CWMA’s Knotweed Comparison Chart)

Bohemian knotweed is a hybrid between Japanese and Giant knotweed. The features are in between both parents and can sometimes be confusing.

Reproduction includes vegetative through stem and rhizome fragments. There is some seed production but it is complicated. Plants may have perfect or single sex flowers that may or may not be fertile.

Control requires repeated treatments because the extensive roots and rhizomes will resprout. The roots can be 6 feet deep and the rhizomes 65 feet long. First reported in Colorado in 1939, the knotweeds are now found in about 10 of our 63 counties.

RESOURCES

EDDMaps – Japanese knotweed

Invasive Species Centre – Bohemian knotweed

Michigan Department of Natural Resources

Jones, Daniel, Stephen Pike, Malcolm Thomas, and Denis Murphy. 2011. “Object-Based Image Analysis for Detection of Japanese Knotweed s.l. Taxa (Polygonaceae) in Wales (UK).” Remote Sensing 3 (2): 319–42. https://doi.org/10.3390/rs3020319.

Japanese knotweed – CABI

Bohemian knotweed – CABI

Giant knotweed – CABI

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