September’s Weed of the Month!

Its September, and that means its time for a new Weed of the Month!

We are working with our partners at the Columbia Gorge Cooperative Weed Management Area to promote invasive knotweeds  (Fallopia sp.) as our weed-of-the-month for September.

Knotweed is a horribly invasive plant unleashed on the countryside by well-intentioned gardeners attracted to its ornamental qualities.  Often mistaken for bamboo, knotweed grows up to 15 feet tall, with large triangular to heart-shaped leaves.  Stems are hollow with swollen joints and plumes of white flowers emerging in late summer. There are four species of knotweed invading our area: Japanese (Fallopia japonica), Giant (Fallopia sachalinensis, hybrid Bohemian (Fallopia × bohemica), and Himalayan (Persicaria wallichii). While it spreads most easily in streamside areas, it can also be found along roadsides, home landscapes and other upland sites.

Imagine an invasive weed that you can’t cut, mow or pull because doing so will encourage it to spread.  Knotweed reproduces readily from small pieces of its rhizomes (lateral roots) and stems as well as by seed.  Knotweed can grow up through asphalt and concrete.  It outcompetes native vegetation, and has taken over large tracts of riverbank across much of Clackamas County.

If you have this plant, please help keep it from spreading!  Chemical control and diligent multi-year monitoring and retreatment are often the only effective ways to stop this invader.  August to October is the best time to spray an herbicide for knotweed control.  That said, if you do nothing else, please cut, bag and dispose of flowers in the garbage before they go to seed.  Information on knotweed and how to control it is contained in the Oregon State University publication EM 9031 or online at:

If you find knotweed, be sure to report it to the Oregon Invasives Hotline at  The WeedWise program is working to control populations is several targeted watersheds, as well as track all known locations to help with future control efforts.  So help stop the spread of this damaging invader.




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WeedWise Program