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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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Alien Plant Invader: Japanese and giant knotweed


close up of knotweed flowersWith its fluffy white flowers and bamboo-like stalk, Japanese and giant knotweed seem like beautiful plants to have in your yard. But be careful with these invaders! They are fast growing plants that can quickly dominate and displace other plants.

Both species are herbaceous perennial plants, dying back each winter and blooming in mid-summer with clusters of small greenish white flowers. Japanese and giant knotweed can grow 6-12 feet tall by mid-summer. Right now, plants may be turning yellow and the white flowers may not be present.  See more photos below.


Why is Japanese knotweed bad? These plants are especially invasive along streams and rivers. Japanese and giant knotweed can displace the native trees that protect our water quality. Native trees provide shade that keeps streams cool, stabilize stream banks and reduce erosion and sediment.

yellow knotweed fall leavesCloser to home, unmanaged knotweed in your yard can quickly overwhelm landscaping, garden beds, fences, and your free time. The plant largely reproduces vegetatively, meaning that root fragments from these plants can break off and establish elsewhere, causing new infestations.  This makes digging it up difficult.


Visit this page for more information about control, removal, and identification of knotweed. The East Multnomah Soil and Water Conservation District and West Multnomah Soil and Water Conservation District also have information available. 


Knotweed may start out small, but quickly starts to take over!

The stems look like bamboo, but the leaves don't.

 small knotweed plant in driveway knotweed in bloom

  knotweed leaf compared to hand distinctive stems of knotweed 

knotweed stemsInvasive species affect us all. They damage our forests, our streams and rivers, and our property.  Nationwide, damages associated with invasive species are estimated to be $120 billion each year. In Oregon, the control of invasive weeds and the cost of the damages they create amounts to about $125 million each year.  We know that it costs a lot less to control new invasive plants before they become infestations, so we need everyone’s help. Find out more about the problems caused by invasive species and why Environmental Services works to stop their spread.


See our other Alien Plant Invader posts:




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