patch of knotweed invading a pasture

Knotweed (Polygonum spp.) is a highly invasive, herbaceous perennial plant native to Japan, China, and Korea. It is listed by the World Conservation Union as one of the world’s 100 worst invasive species for good reason. It frequently colonizes riparian areas (near stream and river banks), roadsides, and areas of human disturbance where it easily replaces native vegetation due to its fast growth, and expansive root mass.

Riparian areas are where knotweed causes the biggest problems. Therefore, knotweed control in these areas is of the highest priority. Knotweed is a problem in riparian areas because its root system is primarily made up of thick underground stems called rhizomes with very little fine root material. This root structure does not hold soil well compared to the native riparian trees it replaces, and therefore increases erosion along stream banks. It also does not grow as tall as the native trees it replaces greatly reducing shade on streams allowing more sunlight to penetrate and raise water temperatures.

Knotweed spreads very rapidly through streams and rivers because it regenerates vegetatively and very small fragments of rhizome and stem material are able to produce viable shoots and roots. After a flood event, where one established knotweed plant can be broken up and transported downstream, a watershed wide outbreak can result. This in turn can result in degraded water quality and ultimately an ecosystem with decreased biodiversity. Of particular importance here in Multnomah County is the Sandy River. Being one of few remaining Salmon bearing river systems that flows freely from source to sea, we find the eradication of knotweed critical in maintaining the health of this watershed.


The three main varieties of Knotweed are Japanese, Himalayan, Giant and hybrids. Himalayan knotweed grows up to 6 feet tall, Japanese can reach 12 feet tall and giant knotweed can grow up to 15 feet tall. Japanese knotweed is by far the most common type of knotweed in the Pacific Northwest. Knotweed has hollow stems with distinct raised nodes that give it a very similar appearance to bamboo, though it is not closely related. While large mature plants have stems that can reach 9-12 feet each growing season, it is typical to see much smaller plants in the shade or when they’re young. The flowers are small, creamy white, blooming in late summer and early autumn. It is a herbaceous plant which dies back to the ground every winter.


Free control is available for knotweed found in streamside areas.  If you have found a knotweed patch in an upland area and wish to control it, first off understand that knotweed is not an easy plant to control. The extensive underground rhizome system can sustain the plant even when the above ground growth is removed. Therefore, targeting the rhizomes is critical to the eradication of knotweed. This is best achieved using a chemical treatment. Currently, our treatment protocol consists of a foliar application of an aquatically formulated herbicide containing Imazapyr in late summer or early fall.

Manual control of knotweed is not recommended due to the lengthy time commitment necessary. Also, digging and disturbance is known to increase stem density. Digging can also result in the formation of new colonies if the root material is not disposed of carefully.

If you choose to use herbicides, always read the entire label and carefully follow the instructions. Adhere to the label requirements for application, mixing, and loading setbacks from wells, perennial and intermittent streams and rivers, and other water bodies. Always wear appropriate protective clothing and gear.

Report Sightings!

If you think you have found this plant please report it. This plant is on our EDRR list, and we provide free control in streamside areas. Report a sighting!

More Photos

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