Weeds are troublesome, but not all are as invasive as woody knotweed (Polygonum species). Knotweed species love water and lots of sun, so they thrive along streambanks and other riparian areasRiparian Areas Areas of land that occur along the edges of rivers, streams, lakes, and other bodies of water..
Above ground, knotweed forms dense, bamboo-like thickets, which completely shade the ground. Many native plants can’t tolerate complete shade, so once it is established in an area, nothing else can grow underneath it.
Below ground is where the real trouble begins. Knotweed’s rhizomatous root system can grow over 30 feet long! While knotweed’s roots are extensive, they lack fine root material needed to hold onto soil. This means that large stretches of streambanks that have knotweed are at risk of erosion, especially during the Pacific Northwest’s rainy seasons.
Not only is knotweed a pest in natural habitats, but it is also capable of growing through asphalt and damaging building foundations and roads! Its roots can establish in drainpipes, cracks in concrete, and home foundations. This can break up driveways, clog pipes, and cause structural stress.
Knotweed doesn’t just cause harm; it also spreads rapidly. New plants can regrow from the smallest root or stem fragments, often as small as half an inch. This allows a single plant to quickly give rise to new plants or entire colonies. Because of this, knotweed is an enormous threat along streams, creeks, and rivers where root fragments can be washed downstream.
Knotweed has been found across the Tualatin River watershed but is particularly a problem along Fanno Creek, Gales Creek, and East and West Fork Dairy Creek. If you live near these waterways, keep a close watch out for this problematic weed.
Knotweed is notoriously hard to control.
Its extensive root system can sustain the plant even when its above ground growth has been removed. Therefore, focusing on the root system is critical in managing knotweed.
Unfortunately, digging and cutting is known to increase the density of an infestation and can even spread it to new locations. Knotweed’s roots grow deep into the ground, over 10 feet deep. Even when removal is done carefully and thoroughly, it will take years of repeated control to fully remove it from a site. The only dependable way to remove it is by applying appropriate and carefully mixed herbicides in the late summer or early fall.
Let Tualatin SWCD treat it for you!
Tualatin SWCD has been successfully treating knotweed infestations for years. Since 2019, we’ve treated more than seven acres within our community. If you think you have it on your property, let us know! If found, we will work with you to determine the best treatment plan that aligns with your schedule. All weed treatment is done on a voluntary basis and is free of charge.
If you want to treat knotweed yourself, we provide free consultation to help develop your treatment plan. If you choose to use herbicides, read the entire label and carefully follow the instructions. Adhere to the label requirements for application, mixing, and always wear appropriate protective equipment.