Weeds are always adapting and changing. Species crossbreed (with native or non-native species), become resistant to herbicides, and infest locations where we might not expect them. At the forefront of all these variations is the hardy woody knotweed species: Japanese knotweed (Polygonum cuspidatum), giant knotweed (P. sachalinense), and Bohemian knotweed (P. bohemicum).
If you thought those stubborn thistles were tricky to deal with, knotweed proves to be even more challenging and takes years to fully remove from a site. It’s so hardy that knotweed is known to grow in the craggy lava fields of its native range in Japan! However, in Japan knotweed’s spread is limited due to poor soils, repeatedly being covered by volcanic ash, natural enemies that feed on it, and soil fungi that restrict its growth. Unfortunately, this is not the case here in Washington County. North American wildlife do not eat knotweed, allowing it to spread unhindered along streams and in disturbed areas. These dense stands easily outcompete native streamside vegetation, causing erosion and altering wildlife habitat. Not only is knotweed a pest in natural habitats, but it is also capable of growing through asphalt, affecting building foundations and roadways.
Defending the Tualatin River Watershed against this troublesome weed is no easy task. Invasive Species Technician Trevor Norman shares his experience of surveying for knotweed along Gales Creek:
“It’s muggy in the riparian undergrowth, almost like you’re walking through a jungle. The blackberry, reed canary grass, and willow are so thick you need a machete to cut your way to the creek. Every step you take requires a glance downward to scan for nutria holes. Stepping in one of these will end in a short field day. … A family of ducks float by you downstream, quacking loudly as if poking fun at your clumsy missteps. You look down at your iPad to figure out where you are, only to look up and find a black-tailed deer staring into your soul from a few yards away.
Moving slowly, you continue your journey downstream probing the undergrowth for the bamboo-like stems indicative of knotweed. It seems like you’ve slogged miles and miles; however, looking back, the truck is still in view. Every hundred feet you drop a data point cataloging the amount of knotweed you’ve found growing along the stream, and it’s a lot. You start noticing it, sometimes just a small sprout, sometimes 10-feet tall! There are no native species growing in these knotweed thickets … just an endless sprawl of tangled green stems erupting from clumps of last year’s dead growth.”
Knotweed has been actively managed along streams in the Tualatin River Watershed by Clean Water Services since 2009, with Tualatin SWCD taking the lead in 2018. In 2009, a whopping 55 acres of knotweed were treated throughout the watershed! Having started the 2020 treatment season in early August, Tualatin SWCD’s Invasive Species Program has been working with local partners, landowners, and contractors to survey and treat knotweed infestations. In 2020, only 1.78 acres of knotweed were surveyed and treated, a 97% reduction watershed-wide knotweed infestation area since 2009!
Often, knotweed takes three to four years to be completely eradicated from a site with proper management and monitoring techniques. Woody knotweed species are known to sprout from small fragments. Digging, cutting, or improper herbicide use will only spread these plants by disturbing their robust network of underground roots and above-ground stems. Appropriate and carefully mixed herbicides applied in the late summer/early fall (before the first frost) are the only dependable way to treat knotweed.
If you have knotweed on your property, you can reach out to the Tualatin SWCD Invasive Species Program at Invasives@TualatinSWCD.org with a photo of the infestation. Once the infestation is confirmed, a certified herbicide applicator can survey and treat the knotweed for free.
It is only through our contractors’ dedication and hard work, property owners’ cooperation, and the community’s helpful observations that we have made huge progress in containing and managing this troublesome weed. Thank you for your continued vigilance and support!