The Scappoose Bay Watershed Council and the Columbia Soil and Water Conservation District (CSWCD) have been working for the past several years to treat invasive weeds throughout our watershed. Although there are numerous weed species in our watershed, efforts have been focused on a few of those species that are new invaders with a high potential to spread and cause damage.
Two species that have been of greatest concern in our watershed are garlic mustard and knotweed species. Knotweed is found along approximately 18-miles of creek throughout Scappoose and along roadsides and sections of creek in St. Helens. So far, garlic mustard has been found on only 3.5 miles of lower North Scappoose Creek and along Scappoose Creek.
Since these weeds are treated mainly along the creeks, only aquatic-safe herbicides are used by licensed professionals who walk the creeks and treat only the target weeds to minimize damage to surrounding native plants. Herbicide treatment is the most feasible way for treating these particular weeds because knotweed has the ability to sprout from root and stem fragments, so manual removal on a large scale is not practical. Garlic mustard does not spread from fragments but from seed, potentially thousands per plant, so large-scale manual removal is also impractical.
The majority of the land treated is privately owned, so permission is obtained from willing landowners to access and treat their properties for the targeted weeds at no cost to the landowner. Treating these invasive weeds is a community effort and the participation of all landowners is needed to effectively eliminate these weeds and the threats they pose in our watershed. If landowners are opposed to herbicides but want to help the effort to eradicate the weeds, they can contact the Watershed Council or CSWCD for technical assistance on alternative control methods.