The Weed Wire: Knotweeds
A dense patch of Himalayan knotweed in Lincoln City
History and Native Range: Knotweeds- currently classified within the genus Reynoutria- are native to eastern Asia, including Russia and Japan. Knotweeds are generally very hardy and fairly drought-tolerant, despite preferring to grow in wet areas like riparian zones. They are prolific growers, with some species growing up to one foot per day in ideal conditions. They are capable of growing in nearly any type of substrate, including through pavement; in fact, in its native range Japanese knotweed is an important pioneer species that is often the first to grow back in lava fields following volcanic eruptions.
Due to past cultivation as an ornamental species or experimental forage crop, several varieties of knotweed can now also be widely found across Europe and North America- particularly giant, Japanese, Bohemian, and Himalayan knotweeds.
Identification: While leaf shape can differ greatly between varieties of knotweeds, all knotweeds introduced to Europe and North America have erect, segmented stems with bamboo-like nodes, as well as light green to white or cream-colored inflorescences made up many small individual flowers. Stems are usually green, often mottled, and may take on a reddish tinge with sun exposure. Branches often take on a zigzagging shape.
From PNWSalmonCenter.org
The Problem Here: Being such hardy and aggressive growers, knotweeds are often considered to be one of the most problematic of invasive plants. While not all introduced varieties of knotweed are capable of spreading by seed, all of them can easily propagate vegetatively via fragments of roots, stems, or leaves. Knotweed plant tissue and seeds often contaminate fill material used in construction projects, facilitating their transportation to new sites. Floods, landslides, and other means of soil disturbance can cause knotweed fragments or seeds to be transported from one site to another. In this way, a single knotweed infestation may quickly spread throughout an entire watershed.
Once allowed to establish, knotweed’s fast and dense growth habits may quickly crowd out other plant species present and form a monoculture. This imperils many sensitive native plant and animal species that inhabit riparian ecosystems, but also poses a structural threat to river banks- as knotweeds go dormant during the winter and die back, the bank’s protection from erosion is lost during the time of year in which waters are highest and fastest. This increased erosion, again, leads to the transport of root fragments downstream and allows for new infestations to form. Additionally, because knotweed is capable of even growing through pavement, manmade structures can also be threatened by an infestation left unchecked.
Due to the immense environmental damage posed by uncontrolled knotweeds, Lincoln SWCD has been approved for a US Forest Service-funded program to provide free chemical knotweed treatment to qualifying public or private properties in Lincoln County. If you believe knotweed is occurring on your property, please contact the District for more information!
Manual Control: Manual control of knotweed can be very difficult. It is important to note that mowing is NOT an effective means of control- fragmenting the plant will only cause the infestation to spread vegetatively and worsen. Manual control is most effective when an infestation is caught early on and limited to a few small plants. Care must be taken to dig up the entire plant and root system. Both the removed plant material and soil should be bagged and thrown away in landfill garbage, not composted. The site of removal and area within at least 30 feet of the site should be monitored for the next few years for signs of re-emergence.
If for any reason knotweed is cut or mowed, care should be taken to ensure that no cut fragments reach a waterway or wet soil. Cut material should be bagged, burned, or allowed to thoroughly dry out.
Herbicides: In most cases, chemical control is the only effective mechanism of controlling knotweed. A single application may sometimes be enough to eliminate an infestation, but oftentimes repeat applications are required. Glyphosate and imazapyr are two effective chemicals that may be used for treatment and can be found in formulations approved for aquatic use. If choosing to conduct herbicide treatment, care must be taken to read and understand the label of your chosen product and follow it fully in regards to methods of use and environments in which it is safe to apply.
Biological/Cultural Control: No known biological control methods currently exist for knotweeds. Cultural control in the form of grazing may be possible, as knotweeds are not toxic and may be palatable for livestock, but rarely eliminate infestations. Due to knotweed’s shade tolerance and tenacious growing ability, shading or covering infestations will not effectively control them.
Links to Information and Studies:
Partner Spotlight
Join the MidCoast Watersheds Council and our partners at the Mid-Coast Water Planning Partnership and the Lincoln Soil and Water Conservation District for an evening of films and conversation on the efforts of these organizations to conserve and enhance land and water for future generations.
A HUGE thank you to Pelican Brewing Company for sponsoring this event!
The Planning Partnership is moving into the implementation of a large water plan. As part of these efforts the Partnership is sharing success stories and engaging with landowners, businesses, communities and agencies to continue the good work of securing water for our future in Lincoln County.
Agenda:
5pm - Doors Open
5:15pm - Greetings and Introductions
5:30pm - First showing of films from the Mid-Coast Water Planning Partnership and Lincoln Soil and Water
6pm - Break for refreshments and conversation
6:15pm - Second showing of films from MidCoast Watersheds Council
7pm - Final remarks and end of event
Upcoming Board Meetings
Our next board meeting will be held on April 9, 2026 via Zoom. These meetings cover the general business of LSWCD, while also convening to decide the next steps the District will take in overall operations or within specific programs.
For registration assistance or special accommodation call the office (541-265-2631) or email Tyler Clouse (info@lincolnswcd.org).
Board of Directors Meetings take place at least once a month, generally on the second Thursday of the month, from 6 to 8 p.m. All meetings are open to the public, with the exception of occasional Executive Sessions.
The Lincoln Soil & Water Conservation District (LSWCD) complies with the American with Disabilities Act (ADA). The ADA prohibits discrimination against persons with disabilities. If you would like to attend a meeting but need special accommodations, please contact us at least 72 hours prior to the meeting.