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Why is Knotweed a threat?
- Knotweed is invasive, fast growing, and aggressive! Manual and chemical controls are difficult.
- Knotweed spreads by rhizomes, stem or root fragments, and seed. Nodes of the cane may sprout roots when in contact with water. A 1-inch root fragment can produce a new plant!
- Knotweed is allelopathic; exudes toxins through root and rhizomes to inhibit germination and growth of other plants, native and ornamental.
- Knotweed creates a dense canopy, excluding the establishment of tree seedlings along riverbanks, which contribute woody debris to the river system and are important to fish habitat and survival.
- Knotweed creates monocultures, excluding native vegetation, thus creating poor habitat for animal species and insects, and causing a break in the food chain.
- Knotweed may fill and choke small streams, tributaries, and channels used by salmon and other fish.
- Knotweed can negatively impact transportation right-of-ways, damage pavement and concrete, as well as creating flood hazards. Knotweed Facts - What is it?
- Dead winter stalks can create fire hazards, as well as aesthetically displeasing.
What kinds of knotweed are there?
Japanese Knotweed (Polygonum cuspidatum) - Herbaceous perennial plant from long creeping rhizomes, which can be 60 feet long, and develop into dense mats. Shoots can generate from the rhizomes and from the roots when near the soil surface or when buried up to 6 feet. Stems are stout, hollow, reddish-brown, 4 to 9 feet tall, semi-woody but die back at the end of the growing season. The plant has a bamboo like resemblance. The nodes are slightly swollen and surrounded by thin papery sheaths. Leaves are alternate in arrangement, usually ovate, narrowing to a point. The flowers are greenish white to cream, borne in large plume-like clusters at ends of stems and in leaf axils.
Giant Knotweed (Polygonum sachalinense) - Very similar to Japanese in appearance, the main difference is leaf and plant size. It can grow to over 12 feet tall. Leaves are heart-shaped, can exceed one foot long and are twice the size of Japanese. The flowers of Japanese increase in size with maturity, but the giant does not. Both knotweed species were introduced from Japan as ornamental plants, are equally invasive and difficult to control.
Hybrid Knotweed - Japanese and giant knotweed are able to hybridize with one another. Often times with characteristics of both species on the same plant.
Himalayan Knotweed (Polygonum polystachyum) - Himalayan can grow up to six feet tall, with red stems and leaf stalks. The leaves are oblong, lance-shaped, with brown persistent sheaths at the base of the leaf stalks. The flowers are white to pink, and occur in loose, branched clusters.
How Can I Help?
When weed infestations are identified, it is important to work with your neighbors to control the problem. The City is sponsoring an Adopt-a-Patch program. Knotweed infestations have been identified in the following locations: S-Curves, Elk Creek Road, Siuslaw Street, Tanana Avenue, Spruce Street, 2nd Street, Les Shirley Park, as well as on private property. If you would like to participate in the Adopt-A-Patch program, please call City Hall. We will supply disposal bags and arrange for pickup of the bagged knotweed. The City is also available to identify Knotweed and answer various questions.
How do I control it?
Control of knotweed relies on the death of the extensive rhizome system, which usually takes a number of years. Cutting or pulling. Regular cutting or pulling will, after a number of years, eventually exhaust the rhizome and kill the plant. It is important that all cut or pulled stems of knotweed are disposed of in the landfill or burned.
Knotweed Not’s
- Do not flail or mow Knotweed as this will cause it to spread. Cutting with sharp hooks, slashers etc is recommended.
- Do not spread Knotweed stem and crowns. If you cut down Knotweed, it is best disposed of on site (burned or placed in a garbage bag for disposal).
- Do not spread soil contaminated with Knotweed rhizome. Any soil that is obtained from ground within 7 m of a Knotweed plant could contain rhizome. The rhizome is highly regenerative and will readily grow into new plants.
- Do not chip Knotweed material. Mechanical chippers do not kill Knotweed. If you spread the chipped material on soil, Knotweed could re grow.
- Do not add Knotweed to compost.
- Do not waste time. If Knotweed appears in your area, treat it immediately. Do not allow it to become established.
Please help to prevent the spread of Knotweed by following these guidelines. If you intend to use a herbicide in or near water, you need to obtain the approval of the Environment Agency prior to treatment.
Follow the manufacturer's instructions regarding protective clothing and the safe and effective use of herbicides. When applying herbicides, take care to avoid drift, and any damage to non-target plants.