Japanese knotweed in bloom Japanese, Giant, Himalayan leaves and stalk

Why is it 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, and weaken retention dams and dikes, as well as creating flood hazards.

· Dead winter stalks can create fire hazards, as well as aesthetically displeasing.

Japanese, Giant, Hybrid, Himalayan Knotweed

Other names; Mexican, Japanese or false Bamboo, Fleeceflower

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. Male and female flowers are on separate plants.

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.

Contact Skamania County Noxious Weed Department if you have seen this weed.

Information Resources: www.wa.gov/agr/weedboard

Knotweed project funded by PL 106-396, Title II funds administered by Gifford Pinchot National Forest.