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Knotweed Complex

(Japanese Knotweed (Polygonum cuspidatum)/Giant Knotweed (P. sachalinense))

Common Names

Mexican bamboo, fleeceflower, Mexican bamboo, huzhang, Sakhalin knotweed

Description

Japanese and Giant knotweeds are members of the buckwheat family and were introduced into Montana as a ornamentals. These knotweeds are shrub-like perennials that can grow over ten feet in height. Young shoots grow from spreading rhizomes that can reach sixty five feet in length,  and shoots are fat and red or red and green speckled. The stems are smooth and swollen at the join where the leaf meets the stem and resemble bamboo. Leaf size is around six inches long by three to four inches wide, they are broadly oval to heart-shaped, and leaves are alternate on the stem. From August to September, the small pale green to white flowers occur in attractive sprays about four inches long. Seeds are triangular, shiny and about 1/10 inch long. As with many invasives, the knotweeds reproduce by seed and by extensive root systems. Japanese and Giant knotweeds easily hybridize. The plant does not tolerate frost and after the first hard frost, rapidly turns brown and dies for the season.

Key Features

Large, broad heart-shaped leaves, bamboo type stems and white-ish green, showy flowers. Also look for a great deal of height from this plant.

Habitat

Knotweeds  tolerate a variety of conditions. They prefer full sunlight, but can tolerate full shade as well. They thrive in the warm weather and riparian areas and wetlands, but will tolerate dry soil and salt as well. Knotweeds can often be found near water sources, such as streams and rivers, floodplains, low-lying areas, and wetlands.  Knotweeds are also found in  abandoned fields, forest edges, disturbed soils, roadsides, right-of-ways, vacant lots, waste places, and around old home sites.

Currently found in the following counties:

Beaverhead, Broadwater, Jefferson, Judith Basin, Lake, Lincoln, Minneral,  Missoula, Ravalli, Sanders,  Silver Bow, Wheatland

IWM

Interesting Facts

Japanese knotweed is edible at the sprout stage and is reputed to taste like mild rhubarb. Bees enjoy knotweed pollen and the honey is very good.

Commonly Confused Plants

  • Other knotweed species

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Photo credits:Photo Credits: Tom Heutte, USDA Forest Service, Bugwood.org; Kellieanne Morris