Description

Japanese knotweed Polygonum cuspidatum Sieb. & Zucc. is a member of the buckwheat family (Polygonaceae) and is referred to by several common names including Japanese fleeceflower, Japanese bamboo, Mexican bamboo, crimson beauty, or Reynoutria. It has several scientific name synonyms including Fallopia japonica, Pleuropterus cuspidatus, Pleuropterus zuccarinii, Polygonum cuspidatum var. compactum, Polygonum zuccarinii, and Reynoutria japonica. Japanese knotweed is a herbaceous perennial plant with an upright, shrub-like habit (Figure 1).

Figure 1.

It has a rhizomatous root system and can spread by stolons. Japanese knotweed can reach 10 feet in height on moist, fertile sites, although it normally attains a mature stature of 3 to 6 feet in Montana. A distinguishing identification feature is the membranous sheath that surrounds the joint at the base of each stem. The hollow stems are smooth, stout, and swollen where each leaf is attached. Stems have a characteristic reddish-brown color that is obvious after leaf drop in the fall (Figure 2).

Figure 2.

In Montana, the leaves are normally 4 to 5 inches long and 2 to 3 inches wide (Figure 3).

Figure 3.

Leaf shape is ovate to somewhat triangular with a pointed tip. Japanese knotweed is dioecious, with male and female flowers on separate plants. The inconspicuous greenish white to white panicles appear in late summer in south central Montana (Figure 4).

Figure 4.

Fruit set has not been observed on plants at five sites in Carbon and Yellowstone Counties, Montana, presumably because only one sex of plant is present, or because conditions are unfavorable for fruit set and/or maturation. Fruit, when present, are 1/8 inch, brown, shiny, triangular achenes.

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Last Modified: 11/14/2007