Japanese knotweed is a non-native, invasive plant from Asia that spreads quickly to form dense thickets that exclude our native vegetation. Displacement of native plants can alter the quality and quantity of wildlife habitat. Japanese knotweed can also alter soil chemistry and nutrient cycling. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands. Once established, populations are extremely persistent. It shades out everything underneath it, preventing forest regeneration, eliminating populations of understory plants, and essentially stopping natural succession.
Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, and drought. It is often found near water sources, such as along streams and rivers, in low-lying areas, waste places, along roadsides, in utility rights-of-way, and around old home sites. This plant can quickly become an invasive weed in natural areas.
Japanese knotweed was probably introduced to the U.S. in the late 1800's. Also known as crimson beauty, Mexican bamboo, Japanese fleece flower, or Reynoutria, it was first used as an ornamental and has also been used for erosion control and for landscape screening. It is now found throughout the eastern U.S., in several western states, and Alaska.
Japanese knotweed spreads primarily by vegetative means through the transport of its long, stout rhizomes or underground stems. The seeds are often introduced to new sites as a contaminant in fill dirt, sometimes distributed by water, and carried to a lesser extent by the wind. Neglected gardens and discarded cuttings provide a source of dispersal from urban areas.
What does Japanese knotweed look like?
Japanese Knotweed is an upright, shrub-like, semi-woody perennial that can grow to over 10 feet in height. The bamboo-like hollow stems of Japanese knotweed are reddish brown, smooth, stout, and swollen at joints where the leaf meets the stem. The base of the stem above each joint is surrounded by a membranous sheath. Although leaf size may vary, they are normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at the tip. The minute greenish-white flowers occur in attractive, branched sprays in late summer through early fall. Small winged fruits appear later and are triangular, shiny, and very small, about 1/10 inch long.
Leaves and flowers
Leaves and winged seeds
How can Japanese knotweed be controlled?
This plant is difficult to control. A variety of manual, mechanical, and chemical methods may be applied. Grubbing is NOT effective except for small initial populations or environmentally sensitive areas where herbicides cannot be used. Juvenile plants may be hand pulled depending on soil conditions and root development. Any portions of the root system left behind after grubbing or hand pulling will allow the plant to re-sprout. All plant parts (including mature fruit) should be bagged and disposed of in a trash dumpster to prevent reestablishment. Foliar applications of chemicals can be effective, but also damaging to non-target species if herbicide is applied so heavily that the spray drips off the leaves. The cut stem approach requires less chemical, but more work. Cut stems must be sprayed quickly after stems are cut for the uptake of the chemical into the plant tissue.
Infestation along an Athens County road
For information on how to identify and control Japanese knotweed or how to better manage your trees, contact the Ohio Department of Natural Resources Division of Forestry at 614-265-6366.