INVASIVE PLANTS OF OHIO
Fact Sheet 10 - Factsheet in .pdf format
Japanese knotweed is a non-native, semi-woody perennial that grows in large clumps reaching heights of 3-10 feet. The stout, hollow stems are reddish brown and the nodes are swollen giving them a bamboo-like appearance.
Typical of the smartweed family, nodes are enclosed by a modified leaf-life structure. Stems die back in the winter and new ones are produced each spring. Leaves are alternate and egg-shaped (4-6 inches long and 3-4 inches wide) narrowing to a point at the tip. The tiny (1/8 inch) flowers are creamy white to greenish white and are borne in plume-like clusters in the upper leaf axils. The species is dioecious, producing male and female flowers on separate plants, however male plants are rare, flowers bloom in August - September and female plants produce triangular, shiny black fruits, however, reproduction from seed is infrequent. This plant spreads primarily by its extensive rhizomes creating dense thickets.
The species occupies a wide variety of habitats in many soil types and a range of moisture conditions. It is most common along roadsides and on streambanks, but is also found in lowlying areas, utility rights-of-way, old home sites and along woodland edges and openings. The species requires a high light environment and grows poorly under full forest canopies.
Japanese knotweed was introduced from Asia as an ornamental in the late 19th century because of its unusual bamboo-like growth habit. It has been used as a landscape screening and occasionally for erosion control. It is widely distributed in the U.S., occurring in much of the Midwest and in several western states. In Ohio this species is primarily found in the eastern part of the state.
Japanese knotweed grows quickly and aggressively by extensive rhizomes and forms dense thickets that exclude native vegetation and reduce wildlife habitat. This species represents a significant threat to riparian areas where it can spread easily as small pieces of rhizome are washed downstream and deposited to create new colonies. Transfer of soil containing rhizome or seed may also cause the establishment of new colonies. Establishment can be prevented with careful monitoring and eradication of small patches when they first develop.
Large colonies of this species are extremely difficult to dig up due to their high rhizome densities. Digging of large colonies is not recommended as it is very labor intensive and unlikely that all below ground material can be removed. Small patches may be dug, however care should be used in removing plant material as improper disposal can spread the species further. Repetitive cutting or mowing within a single growing season to deplete stored reserves and remove photosynthetic tissue has been effective. Eradication of the rhizome system is necessary for control of this aggressive invasive species.
Herbicide has been generally effective at controlling this species. Repetitive cutting of stems with spot application of Roundup®, Accord® or Glypro® to the stumps, and foliar spraying in large populations has been reported to be successful.
There are currently no biological controls available for Japanese knotweed.
ADDITIONAL INFORMATION SOURCES:
Seiger, L.A. 1999. Element Stewardship Abstract for Polygonum cuspidatum. The Nature Conservancy.
Seiger, L.A. and H.C. Merchant. 1997. Mechanical control of Japanese knotweed (Fallopia japonica [Houtt.] Ronse Decraene): effects of cutting regime on rhizomatous reserves. Natural Areas Journal 17(4): 341-345.