Impact of Japanese Knotweed on Our Forests

Posted: May 28, 2013

Japanese knotweed is an invasive plant that is causing increased concern in Pennsylvania.
Japanese Knotweed

Japanese Knotweed

Invasive plant species like Japanese knotweed are a considerable threat to biodiversity. Once these species are well established it is sometimes impossible to remove them. When removal is possible, it comes at a high cost financially and ecologically. Researchers at Cornell University estimate that invasive species are costing Americans more than $130 billion every year. Even controlling a single unwanted invader can carry a price tag in the millions.

In some cases, invasive plants are driving our rarest species closer to extinction. According to the U.S. Fish and Wildlife Service, an estimated 42 percent of the nation's endangered and threatened species have declined as a result of encroaching invasive plants and animals. Recent research has shown that some invasive species can cause the populations of even common species to collapse.

According to the USDA Forest Service, 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. It reproduces by seed and large rhizomes, which may reach a length of 40 to 60 feet. A small piece of rhizome can float down a river and begin to grow once it is deposited on land. These buried rhizomes have grown through 2 inches of asphalt!

According to the Ohio Department of Natural Resources, 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” long.

This plant is difficult to control. According to PA DCNR, the key to successful knotweed management is controlling the rhizomes. Mechanical methods alone are largely ineffective. It may be possible to pull single plants if they are not well established and soil conditions allow for complete rhizome removal. Any portions of the root system left behind after grubbing or hand pulling will allow the plant to re-sprout. The herbaceous stems of knotweed can be cut or mowed quite easily. Cutting alone will not control the plant but when performed after June 1 will significantly reduce the height of the regrowth. According to the Penn State Japanese Knotweed Fact Sheet, foliar herbicide applications made after July 1 and before the first killing frost are most effective at injuring the rhizomes. Using both mechanical and chemical controls is one of the most effective forms of knotweed control, particularly for small patches and sensitive areas.

The Cumberland Woodland Owners Association is an organization of private forestland owners and others interested in forestry issues in south central Pennsylvania. The mission of the association is to provide information, education and an exchange of ideas to its members and others about the methods and benefits of proper forest management. For more information about the Cumberland Woodland Owners Association and the meeting, contact Fred Peabody at 717-776-3565 (email:

Article by George Hurd