Tied in Knots by Knotweed

Contact: K. L. Kyde, Maryland Department of Natural Resources | kerrie.kyde@maryland.gov

Japanese knotweed. Photo: Britt Slattery, US Fish and Wildlife Service, Bugwood.org

ANNAPOLIS, MD (September 12, 2009) – August and September are the months when Japanese knotweed (Fallopia japonica) blooms in Maryland. This Asian perennial, which dies back each year but can grow to more than 10 feet tall in a single season, resembles a shrub, but is really an herbaceous plant. It covers creek, river and pond banks all over the state, and can be seen more and more frequently along Maryland highways. Because the plant is rhizomatous, and spreads primarily through vegetative growth, it is very hard to control. MISC has chosen Japanese knotweed as the September Invader of the Month.

Japanese knotweed was brought to England from Japan as an ornamental in 1825. It was subsequently introduced to the U.S. from the U.K. In the U.S., it is widely distributed in the mid-Atlantic states, and can be found from Nova Scotia to North Carolina. It grows mostly in moist to wet, shaded areas, and does not flourish in heavy shade.

A mature Japanese knotweed plant is made up of several to many stout hollow jointed reddish, longitudinally ridged canes. Without leaves, it can look similar to bamboo, and is often called “Mexican bamboo.” The petioled alternate leaves are generally 4-6 inches long, pale green with an abrupt pointed tip, and squared off at the bottom. The greenish white flowers form in racemes from the upper leaf axils. They are often branched, forming a series of panicles. The plant is dioecious; male and female inflorescences occur on separate plants. Individual male flowers and their racemes generally point up, while individual female flowers usually droop, but their racemes may be oriented any direction. U.S. populations of Japanese knotweed generally do not produce fertile seed due to low or absent male pollen production. Spread is thus primarily through growth of rhizomes, which have been documented as long as 60 feet! Fragments of rhizome that wash downstream from existing sites or are brought into a site in soil can start infestations in new locations.

Japanese knotweed forms dense thickets that shade the soil and block native plants’ access to light. It emerges early in the spring, usually April in Maryland, and uses its substantial reserved nutrients to grow quickly to shrub size. Although its extensive rhizome system can help stabilize soil banks, it does not develop many fine roots that hold soil well in disturbed conditions like flooding or tidal washing.

The best way to prevent Japanese knotweed from colonizing and taking over a site is to monitor target areas carefully and remove entire plants and their roots before rhizome growth enlarges the infestation. In established infestations, where digging out large plants is at best backbreaking and at worst impossible, repeated manual cutting of the canes can eventually exhaust the root system. Cutting back must be done three times in a growing season to achieve good control. Dense shading may also effect some control over either cut or uncut plants – the shading cloth or plastic must be thick and weighted down. Japanese knotweed has been seen to grow through asphalt paving, however, so this method of control is problematic. Application of a low concentration glyphosate herbicide has been effective in controlling Japanese knotweed. In Maryland and Pennsylvania, land managers have achieved the best results when they allowed plants to attain full size after spring emergence, cut them down in June (or “knocked back” the plants with glyphosate), permitted them to flush, and apply a foliar treatment eight weeks after cutting to these smaller, lower plants. Sequential glyphosate treatments in July and then September can also be effective – the first spray treatment serves the same purpose as an after-emergence cutting. Because Japanese knotweed most often grows next to or near water, aquatic-safe formulations of herbicide are required.

For more information about this and other Invasive Species of Concern, visit www.mdinvasives.org

Japanese knotweed clump. Photo: K. L. Kyde