Japanese Knotweed (Fallopia japonica) is native to eastern Asia and was introduced to the U.S. in the late 1800’s. It was brought to the U.S. as a garden ornamental and as a means for erosion control but has become problematic as an invasive species with its ability to grow vigorously in a wide range of conditions. This invader favors roadsides, streambanks, and ditches in parts of the state. Other common names include Mexican bamboo and fleeceflower.

This multi-stemmed herbaceous perennial grows up to 10 feet tall on bamboo-like hollow stems. The dead stalks remain standing throughout the winter. Spade- or heart-shaped leaves are alternately arranged on the stem and up to 6 inches long by 4 inches wide, narrowing to a point at the tip. In late summer the plant produces small, creamy-white flowers in branched clusters persisting into early autumn. In the spring, Japanese knotweed emerges from the ground as reddish-purple, asparagus-like shoots.

This invasive plant can grow in a variety of soil types and habitats including riparian areas, wetlands, roadsides, and woodland edges. Long rhizomes grow horizontally underground allowing Japanese knotweed to spread quite aggressively. Its extensive dense stands shade out native plants. Japanese knotweed is semi-shade tolerant but flourishes in full sun. Reproducing by rhizomes and seed, it is sometimes transported to new sites as a contaminant in fill dirt.

Once established, Japanese knotweed can be quite a challenge to control. Control efforts must target the underground system of rhizomes (roots). Any remaining rhizomes will potentially resprout. Foliar spray may be considered, but due to the extensive rhizome system, multiple applications may be necessary. Cutting alone is not an effective means of control, however cutting just prior to herbicide application can be effective. For more on management, see Iowa DNR resource. The prevention and early detection are vital for managing Japanese knotweed.

Iowa code, Chapter 317 (317.25) lists Japanese knotweed (Fallopia japonica) as a prohibited plant.

***A person shall not import, sell, offer for sale, or distribute in this state in any form of this plant.