Japanese knotweed | The Morton Arboretum

Japanese knotweed is an exotic weed in Illinois. The Illinois Exotic Weed Act prohibits the sale or distribution of this plant in Illinois. This plant mainly spreads by fragments (i.e. stems or roots) being transported to new areas by wind, water, or sometimes animals; this is called vegetative reproduction. Japanese knotweed also has rhizomes that grow horizontally underground and allow the plant to spread further. It commonly grows in wet areas near bodies of water (known as “riparian zones”) but thrives in disturbed areas as well. The plant was introduced from Asia (Japan, China, Korea, and Taiwan) due to its ornamental appeal and ability to withstand erosion. The plant rapidly spread to other areas and today can be found throughout the United States (only absent in the Southwest). The plant is also invasive in other parts of North America, Europe, and Australia.

Japanese knotweed’s dense, thick roots have the ability to penetrate almost any surface, including pavement and waterways. The plant’s ability to spread rapidly allows it to quickly overtake natural and cultivated areas. This plant is also known to change the nutrient composition of soil and make environments favorable to its own growth, while simultaneously making the area more uninhabitable for other plants (allelopathy). The herbaceous plant species Culver’s root (Veronicastrum virginicum) can be a good alternative to Japanese knotweed for its similar blooms.There are a number of native and non-native species that could be used in place of Japanese knotweed. More desirable species may be found using the online Search Trees and Plants page of the Arboretum website. Before purchasing or planting, be sure to check for any local or state guidelines on this species, and ensure that this plant is suitable for its habitat by checking its attributes at mortonarb.org or plants.usda.gov.

More Information

Size and Method of Spreading

The round stems are about 1 inch in diameter at full maturity, with the stems becoming thinner toward the top of the plant. It can reach heights between 3 and 15 feet tall, but typically grows to be 10 feet or shorter, with a width of about 5 feet.

Stem Description

The stems are typically smooth and hollow, with an appearance and texture similar to bamboo. The stems are light green to red, varying with age. It has red coloration that is most prominent at the nodes, but can also have a spotted pattern on the entirety of the stem.

Leaf Description

Japanese knotweed has alternate, simple, oval-shaped (ovate) leaves. The arrangement of the leaves gives the stem a “zig-zag” appearance. The veins of the leaf are pinnate. The leaves are green, with a brighter green color apparent on the underside. Typical of the buckwheat family, the base of the leaf stalk (petiole) has a sheathing membrane (ochrea). The leaves of male plants are typically bigger than their female counterparts.

Flower Description

The small white flowers (⅛ to ¼ inch) form in a cluster on long stalks, and the plant typically has several of these in a cluster (panicle) at each node. In female plants, the racemes tend to sag, whereas the flowers of a male plant are positioned upright. Flowering time is late summer into early fall.