Bohemian knotweed | The Morton Arboretum

Bohemian 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 vegetative propagation, when fragments of stems or roots are transported to new areas by wind, water, or sometimes animals. Bohemian knotweed also has rhizomes that grow horizontally underground and allow the plant to spread further, as well as the ability to spread by seed. It can grow well in virtually any area, but prefers disturbed sites or areas near bodies of water (riparian areas). This plant is a hybrid of Reynoutria japonica (Japanese knotweed) and Reynoutria sachalinensis (giant knotweed). Both of the parent plants are found in Asia, but Bohemian knotweed is thought to have hybridized and established in North America. Today it can be found mostly in northern U.S. states.

Bohemian knotweed’s strong roots have the ability to penetrate almost any surface, including pavement. This, coupled with the ability to spread rapidly, allows this plant to quickly overtake natural and cultivated areas. It can shade out nearby plants, suppress their growth (allelopathic properties), and clog waterways, impacting ecological or human health. Bohemian knotweed is thought to have a greater ability to spread by seed than Japanese and giant knotweed.

As a good alternative, red-osier dogwood (Cornus sericea) can be planted instead for its similar appearance, size, and erosion control properties. There are a number of native and non-native species that could be used in place of Bohemian 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 narrowing toward the top of the plant. It can reach heights between 7 and 12 feet tall, with a width of about 5 feet. The plant has spreading branches that contribute to a substantial width.

Stem Description

The stems are typically smooth and hollow, with an appearance and texture similar to bamboo. The stems are light green to dark brown, 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

Bohemian knotweed has simple, alternate leaves along the stem, giving the stem a “zig-zag” appearance. The shape of the leaf varies between the cordate (heart) shape of giant knotweed, and the flat base of Japanese knotweed leaves. Typical of the buckwheat family, the base of the leaf stalk (petiole) has a sheathing membrane (ochrea).

Flower Description

The small white flowers (less than ½ inch) form in a cluster on long stalks, and the plant typically has several of these in a cluster (panicle) at each node. Red coloration can be noticed at the nodes of these groups of flowers. Flowering time is late summer into early fall.

Fruit Description

The small fruit (achene) is typically less than ½ inch in size, and brown or black. The fruit, which is found at the nodes of the stem, has a winged, papery covering.