Japanese knotweed

Encyclopedia Article

Japanese knotweed                       Fallopia japonica (Houtt.)

Family:  Polygonaceae (Smartweed family)
Life cycle:  Perennial, reproducing by rhizomes and seed
Habitat:  Streambanks, roadsides, waste areas
Native status:  Introduced from Asia.

General description:  Erect, aggressive perennial typically found in dense patches due to spread of rhizomes.  Stems reach heights of 6 ft, hollow, resembling bamboo shoots. Leaves are alternate, egg shaped, up to 8 in long.  White flowers develop in elongated (4-6 in) clusters originating from leaf axils.

Key ID traits:  Forms dense patches of erect plants with bamboo-like stem and egg shaped leaves.  Ochrea surrounds stem at leaf axils.  Leaves come off stems in a zig-zag manner.

Similar species:  The initial introductions of J. knotweed were sterile, but it is believed many of the current populations are hybrids between F. japonica and F. sachalinesis.  Cultivars have been developed for landscaping, including compact varieties that have been planted on the ISU campus.

Miscellaneous:  Plant was introduced as a garden ornamental from Japan, but its invasive behavior was quickly noted.  Once it becomes established it is very difficult to control.  Also called Japanese bamboo. 

Japanese knotweed has alterate, egg-shaped leaves.  The stem typically has purple spots. Stems reach heights of 6 ft.

The ochrea is a key identifying trait, a characteristic of plants in the Polygonaceae family.

Japanese knotweed is usually found in dense patches due to the rhizomes, this patch is spreading along the bank of Skunk River.