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Japanese knotweed

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 the 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 stems and egg-shaped leaves. Ochrea surrounds the stem at the 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: The 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 leaves
Japanese knotweed has alterate, egg-shaped leaves.  The stem typically has purple spots. Stems reach heights of 6 ft.
Japanese knotweed stem
The ochrea is a key identifying trait, a characteristic of plants in the Polygonaceae family.
japanese knotweed in dense patches
Japanese knotweed is usually found in dense patches due to the rhizomes, this patch is spreading along the bank of Skunk River.

 

Authored by:
Bob Hartzler
Bob Hartzler
  • Retired Weed Scientist and Professor of Agronomy
Dr. Bob Hartzler is a former professor of agronomy and a former extension weed specialist with a Ph.D. in Crop Physiology from Iowa State University. He conducted research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also taught undergraduate classes in weed science and weed identification.
Meaghan Anderson
Meaghan Anderson
  • Field Agronomist in Central Iowa
Meaghan Anderson is a field agronomist serving nine counties in central Iowa and an extension specialist with Iowa State University Extension and Outreach. Areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and integrated pest management.

Meaghan holds a bachelor's degree in agronomy and a master's degree in crop production and physiology (focus: weed science) from Iowa State University.
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