Fallopia japonica var. japonica (Japanese Knotweed)
|Also known as:||Mexican Bamboo, Hancock's Curse|
|Habitat:||part shade, sun; moist, disturbed soil, fields, along roads and railroads, gardens|
|Bloom season:||August - September|
|Plant height:||6 to 9 feet|
|Wetland Indicator Status:||none|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Flowers are in branching spike-like clusters in the upper leaf axils. Individual flowers are 1/8 inch across, white to greenish or pinkish, with 5 petals and 8 stamens. Male and female flowers are separate (dioecious).
Female flowers can produce small 3-angled black-brown fruit but seed production is rare, primarily spreading by rhizomes.
Japanese Knotweed goes by both latin names Polygonum cuspidatum (more common in North America) and Fallopia japonica (more common in Europe), with the latter accepted by “Flora of North America” (FNA, our definitive reference). Native to Japan, Japanese Knotweed was introduced to the UK in 1825 by gardeners as an ornamental. In the late 1800's it was brought to North America and quickly escaped cultivation. It is widespread in the eastern US and is likely now present in all of the lower 48 states. It is scattered across Minnesota and is winter hardy. While a major ecological threat to Minnesota's riparian areas, it is still carelessly moved from place to place by gardeners and is even still available in the garden trade. It is possibly in a garden center near you. FNA notes 4 varieties of F. japonica but there are also cultivars in the garden trade. The species described on this page is var. japonica. Dwarf Japanese Knotweed, var. compacta, is smaller with pink flowers and is less common but has also been widely planted in Minnesota. A hybrid between Japanese and Giant Knotweeds—Bohemian Knotweed (Fallopia × bohemica)—is also present in the state.
If you have Japanese Knotweed on your property: We discourage the movement and use of this plant for landscape purposes as it can quickly get out-of-control. We recommend control and removal wherever this plant is found. Control of this plant is extremely difficult once it becomes established. Digging of even small clumps is labor intensive and safe disposal of the rhizomes is problematic. Intensive mowing of tops or mowing in combination with herbicide applications (such as Arsenal®) can deplete this plant's energy reserves over time. There are no biological controls for Japanese Knotweed at this time.
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- Japanese Knotweed plant
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- Japanese Knotweed, fall color
- Japanese Knotweed, stems in fall color
- Japanese Knotweed on MN state government property in St. Paul
- Japanese Knotweed, Arsenal® treated, Winona MN
- Japanese Knotweed at Whitewater Wildlife Management Area (WMA)
- Japanese Knotweed, Arsenal® treated semi-successfully at Whitewater WMA
- Japanese Knotweed at Mille Lacs Reservation
- Japanese Knotweed, on Northshore private property
- Japanese Knotweed, on private property in Askov, MN
Photos courtesy Peter M. Dziuk, taken at locations across Minnesota
Have you seen this plant in Minnesota, or have any other comments about it?