Reynoutria japonica (Japanese Knotweed)
|Also known as:
|Mexican Bamboo, Hancock's Curse
|part shade, sun; moist, disturbed soil, fields, along roads and railroads, gardens
|August - September
|4 to 8 feet
|Wetland Indicator Status:
|GP: FACU MW: FACU NCNE: FACU
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
Pick an image for a larger view. See the glossary for icon descriptions.
Branching spike-like or raceme-like clusters arising from leaf axils and at the tips of branching stems, with 3 to 6 flowers at each node. Flowers are 1/8 inch across with 5 white to greenish or pinkish petals. In the center is a bullet-shaped green ovary topped with a feathery 3-parted style and surrounded by 8 very short, white-tipped stamens, though the stamens are often sterile. Cupping the flower is a pale green to whitish calyx, tapering at the base to a slender stalk.
Leaves are alternate, 4 to 6 inches (to 15 cm) long, 2 to 4½ inches wide, toothless, broadly egg-shaped to nearly round, abruptly narrowed to a pointed tip, straight across to broadly wedge-shaped or occasionally rounded at the base, on a stalk shorter than the blade. Surfaces are hairless; early leaves have minute, blunt hairs along major veins on the underside that can give it a rough texture, though these hairs do not persist.
Stems are hairless, hollow and bamboo-like, light green to blue-green often with red flecks or streaks, and usually clustered. Old stems may persist to the next season before disintegrating. Plants often form dense thickets from long, woody rhizomes.
Japanese Knotweed, also known by synonyms Fallopia japonica and Polygonum cuspidatum, is native to eastern Asia. It was introduced to the UK in 1825 by gardeners as an ornamental, brought to North America in the late 1800s, quickly escaped cultivation, and is now considered one of the top invasive species world-wide. Its tough rhizomes can break through concrete and it is incredibly difficult to eradicate, since root fragments can resprout. It is widespread in the eastern US and is likely now present in all of the lower 48 states. In Minnesota, it was available in the garden trade until 2014, when it was (finally!) designated a Prohibited Control Noxious weed.
Two other related species are also present in Minnesota: Giant Knotweed (Reynoutrias sachalinensis) and their offspring, the hybrid Bohemian Knotweed (Reynoutria × bohemica). Giant Knotweed is uncommon; there are currently no herbarium records for it and very few Minnesota records of it in other databases I've checked; it has quite large leaves (6 to 12+ inches long) with heart-shaped bases and young leaves have twisted, multicellular hairs along the veins. Bohemian Knotweed is more common, possibly more common than either parent, and is intermediate between the two, though it and Japanese Knotweed are frequently mistaken for each other.
There are hundreds of records on iNaturalist and EDDMapS for both Japanese and Bohemian Knotweeds, though the true abundance and distribution of either is murky and the images posted on those sites don't always show characteristics that could result in a positive ID. Thus the county distribution maps are incomplete to say the least. Multiple references note, in early growth, a difference in the hairs on new leaves, but we don't have the evidence to back up that claim. When flowers are present it is much easier: Japanese Knotweed flowers have very short stamens and Bohemian has comparatively long stamens. Late season plants lacking flowers should be distinguished by the leaves: the largest leaves of Japanese Knotweed don't exceed 6 inches (15 cm) long and leaf bases are mostly straight across to broadly wedge-shaped, where Bohemian leaves are distinctly longer than 6 inches and leaf bases are straight across to somewhat heart-shaped. But the bottom line is: both are invasive and should be eradicated.
If you have an invasive Knotweed on your property: the Minnesota DNR has more information, including control measures, that may be helpful. The best alternative for your landscape? The native Spikenard, Aralia racemosa!
About the genus Reynoutria: According to the Centre for Agriculture and Bioscience International (CABI), "Fallopia japonica was independently classified as Reynoutria japonica by Houttuyn in 1777 and as Polygonum cuspidatum by Siebold in 1846. It was not until the early part of the twentieth century that these were discovered to be the same plant (Bailey, 1990), which is generally referred to as Polygonum cuspidatum by Japanese and American authors but, following Meissner's 1856 classification, as Fallopia japonica in Europe (Bailey, 1990). Galasso et al. (2009) proposed transfer back to Reynoutria based on rbcL plastidial sequence analysis and Reynoutria japonica Houtt. is cited as the preferred name in The Plant List (2013)." The trend in databases such as iNaturalist and EDDMapS has been to revert to Reynoutria, and we have followed suit.
Please visit our sponsors
Native Plant Nurseries, Restoration and Landscaping Services ↓
- Japanese Knotweed plant
- Japanese Knotweed forming a thicket
- fall color
- leaves are straight across or broadly wedge-shaped at the base
- fruiting branch
- Japanese Knotweed on MN state government property in St. Paul
- Japanese Knotweed in a residential landscape
- comparison of Japanese and Bohemian Knotweed leaves
- comparison of Japanese and Bohemian Knotweed flowers
Photos by K. Chayka taken in Ramsey and Washington counties. Photos by Peter M. Dziuk taken at various locations in Minnesota. Reynoutria japonica flowers by Paul Rothrock used under CC BY-NC 4.0 (cropped from original photo).
Have you seen this plant in Minnesota, or have any other comments about it?