Flower:

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).

Leaves:

Leaves are simple, toothless, hairless, alternate, broadly ovate with a pointed tip, 3 to 6 inches long, 2 to 4½ inches wide, on a long leaf stem.

A semi-woody perennial, it is fast growing and has hollow, bamboo-like stems that form dense, leafy thickets becoming woody with age.

Shoots arise from coarse, spreading rhizomes that can attain lengths up to 50 feet. 

Fruit:

Female flowers can produce small 3-angled black-brown fruit but seed production is rare

Notes:

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” (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.

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.