Polygonum cuspidatum (syn. Fallopia japonica, Polygonum japonicum) Polygonaceae
Japanese Knotweed, Mexican Bamboo po-LI-go-num cus-pi-DA-tum
- Deciduous perennial, tough, vigorous, spreading rapidly from rhizomes to form very large clumps of red-brown jointed, and hollow stems (bamboo-like), 4-8 ft (1.2-2.4 m) tall. Leaves alternate, simple, 5-7.5 cm wide, heart-shaped with rounded lobes at the base, vein often reddish, upper surface dark green, paler green below; petioles ridged and 2.5 cm long. Mostly dioecious -- i.e., male and female flowers mostly on separate plants. Flowers appear in late summer or fall, they are small greenish or cream white and in elongated clusters, 10-13 cm long. Fruit has a three-winged calyx and contains a single seed, which is dark brown, triangular and 2-4 mm long and 2 mm wide.
- Sun to shade. Grows best on moist, well-drained, nutrient rich soil but tolerates adverse conditions. The above ground parts of plants die with the first fall frost, but rhizome buds over winter just below the soil surface and form new shoots in spring.
- Hardy to USDA Zone Native to Japan, China, and parts of Korea and Taiwan. It was introduced to the United States prior to 1890.
- A very invasive plant, a weed. In the 1960s it was listed as "one of the most persistent and aggressive of all perennial weeds". The plant was and is still used as a landscape and garden ornamental. Through this activity it has spread across the United States and occurs from the Northeastern states to California, as well as in Nova Scotia, Newfoundland, Canada. It is a significant weed in Britain, Europe and Russia, and most of Asia also report having Japanese knotweed. The spread of Japanese knotweed has been swift and unstoppable. Not only an issue in the United States, "the eradication of this weed has stumped many a farmer, gardener, and herbicide in many countries." (Doll and Doll, 1998, Univ. Wisconsin).