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Invasive Species Sheet - Japanese Knotweed

Invasive Species Identification Sheet

Japanese Knotweed (Polygonum cuspidatum Sieb. & Zucc.)

Alternate Latin names: Fallopia japonica (Houtt.) Ronse Decraene, Reynoutria japonica Houtt., Pleuropterus zuccarinii Small

Alternate common names: Japanese or Mexican Bamboo, Japanese Fleece-flower; horticultural varieties include
“Crimson Beauty,” “Variegated” and “compactum”
-perennial, with vigorous, widely-spreading, bushy form; 6'-13' tall; dies back each year
-stems stout but hollow; ringed with a tubular sheath (ocrea) at each leaf base with papery remnants encircling the stem even after lower
 leaves are shed
-leaves alternate, large (up to 6"-9" long); wide at base, tapering abruptly to pointed tip
-lacey-white flower sprays emerge in late summer among leaves on upward side of arched stems
-papery-winged fruits are 3-angled and less than 1/2" long (4 mm –10 mm)
-frost-killed stems turn bronze colored; and may remain upright through winter

Japanese Knotweed and its close relative, the non-native Giant Knotweed (Polygonum sachalinense F.W. Schmidt ex Maxim.), grow from stout underground stems (rhizomes) that spread aggressively and send many shoots up causing the plants to form bamboo-like thickets. Japanese and Giant Knotweed leaves vary in size and shape. Japanese Knotweed leaves tend to be egg-shaped whereas Giant Knotweed leaves tend to be longer and can be very large (more than 13" long). In both species, the smaller leaves have squarish bases while the larger leaves tend toward heart-shaped bases (more so in Giant Knotweed). The upper surface of Japanese Knotweed has an extremely fine-sandpaper feel in contrast to the fine-leather feel of Giant Knotweed.

Japanese Knotweed is spread by seeds and by root fragments that are transported in fill dirt or by moving water. It sprouts out from below ground in early Spring and grows rapidly and profusely on roadsides, streambanks, wetland margins, floodplains, and gravel bars.