Japanese Knotweed (Polygonum cuspidatum) |
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| Habit: Perennial, herbaceous shrub reaching 3 m (10 ft); larger than many woody shrubs, stems die but stalks persist through winter. Leaves: Simple, alternate, broad, up to 15 cm (6 in) long and 12 cm (5 in) wide with an abruptly pointed tip and a flat base. Stems:Upright, round, hollow, glaucous, often mottled; nodes with a papery membrane; persistent dead stalks resemble bamboo. Flowers: Numerous, small, green-white flowers on a slender stalk in leaf axils and near the ends of stems; bloom August-September. Fruit and seeds: Fruits are three-winged, 8-9 mm, seeds are dark and glossy; wind and water dispersed. Habitat: Semi-shade tolerant; found along roadsides, stream and river banks, wetlands, wet depressions and woodland edges; can tolerate a wide array of soil and moisture conditions. Reproduction: Primarily through rhizomes or fragments; some U.S. populations produce abundant fertile seed; forms fertile hybrids with giant knotweed (P. sachalinense); germination rates vary between populations and with timing of seed harvest. Similar species: Non-native giant knotweed (P. sachalinense) has much larger leaves (> 6 in long) with rounded, heart-shaped bases. Comments: This species is prohibited under Michigan law; forms dense thickets that shade out natives; rhizomes can damage pavement; extremely difficult to eradicate; spread by flood waters. Monitoring and rapid response: Monitor riverbanks, stream and pond edges, particularly downstream from known occurrences. This species is difficult to control—research control options thoroughly. On riparian sites, consider upstream and downstream populations and herbicide impacts. Multiple control strategies may be needed for a single population. Resprouts vigorously after cutting, mowing, tilling and digging. Tiny fragments of roots and stem nodes can sprout and form new colonies—remove all cut plant materials and incinerate or place in landfill. Foliar herbicide application may provide effective control. Cutting or spraying early in the season and then spraying later may be easiest as plants will still be short enough to spray efficiently. Wicking or injecting herbicide may be suitable for ecologically sensitive sites but is labor intensive. Follow-up required for years. |
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Species Summary
| Common Name: | Japanese Knotweed |
| Scientific Name: | Polygonum cuspidatum |
| Family: | Polygonaceae (Buckwheat Family) |
| Duration: | Perennial |
| Habit: | Herb |
| USDA Symbol: | POCU6 |
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Resources
MISIN Factsheets |
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Databases |
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| USDA Plants: | Polygonum cuspidatum |
| NatureServe: | Polygonum cuspidatum |
| Weeds US: | Polygonum cuspidatum |
Images |
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| Bugwood Network: | Polygonum cuspidatum |
| Google Images: | Polygonum cuspidatum |