Japanese Knotweed

Information

Polygonum cuspidatum - Polygonaceae Family - Perennial

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Identification

Flowers: Flowers are tiny, greenish-white in color with 5 petals. They grow in large bundles of 8-1- that resemble stems.
Seeds: Fruits are winged on three sides; seeds are triangular, dark brown, shiny and about 0.1 inches long.
Leaves: Leaves are variable; about 6 in. long by 3-4 in. wide, broadly oval to somewhat triangular with a truncated base and a tapered tip.
Flowering Time: Flowers in late summer into the fall.
Life cycle: Japanese Knotweed is a perennial that completes the majority of its growth from early summer through late fall. It flowers in late summer, producing seed shortly after. It dies back at the first winter frost, only re sprouting the following spring.

Impacts

Japanese Knotweed infestations in riparian areas increase flood risk by impeding water flow and creating an excess of debris in stream and river channels.
Knotweed grows extremely quickly and easily shades out many native species, reducing floral biodiversity.
Many native aquatic insects and amphibians only reluctantly feed on Japanese Knotweed, resulting in a decrease in their populations in infested areas.

Most effective control methods

Local infestations can be controlled through manual pulling, although care must be taken to remove the entire rhizomatous root system. Continual mowing can also be effective in controlling population size.
There is currently no biological control agent approved for use against Japanese Knotweed in the US, although several insect species are being researched.
Chemical control can be effective, but proper treatment depends on location and stage of growth. Check with your local weeds supervisor for questions about your infestation.

Japanese knotweed: flowers and foliage

Japanese knotweed: infestation

Japanese knotweed: foliage

Japanese knotweed: seeds

Japanese knotweed: stem

Japanese knotweed: infestation

Japanese knotweed: flowers

Japanese knotweed: seedlings