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Pest Information

Pale Smartweed

Pale Smartweed

  • Latin Name: Polygonum Lapathifolium
  • Common Name: Pale Smartweed
  • Other Names: Common knotweed, pale persicaria, smartweed, willow smartweed, willow weed, wireweed

Pest Details

Pale Smartweed
Pale Smartweed


Introduced from Europe.


A summer annual that grows from seed, or may produce shoots and roots at stem joints that lie in the water. Seed production per plant may be heavy, and seeds may remain viable in the soil for up to 6 years. Seeds germinate in early spring, usually the first year. If top portion of plant is cut off it stimulates production of additional flowering stems from the lower parts of the stem. In highly fertile soils stems are thicker and more multi-branched.


Plants often weak stemmed. Upright growing to around 5 feet tall. Stems are smooth, reddish, and swollen at the nodes. Leaves are 4 to 6 inches long, narrow and pointed at the end. Upper leaf surface is smooth and hairless, while the lower surface is whitish and covered with small dots. At the base of the leaf is a sheath around the stem, that separates as the plant matures and eventually breaks off. Flowers are borne in long, drooping clusters, and may be white to pink. Flowers are tiny, but occur in dense clusters several inches long.

Characteristicts Important to Control:

Smartweed prefers disturbed areas and low competition from other plants. Generally requires high moisture, and is most likely to be found in aquatic to semi-aquatic conditions – shorelines, ditches, and irrigated fields. In dry or drought conditions the plants are stunted, seed production declines, and many seeds are sterile.

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