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Pest Information

Swamp Smartweed

Swamp Smartweed

  • Latin Name: Polygonum Amphibium
  • Common Name: Swamp Smartweed
  • Other Names: Marsh smartweed, swamp knotweed, swamp persicaria, tanweed, kelp, devil’s shoestring

Pest Details

Swamp Smartweed
Swamp Smartweed


Native to Eurasia, and now found throughout much of North America.


Perennial root system, while the foliage dies back in the winter in temperate climates. Plants can grow from either seeds or from the roots. Root system can be extensive and tough, with roots up to ½ inch in diameter


Plants vary considerably in appearance, depending on the moisture available to them. On drier soils they may get to 3 feet tall and be weak and somewhat prostrate. In rich, damp soils they may grow to 6 feet tall. Stems are rough, grooved, and swollen above the nodes. There is the thin sheath around the stem at the leave base, characteristic of smartweed species and other members of the Buckwheat family. Leaves on plants growing in water are smooth, wide and floating, while those on terrestrial plants are smaller and covered with short, fine hairs. Flowers are bright pink and borne in dense clusters as spikes at the ends of their stems. Flower stems may be up to 4 inches long, red, hairy, and often in pairs.

Characteristicts Important to Control:

Most important as a weed pest in crops or other areas that have heavy, moist, overwatered, and poorly drained soil. It grows easily in water filled drainage ditches or irrigation ditches, where it will obstruct water flow.

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