Polygonum amphibium L.
Water Knotweed, Water Smartweed
Polygonaceae (Buckwheat Family)
USDA Symbol: POAM8
Freely rooting, long, prostrate stems growing across mud or in water and topped with erect, dense, narrowly egg-shaped, pink flower clusters.
This aquatic or wetland plant is rather showy when growing in colonies. The genus name is from the Greek poly ("many") and gona ("knee" or "joint"), as is the family name, and refers to the thickened joints of the stem where a sheath often surrounds the stem at the leaf axil. The genus includes the smartweeds, with tiny flowers in terminal spikes, and the knotweeds, with flower clusters in the leaf axils. This species also occurs in Eurasia. Its seeds provide food for waterfowl. Some authorities recognize two intergrading, environmentally variable varieties in the West. The variety stipulaceum has short oval flower clusters and is not especially aggressive; the pink flower masses of this variety are very attractive, but since the plants grow quickly, they can become an unwelcome weed in decorative ponds. The variety emersum has lanceolate leaves with narrowly tapering tips and slender flower clusters at least 1 1/2" (4 cm) long; once called P. coccineum, a separate species, this variety is aggressive to the point of being classified as a noxious weed.
From the Image Gallery
Bloom InformationBloom Color: Yellow
Bloom Time: Jun , Jul , Aug , Sep
DistributionUSA: AK , AR , AZ , CA , CO , CT , DC , DE , IA , ID , IL , IN , KS , KY , LA , MA , MD , ME , MI , MN , MO , MS , MT , NC , ND , NE , NH , NJ , NM , NV , NY , OH , OK , OR , PA , RI , SC , SD , TN , TX , UT , VA , VT , WA , WI , WV , WY
Canada: NB , NS , PE
Native Distribution: Throughout North America, except Georgia, Alabama, Florida, and the Arctic.
Native Habitat: Shorelines, wet prairies, swamps, ponds, and quiet streams, in mud or floating on still fresh water.
Growing ConditionsWater Use: High
Light Requirement: Sun
CaCO3 Tolerance: Medium
Additional resourcesUSDA: Find Polygonum amphibium in USDA Plants
FNA: Find Polygonum amphibium in the Flora of North America (if available)
Google: Search Google for Polygonum amphibium
MetadataRecord Modified: 2010-12-17
Research By: TWC Staff