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.
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