Aquatic and Wetland Vascular Plants of the Northern Great Plains
15. Polygonaceae, the Smartweed Family
1. Polygonum L. -- Smartweed, knotweed, tear-thumb3. Polygonum coccineum Muhl. ex Willd.-- Marsh smartweed
Rhizomatous, semiaquatic or terrestrial perennial, somewhat like P. amphibium but the stems rigid, stout and freely branched below when submersed, often with greatly inflated nodes, 3-12 dm long, the secondary branches erect and emergent, not producing floating leaves; in drier situations, the stems procumbent with erect branches or the stems erect throughout, simple or sparingly branched from the base, to 1 m tall. Leaf blades elliptic-lanceolate to lanceolate, 8-20(23) cm long, 2-8 cm wide, glabrous to finely strigose, acute to long-acuminate, semicordate to cuneate at the base; petioles mostly 1-8 cm long; ocreae chartaceous or membranous, 1.5-4 cm long, truncate, strigose or glabrous. Racemes 1-2(3), cylindric, (2.5)4-12 cm long, 0.5-1.5 cm thick; peduncles 1.5-6 cm long, glandular-pubescent. Flowers deeply rose pink; calyx 5-lobed to below the middle, 4-6 mm long; stamens 5, included or exserted; style branches 2. Achenes dark brown to nearly black, lenticular, 2.3-3 mm long. Late Jun--Sep. Marshes, meadows, ditches, streams, ponds and lakes, where water is fresh to brackish; often weedy, especially in low areas previously disturbed by cultivation; very common, often abundant; (Que. and N.S. to B.C., s to NC, TN, TX and CA).
Some authors merge P. coccineum with P. amphibium, considering the former a variety (var. emersum Muhl.) of the latter. At least in our region, the two entities are distinctly different regardless of the depth of water they are growing in. It is common to find the emergent and emersed forms of P. coccineum growing with the floating-leaved and emersed forms of P. amphibium, with no evidence of intergradation between them. P. coccineum is a facultative wetland plant, often spreading into upland habitats adjacent to wetlands. P. amphibium as defined here is an obligate wetland plant.
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