Aquatic and Wetland Vascular Plants of the Northern Great Plains

15. Polygonaceae, the Smartweed Family

1. Polygonum L. -- Smartweed, knotweed, tear-thumb

10. Polygonum ramosissimum Michx.-- Bushy knotweed

Erect, sparingly to usually freely branched, taprooted annual 2-9 dm tall, the stems strongly ribbed. Leaf blades elliptic to mostly narrowly elliptic or nearly linear, 8-40 mm long, 2-15 mm wide, acute to obtuse, often revolute, cuneate and jointed at the subsessile or short-petioled base; ocreae membranous with 2 acute lobes, soon lacerate and breaking into brownish fibers. Flowers in axillary positions near the tips of branches, 1-few per axil; calyx 5(6)-parted, 3-4.5 mm long, the lobes united for about 1/3 of their length or less, green with yellowish, whitish or slightly pinkish margins, the outer 3 cucullate and enclosing the inner lobes; stamens usually 5, included; style branches 3. Achenes dark brown, dull to shiny, sharply trigonous, mostly 3-3.5 mm long and enclosed by the calyx, those produced later in the season often larger and distended beyond the calyx; pedicels mostly enclosed by the ocreae, 1-3 mm long. Jul--Sep. Shores and exposed flats, especially where alkaline; common; (Newf. to B.C., s to VA, IN, OK and CA). P. prolificum (Small) Robins.

Some other polygonums of the knotweed group are sometimes found in the same habitats as P. ramosissimum, but are more characteristic of disturbed upland habitats. Most typical of this group is P. arenastrum Jord. ex Bor. (formerly included in P. aviculare L.) which differs from P. ramosissimum in its spreading to erect habit; calyx lobes pink-margined, more or less flat and not cucullate at the tips; achene unequally trigonous, with 2 convex sides and 1 narrower concave side. Another is P. erectum, a spreading plant with broadly elliptic leaves; calyx united near the middle and constricted above to appear bottle-shaped.

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