Polygonum sagittatum L.
Family - Polygonaceae
Stems - To +/-1m Long, erect to clambering, simple, 4-angled, hollow, becoming pinkish-red in full sun, with retrorse prickles on angles, from fibrous roots. Nodes sometimes slightly swollen.
Leaves - Alternate, petiolate. Ocrea one-sided, sheathing the stem on opposite side from leaf, to 7-8mm long, glabrous. Petiole connected at base of ocrea, to +/-1.2cm long, with retrorse barbs. Blades sagittate, +/-5cm long, +/-1.5cm broad, entire, with slightly revolute margins (the margins antrorse ciliate), glabrous above and below except for retrorse prickles on midrib below. Auricles acute, 7-8mm long.
Leaf and prickly stem.
Inflorescence - Terminal and axillary capitate cluster of 1-10 flowers. Peduncles glabrous. Bracts subtending flowers with scarious margins. Pedicels to 2mm long.
Flowers - Perianth segments white to pinkish, glabrous, 3mm long. Stamens +/-8, erect. Filaments white, to 1.4mm long, some adnate to perianth segments. Anthers white, .2mm long. Achenes 3-sided, glabrous, chocolate to black, to 2.5mm long.
Flowering - June - September.
Habitat - Wet places, roadsides, railroads.
Origin - Native to U.S.
Other info. - This is a simple species to ID in the field because of its square, prickly stems and its sagittate leaves. The plant likes moist areas such as fens or pond margins. The flowers are small and not showy but are not needed to help ID the plant. This species is found nearly throughout Missouri but has not been recorded in some Ozark counties. Our plants belong to variety sagittatum. Another variety, var. gacilenta Fern., has few to no prickles on the stem. This latter variety is not found in Missouri but in a few states to our east.
Photographs taken at the Peck Ranch Wildlife Refuge, Carter County, MO., 7-7-01.