Oddsepal Knotweed

Herbs, compact, often cushion-like. Stems erect, green or reddish, simple or branched near base, not wiry, 1.5-5 cm, glabrous. Leaves uniformly distributed, dense, not articulated to ocreae, basal leaves persistent or caducous, distal leaves gradually reduced to bracts; ocrea 3-6 mm, glabrous, proximal part cylindric, distal part disintegrating almost to base, with whitish, straight, rigid fibers; petiole absent; blade 3-veined, without pleats, linear to lanceolate, 10-20 × 0.6-2.7 mm, margins revolute, smooth, apex spine-tipped. Inflorescences axillary; cymes in most axils, 2-3-flowered. Pedicels enclosed in ocreae, erect, 0.1-1 mm. Flowers closed; perianth 2.3-2.7 mm; tube 3-7% of perianth length; tepals overlapping, whitish with whitish or pink margins, petaloid, oblong, navicular, dimorphic, outer 2 shorter than inner 3, outer 2 0.8-1.2 mm, inner 2.3-2.7 mm, papillose at base, apex acute or acuminate; midveins unbranched; stamens 5-6. Achenes enclosed in perianth, olive brown, narrowly ovate to ovate-lanceolate, 1.5-2 mm, faces subequal, shiny, smooth.

Idaho, Nev., Oreg.

Flowering Jun-Aug.

Dry waste ground, open flats in sagebrush plains, ponderosa pine forests; of conservation concern; 1000-1500m.

Polygonum heterosepalum, common name dwarf desert knotweed or oddsepal knotweed, is a plant species native to the Great Basin Desert in southwestern Idaho, northern Nevada, northeastern California, and southwestern Oregon. It has been reported from 1 county in California (Modoc), 4 in Nevada (Washoe, Humboldt, Elko and Lander), 4 in Idaho (Owyhee, Twin Falls, Elmore and Gooding), and 5 in Oregon (Lake, Malheur, Harney, Grant and Crook). The species occurs in dry, open sites in sagebrush plains and pine woodlands.

Polygonum heterosepalum is a short herb up to 5 centimetres (2.0 in) tall, forming mats that seem rather moss-like. Stems are green or red. Leaves are densely packed, narrow up to 3 millimetres (0.12 in) long. Flowers are white, sometimes pink along margins, borne in groups of 2–3 in the axils of the leaves, slightly zygomorphic with some tepals longer than the others.