Pink-head knotweed

Polygonum capitatum Buchanan-Hamilton ex D. Don, Prodr. Fl. Nepal., 73. 1825

Plants annual or perennial, 0.5-5 dm; roots also often arising from proximal nodes. Stems prostrate, glabrous or glandular-pubescent. Leaves: ocrea brown or reddish brown, cylindric to funnelform, 5-12 mm, chartaceous, base inflated or not, margins oblique, eciliate or ciliate with bristles to 1.5 mm, surface lanate, sometimes also glandular-pubescent; petiole 2-5 mm, winged distally; blade ovate to elliptic, 1.5-4(-6) × 0.6-2.5(-3.3) cm, base cuneate or tapering, margins ciliate with reddish, multicellular hairs, apex acute, faces glandular-pubescent abaxially and adaxially, not glandular-punctate. Inflorescences terminal, 5-20 × 7-18 mm; peduncle 10-40 mm, glabrous or stipitate-glandular in distal 5; ocreolae overlapping, margins eciliate. Pedicels spreading, 0.5-1 mm. Flowers 1-5 per ocreate fascicle; perianth greenish white proximally, pinkish distally, urceolate, glabrous, nonaccrescent; tepals 5, elliptic, 2-3 mm, apex acute to obtuse; stamens 8, filaments distinct, free; anthers pink to red, elliptic; styles 3, connate to middle or distally. Achenes included, reddish brown to brownish black, 3-gonous, 1.5-2.2 × 1-1.5 mm, shiny, smooth or minutely punctate.

Flowering Jun-Sep. Disturbed, urban places; 0-500 m; introduced; Calif., La., Oreg.; Asia (Bhutan, w China, n India, Nepal); introduced also in the Pacific Islands (Hawaii).

Persicaria capitata is planted as a garden groundcover. It escapes infrequently in the flora area; once established outside of cultivation it can be difficult to eradicate.