Marin Knotweed

The origin and taxonomic affinities of Polygonum marinense are uncertain. T. R. Mertens and P. H. Raven (1965) suggested a relationship with P. oxyspermum C. A. Meyer & Bunge or the Mediterranean P. robertii Loiseleur-Deslongchamps. Polygonum marinense may be confused with P. ramosissimum. It can be distinguished by its subsucculent texture, funnelform ocreae, leaves rounded at the apices, and semi-open flowers. Marin knotweed is known from fewer than 15 locations in Marin, Napa, Solano, and Sonoma counties; it is threatened by coastal development.

Plants often reddish tinged, heterophyllous, subsucculent. Stems prostrate to ascending, branching from base, not wiry, 15-40 cm. Leaves: ocrea 4-6 mm, proximal part funnelform, distal part silvery hyaline, soon disintegrating, leaving almost no fibrous remains; petiole 2-5 mm; blade often reddish tinged, elliptic to obovate or oblanceolate; 20-35 × 9-16 mm, margins flat, apex rounded; stem leaves (1.3-)2-2.6(-3.5) times as long as branch leaves; distal leaves overtopping flowers in distal part of inflorescence. Inflorescences axillary; cymes in most leaf axils, 1-4-flowered. Pedicels mostly exserted from ocreae, 2-4 mm. Flowers semi-open; perianth 3-3.5(-4) mm; tube 18-25% of perianth length; tepals overlapping, green, margins white or pink, petaloid, not keeled, broadly rounded, cucullate; midveins unbranched; stamens 8. Achenes exserted from perianth, brown, ovate, 3-gonous, 2.8-3.4(-4) mm, faces subequal or evidently unequal, apex not beaked, edges straight, shiny, minutely roughened; late-season achenes uncommon, 4.5-5 mm. 2n = 60.

Calif.

Flowering Apr-Oct.

Coastal salt and brackish marshes, swamps; of conservation concern; 0-10m.

Polygonum marinense is a rare North American species of flowering plants in the buckwheat family known by the common name Marin knotweed. It is endemic to California, where it is known from just a few locations north and east of San Francisco Bay.

The taxonomy of the plant is uncertain. It has been suggested that the species may be native to the Mediterranean, and that the California specimens may actually be introduced. If, however, it is a true Bay Area endemic, the plant is rare and threatened by habitat destruction and disturbance. It is a resident of salt marsh and other wet coastal habitat.

Polygonum marinense is an annual herb producing a ribbed, reddish stem growing prostrate or erect to a maximum height near 40 centimeters (16 inches). The narrow oval or lance-shaped leaves are alternately arranged along the slender stem. Each reddish leaf has a funnel-shaped stipule that wraps around the leaf base to form an ochrea. Flowers occur in the leaf axils. They are greenish with white or pink-tinged edges.