Prostrate knotweed

Common knotgrass is an annual herb with a semi-erect stem that may grow to 10 to 40 cm (4 to 16 in) high. The leaves are hairless and short-stalked. They are longish-elliptical with short stalks and rounded bases; the upper ones are few and are linear and stalkless. The stipules are fused into a stem-enclosing, translucent sheath known as an ochrea that is membranous and silvery. The flowers are regular, green with white or pink margins. Each has five perianth segments, overlapping at the base, five to eight stamens and three fused carpels. The fruit is a dark brown, three-edged nut. The seeds need light to germinate which is why this plant appears in disturbed soil in locations where its seeds may have lain dormant for years.

Annual, seed-propagated weed with spindle-shaped root. Depending on location P. aviculare is extremely various in habit and leaf form.

Prostrate growth, mostly branching from base, small leaves.

Cotyledons: Very long, evenly slender, bluntly pointed.

Stems: Usually prostrate, heavily ramified, leaves at the nodes up to the tip, up to 50 cm (19.5 inch) long.

Leaves: Elliptical or lanceolate to linear, pinnate-nerved, hardly petiolate, emerging from a thin, membranous, stalk-enclosing, incised tube (stipule sheath).