Polygonum aviculare aka Knotgrass

Polygonum aviculare is known as knotgrass or prostrate knotweed. It belongs to the buckwheat family, Polygonaceae. Kew places its native range from southern Europe to Siberia and the Caucasus. The RHS also lists it as native to Britain and Ireland. It now grows across temperate parts of the world. It occurs in fields, roadsides, waste ground, and pavement cracks. It is mainly a summer annual, though NC State also records perennial plants.

Plants form low mats of thin, tough, branching stems. Some stems rise rather than lie flat. Garden plants reach about 0.25–0.67 feet [0.08–0.20 metres] tall. Their spread is about 2–3 feet [0.61–0.91 metres]. Richer soil can support mats 3.3 feet [1 metre] across. The green stems are smooth and have clear joints. Leaves grow in an alternate pattern along them. They are green to blue-green, with smooth, unbroken edges. A thin sheath, called an ochrea, surrounds each leaf base. A shallow taproot often branches beneath the plant.

Tiny flowers grow in groups at the leaf bases. They appear from midsummer into early fall. Each has five sepals but lacks petals. The sepals may be green, pink, or purple, with white edges. Flowers open briefly during hot, sunny spells. They offer little scent or nectar and attract few insects. Plants usually fertilize their own flowers. Each flower can form a brown, three-angled achene. These small fruits taper toward one end.

Full sun suits this plant, though it also tolerates partial shade. It grows in ordinary garden soil and copes with poor ground. It tolerates clay, sand, and shallow, stony soils. It accepts acid, neutral, and alkaline conditions. Moisture-retentive soil suits it, but established plants withstand dry ground. It also tolerates compacted soil and foot traffic. NC State lists USDA zones 2–10. These traits help it persist in lawns and crop fields. Mowing gives poor control because the stems stay low. One plant can shed thousands of seeds. Those seeds can remain viable for several years.

Its main landscape value lies in feeding wildlife. Caterpillars eat the plant, and birds feed on its seeds. Young leaves and plants have recorded raw and cooked food uses. The leaves also serve as a tea substitute. People eat the seeds whole or grind them into flour. Their small size makes harvest awkward. The plant has traditional medicinal uses and yields several dye colours. Its tendency to seed freely makes it a troublesome garden weed.