Polygonum arenastrum aka Oval Leaf Knotweed

Polygonum arenastrum belongs to the knotweed family, Polygonaceae. It is an annual, so it lives for one year. Its common name in English is small-leaved knotgrass. It is native to Macaronesia and northwest Africa. Its range also spans the temperate parts of Eurasia. The RHS lists Britain and Ireland as native areas too. It grows by roads and on waste ground throughout Britain. Records also place it in Canada, Mexico, and the United States.

Plants reach about 1 foot [0.3 metres] in height. In rich garden soil, one plant can spread 3.3 feet [1 metre] across. Its spread can thus far exceed its height. Flowers appear from June to October. Each flower has both male and female organs. Insects can carry pollen, but few visit the flowers. The blooms offer little scent or nectar. Flowers set most seed through their own pollen. Seeds ripen from August to October, while flowers still bloom.

This plant grows in many kinds of ordinary garden soil. It accepts sand, loam, and clay. It prefers soil that stays moist. It favours acid soil, even soil that is very acid. It also grows in neutral or slightly alkaline soil. Sun is the clearest choice, though it tolerates light shade. PFAF gives mixed advice on shade and soil richness. It calls for soil with modest levels of plant food. Yet it reports that plants spread more in rich soil. The plant can also cope with coastal exposure. PFAF lists USDA hardiness zones 4–8.

Its main garden fault is its strong chance of becoming a weed. PFAF calls it an invasive weed of land used to grow plants. It makes many seeds, which birds eat. The young of several kinds of butterfly eat the plant too. Rabbits seem to leave it alone. These traits help wildlife, though the plant grows as a weed. Reported uses include making blue, yellow, and green dyes. The roots hold tannins. The evidence names no specific pests or diseases.

Records describe eating young leaves and young plants raw or cooked. Leaves also serve in place of tea. People can eat the small seeds whole or grind them into flour. Their small size makes them hard to gather and prepare. PFAF records their use in folk medicine for wounds and gut problems. These records do not prove that such uses are safe or work in clinical care. Its hazard notes concern related plants, not proven effects in this species. They do not show how toxic this species is.