- Name also: Red Sorrel, Sour Weed, Field Sorrel, Common Sheep Sorrel
- Family: Dock Family – Polygonaceae
- Growing form: Perennial herb.
- Height: 10–40 cm (4–16 in.) Stem reddish, ridged, branched from inflorescences on.
- Flower: Regular, small, unisexual. Male flower: Perianth-segments 6, with membranous margins, often red, under 2 mm long. Stamens 6. Female flower: Perianth consists of 2 whorls of 3. Segments of outer whorl small, green, and erect. Inner whorl forms the fruit-enclosing valves whose segments are elliptic and thin, greenish or red, with entire margins, and lacking pimples. Valves and fruit of equal length. Pistil formed from 3 fused carpels, styles 3. Inflorescence a raceme.
- Leaves: Hairless. Stalk of basal leaves usu. longer than blade. Blade narrowly elliptic to linear, and usu. hastate. Basal lobes lanceolate to linear. Stem leaves alternate.
- Fruit: Brown, roundish, three-edged, glossy achene, ca. 1 mm across.
- Habitat: Dry fields, slopes, roadsides, sandy areas, and forest clearings.
- Flowering time: June–August.
Sheep’s sorrel belongs to an intrageneric section in which the unisexual flowers are on different plants (the species are dioecious) and the fruit valves are inconspicuous. This species thrives on dry and acid soils. Indeed, it has been noted to avoid calcareous sites. It is not capable of dissolving phosphates from lime-rich minerals.
Sheep’s sorrel is a troublesome weed, because it can grow a tap root down to a depth of half a metre (almost two feet) and the superficial root system can extend a metre away from the shoot. In addition to this, the roots develop bulbils from which new individuals arise. The seeds are also long-lived, and germinate vigorously on dry and warm sites. Sheep’s sorrel’s leaves contain plenty of oxalic acid which function as defence against herbivores.
A rather similar species is common sorrel (R. acetosa). Its leaves are sagittate (rather than hastate), and fruit valves clearly larger than the fruit.