- Name also: Common Wood-sorrel
- Family: Wood Sorrel Family – Oxalidaceae
- Growing form: Perennial herb. Rhizome fragile, creeping, with scales. Leaf-stalks and flower stalk(s) straight from rhizome. 2 small bracteoles in the middle of flower-stalk.
- Height: 5–10 cm (2–4 in.).
- Flower: Regular (actinomorphic). Petals 5, white, purple-veined, sometimes entirely reddish–purple, 8–12 mm (0.32–0.48 in.) long. Sepals 5, narrowly ovate. Stamens 10. Pistils 5. Flowers solitary terminating scape. Also with closed dwarf flowers.
- Leaves: In a rosette, long-stalked, evergreen. Blade trifoliolate, thin, pale green. Leaflets triangularly obovate, with notched tips (obcordate).
- Fruit: Roundish, 5-edged, grooved, pale green, 3–4 mm (0.12–0.16 in.) long capsule, which hurls its seeds up to several metres.
- Habitat: Shady spruce copses, leafy forests, broad-leaved forests, rich swamps.
- Flowering time: May–June.
Wood sorrel has adapted to grow in forest shade. Its rootstock is fragile and its leaves are very thin, only a few cells thick. It is sensitive to drying out so it protects itself by folding its leaflets downwards against each other, reducing the amount of surface area that is subject to evaporation. The plant pulls its leaves tightly together at night and also during periods of fierce sunshine, heavy rain and when it is touched. The leaves stay green throughout the winter, like many other undergrowth plants that grow in Finland.
Wood sorrel rushes to bloom early in the spring while other vegetation is still growing and there is plenty light around, and it is easy for nectar-hunting plants to find the plant’s large flowers. Later in the summer the plant refrains from producing large open flowers in favour of cleistogamous flowers that do not open and resemble buds. These summer flowers are self-pollinating but the seeds usually develop well.
The sourly tart taste of wood sorrel’s leaves is produced by the oxalic acid they contain, which protects the plant from e.g. insect grubs and snails. Large amounts are poisonous for humans but snacking on a few leaves on a forest hike won’t do any harm.