Oxalis montana aka Mountain Woodsorrel

Oxalis montana is a small plant that lives for years. It belongs to Oxalidaceae, the wood sorrel family. People call it mountain woodsorrel or sourgrass. It grows wild in eastern Canada and the eastern United States. Its range extends south to mountains in North Carolina and Tennessee. It forms groups in cool, damp woods with rich soil. These include hemlock and spruce-fir forests.

Plants reach 0.25–1 foot (0.08–0.30 m) tall. Each clump spans about 0.25–0.5 foot (0.08–0.15 m). Runners can spread past these bounds over time. Rhizomes or stolons are stems that send out roots and shoots. The plant grows slowly into a low, rounded mound. Its green leaves grow on long stalks from the base. It sheds these leaves each year. Each leaf has three leaflets, each like an upside-down heart. Brown hairs grow on the leaves. The leaflets fold at night and react to changes in sunlight. Flower stalks rise above the leaves, with red near their bases.

Flowers bloom from May to July, each on its own stalk. The blooms nod and have five petals with notched tips. They may be white, lavender, or rose, with deep pink veins. Each petal has a yellow mark near its base. Blooms at lower sites have stronger pink-purple veins. Bees and other insects that spread pollen visit the blooms. Later, cleistogamous flowers form near the plant's base. These have no petals, stay closed, and use their own pollen. Small, upright seed pods, called capsules, then form. They turn brown as they ripen. Each splits into five parts, each with one or two seeds.

Grow mountain woodsorrel in part shade and moist soil that drains well. It prefers acid soil rich in organic matter. NC State lists sand, loam, and clay as suited soils. It lists USDA hardiness zones 1–7. Plants For A Future is unsure of its hardiness in Britain. The plant needs little care. It suits woodland gardens, native plantings, banks, and slopes. Its roots help hold soil in place. It gives wildlife food and cover. NC State reports no major insect or disease problems. It can become weedy where people or other forces disturb soil.

Reported food uses include leaves, flowers, seeds, and roots or tubers. Small amounts give salads and drinks a sour taste. Cooking the leaves cuts their oxalic acid content. Eating any part carries some risk, with greater risk from large amounts. Oxalic acid can leave less calcium for the body to use. Those prone to kidney stones, gout, or related health issues should take care. The plant poses a risk to pets, too. Plants For A Future lists no medicinal uses. It states that boiling the whole plant makes a yellow dye.