Oxalis triangularis aka Wood Sorrel

Oxalis triangularis prefers cool indoor temperatures around 15°C (59°F) and does best when kept away from heat sources. Avoid prolonged exposure to temperatures above 27°C (81°F), which can trigger dormancy or stress. Outdoors, established plants can tolerate brief dips to around -3°C to -5°C in mild coastal areas (USDA zones 8-11).

The species has two main subspecies: ssp. triangularis (smaller green leaves, hardier) and ssp. papilionacea (larger deep purple leaves, more ornamental; RHS Award of Garden Merit). Popular named cultivars include 'Atropurpurea' (deep purple leaves with black border, light pink flowers), 'Charmed Wine' (deep purple foliage, white flowers), 'Ebony' (purple-black leaves, white flowers transitioning to pink), 'Fanny' (olive-green with silver center, soft pink blooms), 'Francis' (bright purple leaves, lavender fall flowers), and 'Mijke' (deep purple leaves, mauve spring-summer flowers).

In USDA zones 8a-11b, Oxalis triangularis can be grown outdoors as a perennial groundcover, border plant, or container specimen. It naturalizes in warm climates (it has become naturalized in Florida and Louisiana). In colder regions it dies back in winter but re-emerges from rhizomes in spring; in zones below 8 it should be grown as a houseplant or the rhizomes lifted and stored frost-free over winter.

No regular pruning is needed. Young plants benefit from being cut back annually during early summer or at the start of dormancy. Established plants can be cut back every 3-5 years to rejuvenate growth. Removing spent flower stems and tatty foliage during dormancy keeps the plant tidy.

Oxalis triangularis grows from rhizomes and should be repotted when the container becomes crowded or root-bound, typically every 1-2 years in spring when new growth begins. Use a well-drained loamy or sandy potting mix and a container with drainage holes to prevent waterlogging.

The trifoliate leaves close at night and may collect dust over time when grown indoors. Gently wipe leaves with a damp cloth or give the plant a light misting to remove dust. Avoid harsh sprays that could damage the delicate foliage.

The most reliable propagation method is division of the underground rhizomes (scale-leafed structures) in spring. Individual scale leaves can also be detached and used to generate new plants. Seed propagation is possible but may not produce offspring identical to the parent subspecies or cultivar. Divide clumps after dormancy, when new growth is just beginning, and replant sections at the same depth in fresh well-drained potting mix.

Yellowing leaves on Oxalis triangularis are most commonly a sign of overwatering, waterlogged soil, or the onset of natural dormancy. Allow the soil surface to dry between waterings and ensure the pot has drainage holes. If the whole plant yellows and dies back without obvious rot, it may simply be entering a dormancy cycle — reduce watering and wait for new growth to emerge.

Brown leaf tips or edges can result from low humidity, excessive direct sun, heat stress, or fluoride/salt buildup in the soil from tap water or fertilizer. Move the plant out of harsh direct sun, ensure good drainage, and flush the soil occasionally with clean water to reduce salt accumulation.

Drooping or collapsed foliage on Oxalis triangularis often indicates the plant has entered dormancy — a normal response to heat, drought, or seasonal change. Reduce watering and do not discard the plant; the rhizomes remain viable underground and new growth will resume when conditions improve. If the plant has not entered dormancy, drooping may indicate underwatering or root rot.

Oxalis triangularis naturally closes its leaves at night (photonasty) and during periods of stress such as strong sunlight or disturbance — this is normal behavior driven by turgor-pressure changes in cells at the leaf base and not a sign of disease. True leaf drop (leaves falling off) can occur during dormancy or under severe stress; reduce watering and allow the plant to rest.

Slow or stalled growth is usually caused by insufficient light, cool temperatures combined with overwatering, or the plant being in dormancy. Oxalis triangularis grows at a moderate rate under appropriate conditions (bright indirect light, temperatures 15-24°C, well-draining soil). If the plant has gone dormant, withhold water and wait; growth will resume once the dormancy period ends.

Oxalis triangularis has no serious insect or disease problems under normal growing conditions. Common minor issues in houseplant settings include fungus gnats (from overwatering), aphids, and spider mites in dry conditions. The plant is deer and rabbit resistant when grown outdoors. Ensure good drainage to prevent root rot, the most significant disease risk.

Oxalis triangularis is pollinated by insects, primarily bees and butterflies. The small funnel-shaped flowers close at night, reopening during daylight when pollinators are active. When grown indoors, the plant may still flower but fruiting is uncommon without insect visitors; hand pollination is possible if seed production is desired.

The leaves, flowers, and roots of Oxalis triangularis are all edible and carry a pleasant sharp, sour flavour due to their oxalic acid content. Leaves can be eaten raw or cooked; flowers are used raw as a decorative salad ingredient; roots are crisp with a sweet, mild flavour. All parts should be consumed in moderation — large quantities can bind dietary calcium and cause nutritional deficiency. People with kidney stones, gout, rheumatism, arthritis, or hyperacidity should avoid or strictly limit consumption. Cooking reduces the oxalic acid content. Note that the plant is also classified as toxic to pets.

No medicinal uses are documented for Oxalis triangularis in the standard ethnobotanical references. PFAF, Useful Tropical Plants, and Useful Temperate Plants all explicitly record "none known" for medicinal applications.

No significant other uses are documented for Oxalis triangularis beyond its ornamental value as a striking foliage plant. PFAF, Useful Tropical Plants, and Useful Temperate Plants all record "none known" for other uses; the plant is grown almost exclusively for its decorative deep-purple trifoliate leaves and dainty flowers.