Purple shamrock

Identification and origin

Oxalis triangularis belongs to the Oxalidaceae, the wood sorrel family. Its native range covers much of South America: Peru, Bolivia, Paraguay, most of Brazil, and northern Argentina. It is still often sold under its former name, Oxalis regnellii, which is now treated as a synonym.

It is a herbaceous perennial that forms clumps from unusual rhizomes: covered in fleshy scale leaves that store water and nutrients, they look like small, elongated bulbs. In cultivation it reaches 6–18 in tall and 12 in – 2 ft wide; in flower, most potted clumps stand 6–12 in tall.

Each leaf, 1–3 in long, is made of three triangular leaflets, hence the epithet triangularis. The color ranges from green to deep maroon-purple, sometimes variegated. In the evening, on overcast days, or when touched, the leaflets fold downward: this movement, called nyctinasty, is perfectly normal. The five-petaled flowers, white, pale pink, or lavender and under 1 in across, open in small clusters above the foliage from spring into fall, and they also close at night.

Despite its common name, it is not a clover: it is unrelated to the genus Trifolium, even though it is widely sold around St. Patrick’s Day. Common cultivars include ‘Atropurpurea’ (nearly black foliage with a lighter V-shaped blotch), ‘Charmed Wine’ (dark purple, white flowers), ‘Ebony’ (purple-black), ‘Fanny’ (olive green with a silver center), ‘Francis’ (bright purple), and ‘Mijke’ (deep purple, mauve flowers). With good care, a single plant can live for decades.

Light

It needs plenty of light. The ideal spot is an east-facing window, which gives it gentle morning sun, or a south-facing window behind a sheer curtain. A west-facing window works if the afternoon sun there is not too intense. Avoid harsh afternoon sun, which scorches and browns the leaflet edges.

Outdoors, give it full sun to partial shade.

Watering

During active growth, keep the potting mix barely moist, never soggy. Let the surface dry between waterings, then water and do not leave water standing in the saucer. Constantly wet soil is the quickest way to lose this plant. The pot must have a drainage hole.

Watch the plant rather than the calendar: leaves that fade and stems that collapse usually signal the start of dormancy, not a lack of water.

During active growth, humidity above 50% gives nicer foliage: a tray of damp pebbles helps during the heating season. Feed once a month with a diluted balanced fertilizer, only while the plant is actively growing; never during its rest.

Potting mix

Choose a soil-based, rather loamy potting mix lightened with perlite so it drains well. The plant tolerates sandy, loamy, or chalky soil and an acid, neutral, or alkaline pH, as long as water drains freely. In the ground, it does well in moist but well-drained soil.

Temperature

Purple shamrock prefers cool conditions: aim for 61–70 °F during the day, no higher than 75 °F, with distinctly cooler nights around 50–55 °F. Cool nights prolong flowering. In a pot, do not let it drop below 50 °F while it is in active growth, and keep it away from heating and cooling vents. Above 75 °F, it tends to drop its leaves and go dormant.

Outdoors, it is grown in the ground in USDA zones 8a to 11b. There it dies back in winter and returns in spring; it only tolerates light frost, down to about 23 °F, and only in mild regions in a sheltered spot. Elsewhere, move the pot outside in spring once the danger of frost has passed and bring it back in for fall.

Dormancy. After a few months of growth, often in summer after flowering, sometimes in fall or after a spell of heat or drought, the foliage declines. Stop watering, let the leaves dry out and turn brown, then cut them off. Set the pot in a cool, dark place: purple-leaved forms often need only about a month, but the rest can last up to three months. As soon as new shoots appear, bring the plant back into the light and resume watering and feeding.

Repotting

This plant likes being slightly crowded in its pot. Repot once a year as it comes out of dormancy and resumes growth: fresh mix prevents compaction and salt buildup. Move up to a larger pot, only one size up, and only if the clump looks truly crowded. Replant the rhizomes just below the surface of the mix.

Pruning

Maintenance is limited to light cleanup. Remove old leaves as they yellow, which is natural aging, and at the start of dormancy cut back the foliage once it is completely dry.

Propagation

Division is the simplest method, done during dormancy or at repotting time. Slide the clump out of its pot: the many small rhizomes separate easily. Replant them in small groups just below the surface of a well-drained mix, then water sparingly as growth resumes and place them in bright light.

You can also break off the fleshy scale leaves of the rhizomes: each one can produce a new plant. Seed is another option, but division remains the recommended method.

Common problems

• Foliage fading and stems collapsing: the start of dormancy, triggered by the end of flowering, heat, or drought; follow the rest routine.
• Plant declining while the mix stays wet: overwatering, or a pot with no drainage hole.
• Scorched leaflet edges: afternoon sun that is too strong.
• Old leaves yellowing one by one: simple natural aging.
• Leaves and flowers folded at dusk: normal behavior, no action needed.
• Pests: the plant has few pests, but aphids or whiteflies can occasionally move in.

The same symptom can have several causes: always weigh light, watering, temperature, and the time of year together.

Toxicity

All parts of the plant contain soluble calcium oxalates. It is toxic to dogs, cats, and horses: eating it can cause drooling and tremors, colic in horses and, rarely in dogs and cats, kidney failure. Serious effects usually occur only after large amounts are eaten. For people the toxicity is low, and the plant does not cause contact dermatitis. If any is eaten, contact a veterinarian or a poison control center.