Iron Cross

About

Oxalis tetraphylla, long sold as Oxalis deppei, is a small bulbous plant from Mexico, grown for a leaf that looks like a lucky four-leaf clover marked with a dark cross. Each leaf is divided into four broad leaflets arranged like the arms of a cross, and a band of deep purple-brown marks the base of every leaflet, so the four marks meet at the centre in the pattern that gives the plant its name. Sold as a good-luck plant and often as a novelty for a bright windowsill, it grows from clusters of little scaly bulbs, spreads into a fresh green mound and carries nodding pink flowers over the foliage through the warm months.

Leaves that move

The four-part leaves are the main reason to grow it, held on slender stalks that rise straight from the bulbs at the compost surface. Fresh apple green with their crossing purple markings, they fold down at dusk and open again with the morning light, a daily rhythm easy to watch on a windowsill. The flowers are small, five-petalled and carried in loose sprays well above the leaves, rosy pink with a paler throat. A well-grown clump makes a dense mound of clover-like leaves with pink flowers over a long season.

Its origin

This oxalis grows wild in the seasonally dry uplands of Mexico, where it grows from its bulbs with the rains, flowers through the warm, moist season, then dies back and rests as dry bulbs through the dry months. That wet-then-dry rhythm matters most: a growing spell that needs steady moisture and light, and a resting spell that should be kept dry. Understanding that cycle is most of what it takes to keep the plant for years.

Light and position

Give it bright light, with a few hours of gentle direct sun helping the flowers to open and the leaf markings to darken. A sunny sill screened from the strongest midday glare is close to ideal. In too little light the stalks stretch, the leaves pale and the flowers fail to open properly. The nightly folding of the leaves is normal and should not be mistaken for a light problem. Turn the pot now and then so the clump grows evenly instead of leaning.

Watering through the cycle

While the plant is in leaf and growing, water once the top few centimetres of the compost have dried, wetting it through and letting the surplus drain, since the bulbs rot in cold, standing water. Keep it evenly moist but never sodden through spring and summer. When the leaves begin to yellow and die down towards the end of the season, cut watering right back and let the pot dry out, resting the bulbs almost dry until new shoots appear. Overwatering a dormant pot is the main way to lose the bulbs.

Warmth and air

Ordinary warm room conditions suit it in growth, and it should be kept above about 5 deg C. The dormant bulbs survive light frost outdoors in free-draining soil. It is less demanding of humidity than many houseplants, tolerating normal room air, though very dry heated air can brown the leaf edges, so a spot away from a hot radiator is wise. Light airflow around the clump keeps it healthy and helps the flowers.

Feeding

Feed while the plant is in active growth, using a balanced liquid fertiliser at half strength every two or three weeks from the time the leaves appear until they start to fade. Feeding builds the bulbs for next season. Stop entirely once the plant dies back, and give nothing to a resting pot of dry bulbs.

Propagation

The plant multiplies underground, each bulb splitting into a cluster of small offset bulbs over a season. Tip out a dormant pot, separate the little bulbs and replant them a few centimetres deep in fresh, gritty compost to make several new pots from one. This is best done at the end of the dormant rest, just before growth restarts. Division like this is quick, reliable and the usual way gardeners increase it, and a large clump will often provide enough bulbs to fill two or three fresh pots.

Keeping it well

Grown on its natural cycle, the iron cross is easy and healthy. The most common failure is rot, from watering the dormant bulbs or from cold, wet compost in growth, so match the water to the plant’s phase. In dry air spider mites may stipple the leaves, and a rinse with raised humidity helps deter them. A powdery grey coating in damp, still air is a fungal problem; improve airflow. Leaves that flop and yellow out of season usually mean the pot has been kept too wet.

The year in brief

Spring starts bulb growth; replant divided bulbs, resume watering and begin to feed as the leaves rise.

Summer is full leaf and flower; keep the compost evenly moist, feed regularly and give it good light.

Autumn moves the plant towards rest; as the leaves fade, ease off water and stop feeding.

Winter is dormancy for the dry bulbs; keep the pot dry and cool, and wait for spring instead of watering it.

Common questions

Why do the leaves close up in the evening? That is normal for this plant. The four leaflets fold down at night and reopen with the morning light.

My plant died down, is it finished? Almost certainly not. It rests as dry bulbs through its off-season and regrows when watered again; keep the pot dry meanwhile.

How much light does it need? Bright light with a little gentle sun; too little and the stalks stretch and the flowers stay shut.

How do I make more of it? Tip out a dormant pot, separate the offset bulbs and replant them in fresh, gritty compost.

Is it safe around pets? Not entirely. The leaves and bulbs contain soluble oxalates, which harm cats and dogs in quantity, so keep it away from pets that chew foliage.