Desert rose

Identification and origin

Adenium obesum belongs to the Apocynaceae, the oleander family. Its native range covers a broad band of tropical Africa, from Mauritania and Senegal to Chad, Sudan, Ethiopia and Somalia, and south to Kenya, Uganda, Burundi and Tanzania, as well as the Arabian Peninsula (Saudi Arabia, Yemen, Oman) and the island of Socotra. It has become naturalized in Polynesia, in the Society Islands.

It is a succulent shrub or small tree. Its swollen base, the caudex, is a thick, woody trunk that stores water. It carries many short, fleshy branches arranged in spirals, with bark that is smooth and light gray when young and more textured with age. In cultivation the plant usually reaches 3–9 ft tall and 3–5 ft wide, and it grows slowly.

The leaves, clustered at the branch tips, are thick, leathery and glossy, bright green to gray-green, and usually 2–4 in long. They often drop during the dry season, even in cultivation. In summer, funnel-shaped flowers with five petals open, about 2 in across, pink, red or white with a paler throat; each lasts only two or three days, but they follow one another. The fruits are paired follicles, gray to brown, joined at the base like ox horns; when ripe they split open to release small seeds tipped with tufts of hair that help the wind carry them. There are a great many hybrids, with single, double or triple flowers.

Light

Desert rose is a full-sun plant: it needs at least six hours of direct light a day to bloom. In low light it will not flower, and in shade its stems stretch and become sparse. Indoors, set it in front of the brightest window in the house.

In very hot, sunny regions the foliage can scorch, and light shade or filtered light during the hottest hours then gives better results. In a temperate climate it benefits from a summer outdoors, on a sunny patio or deck, before coming back inside for winter.

Watering

Its water reserves make it drought tolerant, and too much water is its main enemy. Let the mix dry between waterings, then water thoroughly until water runs from the drainage holes, and never leave water standing in the saucer. During growth, in spring and summer, the plant uses more: check the mix more often and water preferably early in the morning.

In winter, cut back sharply. If the plant is kept cool and loses its leaves, it goes into rest: you can then stop watering almost entirely for three to four months, and resume gradually as warmth returns in spring.

During the growing season, a weak liquid fertilizer every week, following the label, supports flowering. A plant grown outdoors can instead receive a balanced slow-release fertilizer in summer.

Potting mix

Use a cactus and succulent mix, or an ordinary potting mix lightened with plenty of pumice, perlite or coarse sand, to get a very porous blend. A slightly acidic to neutral pH, closer to 6, suits it. The pot needs several drainage holes: this is the key condition for avoiding root rot.

Temperature

This tropical plant loves heat, ideally between 75 °F and 95 °F. It handles peaks of about 100 °F, but stops flowering above that. It does not tolerate frost and must never freeze.

Between 36 °F and 55 °F it survives, but its leaves yellow and drop. Avoid prolonged exposure below 50 °F, and keep it above 61 °F if you want to limit leaf drop in winter. If it spends the summer outdoors, bring it in as soon as nights fall to about 55 °F, to a bright spot. In the ground it can only be grown in a tropical climate, in USDA zones 10b to 12.

Repotting

Do not choose an oversized pot: too much room favors roots at the expense of flowers. A deep pot produces an elongated, carrot-shaped caudex. Plant so that the roots stay below the caudex line.

To show off the caudex, the plant is often raised at repotting, sometimes several times, exposing the top of the roots, which take on a sculptural look. Repot in late spring, with the mix dry, then wait a few days before watering.

Pruning

Desert rose takes pruning well. Cut just above a node or a branch junction, and remove damaged parts. Pruning makes the plant branch, and since flowers form on young shoots, a well-branched plant blooms more. It also lends itself to bonsai.

Wear gloves: cuts ooze a milky sap that is poisonous and irritating.

Propagation

Seed is the most common method of propagation. Stem cuttings also root, but they seldom develop a swollen caudex, or only below ground. Hybrids are usually propagated by grafting onto young seedlings, a technique widely used in large nurseries in Southeast Asia.

Common problems

• Aphids, scale and mealybugs: mealybugs are common indoors, mites and aphids outdoors.
• Root and caudex rot: the usual cause is a mix that stays wet, especially in cool conditions.
• Leaf spot: an occasional fungal disease.
• Leaves yellowing and dropping: in winter this is often the natural rest brought on by cold or dryness; in the warm season, suspect overwatering or a cold snap.
• Few or no flowers: too little direct light, excessive heat or an oversized pot.

A single symptom can have several causes: look at watering, light and temperature together before acting.

Toxicity

Every part of the plant is poisonous: roots, stems, bark, leaves, flowers, fruits, seeds and sap. They contain cardiac glycosides, and the sap has been used in Africa as an arrow poison and to stun fish. In dogs, cats and horses, ingestion causes vomiting, diarrhea, loss of appetite, depression and an irregular heartbeat, and can be fatal. In people, the sap irritates the skin. Wear gloves when pruning or repotting, and keep the plant out of reach of children and pets.