Desert rose bonsai

Placement

Adenium obesum is a tropical succulent that cannot take frost. In warm regions the bonsai can stay outdoors all year. In a temperate climate it spends the warm season outside, from late spring to the end of summer, in a sunny or lightly shaded spot. It only blooms with enough sun, so aim for at least six hours of direct light a day. In intense heat, filtered light during the hottest hours keeps the foliage from scorching, and above about 100 °F flowering stops.

From fall to spring, keep it in a warm, very bright room or a heated greenhouse. Bring it in as soon as nights drop to about 55 °F. It must never go below 41 °F, and prolonged exposure below 50 °F makes it yellow and lose leaves. It often drops its leaves in winter: this is normal and does not mean it is failing. To limit leaf drop, keep it above 61 °F.

Soil mix

Drainage comes first: standing moisture can kill the tree. Use a very porous succulent mix, or a potting mix amended with plenty of pumice, perlite or coarse sand. A slightly acidic to neutral pH, close to 6, suits it. Choose a pot with several drainage holes that is not too large: too much room favors roots over flowers, and a deep pot produces an elongated, carrot-shaped caudex.

Watering

Like any succulent, desert rose stores water in its caudex. Let the mix dry partway before watering, then water so the whole root ball is moistened, and empty the saucer. In the height of the season, in sun and heat, the pace picks up markedly; in shade or cool weather it slows a great deal. Always check the mix before watering rather than following a schedule, and water preferably in the morning.

In winter, water very little. A tree kept cool and leafless goes into rest: you can then stop watering almost entirely for three to four months, and ease back into it as warmth returns.

Fertilizer: from spring to fall, give a half-strength liquid fertilizer once a month. For a tree grown outdoors, a solid organic fertilizer can take its place. Do not feed in winter unless the tree keeps producing new shoots.

Pruning

Desert rose can be pruned year-round, but keep the young shoots in mind: that is where the flowers form. Pruning makes the tree branch and so multiplies the flower buds. Cut just above a node or a branch junction, and remove any damaged parts.

Cuts ooze a milky, poisonous sap: wear gloves, and keep the tree out of reach of children and pets. The design rests mostly on the caudex and roots. The roots can gradually be brought up above the soil, which gives them a sculptural look, and the tree can even be grown upside down to show off its roots.

Wiring

Always wire loosely. The bark marks easily, and wire that is too tight leaves scars as soon as the branches thicken. Choose aluminum wire about one third the thickness of the branch, wound at roughly 45°, and check it often during growth: the bark can begin to swallow the wire within one to four months. Remove it before it bites in, cutting it at every turn rather than unwinding it. Guy wires are an alternative to coiled wire, as long as the bark is protected where they attach.

Repotting

Repot every two or three years, in late spring, pruning the roots but removing no more than about a third. Let the root ball dry out before you start, and wait a few days after repotting before watering again. This is the time to raise the caudex slightly to expose the top of the roots, while always keeping the roots below the caudex line. Afterward, set the tree in a stable spot sheltered from harsh sun and wind, and wait for it to recover before any heavy pruning or feeding.

Propagation: seed is the most common method; grafting, widely practiced in large Southeast Asian nurseries, is used mainly for hybrids. Cuttings root, but seldom develop a swollen caudex.

Pests: aphids, mealybugs and spider mites mostly attack trees weakened by poor growing conditions; mealybugs are more common indoors, spider mites and aphids outdoors. Good conditions prevent most infestations. A mix that stays wet, especially in cool weather, leads to root rot.