Orbea
Orbea is a genus of clumping, leafless succulent plants in the milkweed family (Apocynaceae, subfamily Asclepiadoideae), native to Africa and the Arabian Peninsula. Its members are stapeliads — low, soft-fleshed stem succulents that trade leaves for photosynthetic stems and produce some of the most striking flowers in the succulent world: flat, five-pointed, star-shaped blooms, often intricately banded or spotted, with a raised, ring-like annulus at the centre. Because many species emit a carrion-like scent to attract pollinating flies, they share the common name starfish flower with several related genera.
Description
Orbea are clump-forming perennials with fleshy, four-angled (occasionally more) green stems that branch from the base to build up dense, spreading mats. The stems are typically toothed along the angles and lack true spines; the "teeth" are soft, leaf-derived points rather than the hardened spines of a cactus. In strong light the stems often flush purple, red or bronze, and in shade or with generous water they stay green and plumper.
The flowers are the genus's calling card. Each is a five-lobed star, flat to shallowly bowl-shaped, ranging from a couple of centimetres to around ten centimetres across depending on the species. The surface may be smooth, wrinkled, papillate (covered in tiny bumps) or fringed with vibrating hairs, and is frequently patterned in yellows, maroons, purples and creams. At the centre sits a distinctive raised annulus — a fleshy ring or collar around the corona — which helps separate Orbea from its many stapeliad relatives. Most species carry a strong scent, from faintly musty to frankly reminiscent of rotting meat, which draws the flies that pollinate them.
After successful pollination the plant produces the paired, horn-like follicles typical of the milkweed family, which split to release flat seeds each tipped with a silky parachute of hairs.
Distribution
The genus is centred on eastern and southern Africa, with species ranging from Ethiopia and the Horn of Africa down through Kenya and Tanzania into South Africa, and across into the Arabian Peninsula. Typical habitats are dry, warm and rocky — bushveld, semi-desert scrub and stony slopes — where the low clumps grow tucked among rocks, in the light shade of shrubs, or scrambling through leaf litter. This preference for a little overhead cover, rather than blazing full sun, carries over usefully into cultivation.
Notable species
- Orbea variegata — the type species and long-time favourite, with boldly spotted, star-shaped yellow-and-maroon flowers; the source of the "starfish flower" name in cultivation.
- Orbea semota — an East African species (Kenya and Tanzania) with comparatively large, richly coloured flowers ranging from chocolate-brown to pure yellow.
- Orbea schweinfurthii — bears small, densely papillate flowers, deep yellow spotted with dark maroon.
- Orbea lutea — noted for its bright yellow flowers fringed with vibratile marginal hairs.
Many further species are grown by specialists, and the boundaries of the genus have shifted over time as botanists have moved species between Orbea and related stapeliad genera such as Stapelia, Huernia and Caralluma.
Cultivation
Orbea are among the easier stapeliads and make rewarding, fast-growing subjects — provided their one weakness, a strong tendency to rot, is respected. Grow them in a very free-draining, mostly mineral mix in a wide, shallow pot that suits their spreading habit. Give bright light with some shade from the fiercest afternoon sun; unlike many cacti they resent being baked, and scorched stems disfigure easily.
Water generously during the warm growing season, letting the mix dry out between waterings, then keep the plants much drier and frost-free through winter. Warmth suits them: they are tender and are best kept above about 10 °C, though established clumps tolerate a brief cooler, dry rest. Overwatering, a cold wet winter, or a soggy mix are the classic causes of collapse, so err toward keeping them lean. See Watering and Repotting for general technique, and Pests and diseases for the mealybugs and stem rots that most often trouble the group.
Propagation
Stem cuttings are the simplest and most reliable method. A detached stem segment should be left to dry and callus for a few days, then set in a gritty, barely moist mix where it roots quickly during warm weather; clumps can also be lifted and divided. Fresh seed germinates readily on a warm mineral surface, though seed-grown plants are slower and, being frequently cross-pollinated, may not come true to a parent's flower pattern. See Propagation — seed and Propagation — cuttings for full walkthroughs.
Hobby and cultivar notes
Stapeliads like Orbea have a devoted following, prized less for their modest stems than for their extraordinary, almost artificial-looking flowers. They are often grown as a group alongside Stapelia, Huernia and Caralluma, and natural and garden hybrids occur within and between these genera, sometimes making named plants hard to pin down. Because vigorous clumps root so freely from cuttings, choice forms pass easily between growers. The carrion scent, though off-putting up close and a magnet for the occasional fly, is only released while the flowers are open and is a small price for the display.
See also
- Stapelia · Huernia · Caralluma — related stapeliad genera
- Soil and potting mix · Watering · Repotting
- Propagation — cuttings · Propagation — seed · Pests and diseases