Orbea Haworth 1812
The genus Orbea consists of around 56 species of leafless stem succulents forming clumps and mats of vegetation. The green stems are four-angled with prominent pointed tubercles and may develop attractive purple blotching in bright sunlight. Stems generally branch from their base.
Orbea have unusual five-lobed flowers, generally with a raised ring or annulus around the centre. This ring (Latin: ring=orbis) accounts for the name of the genus. The flowers of many, but not all species produce a putrid odour of rotting carrion to attract blow flies as pollinators. The petal edges of some species carry numerous hairs that vibrate with the slghtest air motion. As with other Asclepiads, pollen is contained in pollinaria which must be transferred to the female part of the flower by the pollinator. Following fertilisation of flowers, pairs of seed horns appear containing many seeds, each with its own silken parachute.
For many years Orbea was merged with Stapelia until separation by Leach (1975). The genus has been much revised, with several species moved recently from Stapelia, Caralluma, Diplocyatha, Orbeopsis, Orbeanthus and Stultitia to Orbea. Names on labels have often not caught up with these changes. The genus is distributed throughout Afria and into Arabia. Orbea variegata is regarded as an exotic invasive species in S. Australia.
Cultivation of many Orbeas is not difficult. Orbea variegata is a well known cottage windowsill plant. Most species require careful watering (rain water with some fertiliser) during the growing season and complete withdrawal of water during the winter months. A minimum winter temperature of 10°C is acceptable, providing that plants are kept absolutely dry. A heated growing bench or incubator may help delicate plants to get throught the colder months. Many species of Orbea live under shrubs in habitat and prefer light shade rather than full sun, although stems may not colour up under shady conditions.
A gritty free-draining compost is essential, and clay pots are advisable for the more delicate species. Some growers prefer a mineral-only compost to minimise the chance of fungal attack on the roots. A layer of grit on the surface of the compost prevents moisture from accumulating around the base of the stems.
Keeping Stapeliads and their roots free of pests such as mealy bugs is the real key to success, as fungal attack often occurs as a result of damage to stems by insects. Watering with a good systemic insecticide such as those based on imidachloprid should help to keep plants free from insects.