Orbea

Haw.
Family : Apocynaceae
Common names : carrion flower ( Eng. ); aasblom, bokhoring (Afr.)

Orbea hardyi

Stapeliads are one of the most interesting groups of succulents-the diversity in shape, colour, size and smell make them an enticing group to collect and grow. The genus Orbea has beautiful flowers and is one of the easier groups to grow successfully. Even sterile plants with their purple-blotchy stems against various shades of green backgrounds are very attractive. In a rockery, mats of the plants among exposed rocks give a good backdrop for other accent plants such as euphorbias and larger aloes.

The genus Orbea is probably one of the stapeliad genera that have undergone the most taxonomic changes. It was described by Haworth in 1812, and then sunk under Stapelia soon after. Until 1975 when Leach resurrected the genus, most of the species were allocated to either Stapelia or Caralluma.

The name Orbea is derived from the Latin word orbis, which refers to the central raised disc or annulus, found in the flowers of most species.

Shows annulus,central raised disc

Description
Orbeas are leafless, glabrous, succulent perennials that form compact to diffuse clumps. They branch from the base and often arise from rhizomatous rootstocks. The stems) are erect to prostrate and sometimes exhibit a creeping nature. The four-angled stems are usually prominently sharp-toothed, with a soft tip. They usually have well-developed, tooth-like projections (tubercules) on the flanks and are always mottled purplish maroon on a green background, especially distinct when exposed to the sun. At the base of the tubercles a pair of stipular denticles are found and the stems are without any well-developed leaves.

Tubercules

The flowers are 10-100 mm in diameter and are borne in few- to many-flowered inflorescences, appearing together or in quick succession. One to five inflorescences usually develop at any place along a stem. They often have an unpleasant odour, which attracts pollinators. The seed-carrying follicles (fruit) look like two horns, hence the common name bokhoring. Like the stems, they are also smooth, blotched and dotted purple.

Conservation Status
Most of the 15 species of Orbea on the most recent Red Data List for southern Africa are in the category, Least Concerned (LC), indicating that they appeared on previous lists and have been deemed as LC in the latest assessments. Orbea elegans is listed as Critically Endangered (CR), O. macloughlinii and O. woodii as Vulnerable (VU) and O. pulchella as Near Threatened (NT).

Distribution
The genus is distributed widely throughout Africa and enters the Arabian Peninsula . About 56 species are recognized world-wide and are more or less equally divided between the northern and southern hemispheres. Two broad centres are recognized, the northeastern centre north of the equator and the centre in southern Africa south of the equator. About 28 species are found in southern Africa . Strangely, Orbea is not represented in Madagascar , and in southern Africa it is absent only from Lesotho .

Unlike most of the other stapeliad genera in southern Africa, which have their highest degree of diversification on the border between the winter rainfall area and the dry karroid regions, Orbea has its highest diversification along the eastern escarpment, with a peak in the Soutpansberg and Blouberg areas.

Ecology
The succulent nature of this group enables the plants to go into a dormant state during part of the year, making use of stored food reserves. It is important for growers to establish from which region the plants originally came, i.e. whether they are from summer or winter rainfall areas, to know when to stop watering and give the plants the opportunity to go into a rest phase.

Orbeas do not have any leaves and the stems, which contain the chlorophyll, perform the process of photosynthesis.

Vibratile hairsThe complexity of the flower is narrowly associated with the process of pollination. The genus is florally very diverse and flowers are adapted to a wide range of different flies that act as pollinators. Many orbeas have vibratile hairs, usually at the edge of the corolla , presumably drawing the attention of flies, which are attracted by the odour of the flowers as well as the movement of the hairs.

After pollination, one or two follicles or 'fruit-horns' develop from a flower, with the tightly packed seeds inside. Each seed has a tuft of white hairs (called a coma) on the tip. When the follicles dry out and burst open, these tufts of hair dry and fluff out, ensuring that the seeds become airborne and are dispersed. Young plants are found mostly under small bushes or shrubs-probably because this is where the coma becomes stuck and the seeds are deposited. These microhabitats offer protection for seedlings to survive and establish.

Economic and cultural value
Apparently not used medicinally, Orbea is mainly of horticultural value and, like most stapeliads, the plants are greatly sought after by succulent collectors and are grown and exchanged world-wide. In Europe and America they are cultivated mainly in greenhouses. Only a few species are used as food, for example the stems of O. lugardii and O. maculata , which are eaten as a vegetable and taste like lettuce. O. namaquensis is also eaten occasionally, but is very bitter.

Species

O.carnosa subso.keithii O. carnosa (Stent) Bruyns subsp. keithii (R.A.Dyer) Bruyns is stoloniferous and grows to a height of 60 mm. This plant has very prominent marginal teeth and small, fleshy flowers, which appear successively between the stem angles from October to January.
Orbea cooperi

O. cooperi (N.E.Br.) L.C.Leach was formerly described in the genus Stultitia . It is widespread in the Eastern Cape , extending northwestwards through the western Free State into the Northern Cape . The plant has erect or ascending branches, up to 50 mm tall. One to three flowers arise from the sessile inflorescence near the base of the stems. The flowers are flattish, with a deep reddish colour and a lighter central annulus.

 

O. elegans O. elegans Plowes is the most recently described species. It occurs on the southeastern foothills of the Blouberg in Limpopo Province . The flower morphology is extremely distinct, differing from other species in the handsome ( elegans ), spotted corolla with its dark purplish maroon, raised annulus. The species is currently listed as Critically Endangered (CR) on the Red Data List.
O. hardyi O. hardyi (R.A.Dyer) Bruyns is endemic to the northern parts of South Africa , from the Soutpansberg in Limpopo to about Pelgrim's Rest in Mpumalanga , growing in deep shade and humus-rich, littered pockets of soil. It is extremely difficult to detect when not in flower. The sprawling, perennial, mottled stems can reach a length of 0.3 m, branching low down near the base. Flowering is in late summer, December to March, when one to four flowers are borne successively near the upper parts of the stems. The corolla tube is shallow-flattish, with an annulus. The corolla lobes are yellowish to cream-coloured with numerous red markings and spots, becoming lighter towards the centre. This species was named in honour of D.S.Hardy who was a well-known horticulturist of especially succulent plants.
O. lutea (N.E.Br.) Bruyns subsp. lutea grows up to 100 mm high and, although usually erect, tends to sprawl when growing in shade. The stems are angled and also have prominent teeth. The flowers are produced in clusters of a few up to 17 and are carried at the base of the branches. The bright mustard-yellow to yellow corolla lobes have marginal vibratile hairs that dangle and move in the slightest breeze. The name is derived from the Latin word luteus which means yellow. Open flowers have a strong foul-smelling odour that attracts large numbers of fly pollinators. Flowering is mainly between December and February. This is the most widespread species of Orbea in South Africa found in all the northern provinces , Free State , KwaZulu-Natal and the Northern Cape . The subspecies vaga (N.E.Br.) Bruyns has brown flowers.
O.melanantha O. melanantha (Schltr.) Bruyns has somewhat shorter and thicker stems than O. lutea, with the marginal teeth less prominent. It also has a strong, foetid odour that attracts large numbers of carrion flies, which are sometimes even deceived into depositing their eggs on the flowers. Flower colour varies from black to almost yellow and flowering occurs from October to February. This species is found north of Pretoria in the Waterberg District and is abundant in the Polokwane area of Limpopo Province where it usually grows on rocky plains. It is also found in the Lydenburg-Middelburg areas in Mpumalanga. The epithet melanantha refers to the dark maroon, almost black flowers.
O. namaquensis O. namaquensis (N.E.Br.) L.C.Leach has short, stout stems that are sometimes almost cylindrical. Is restricted to Namaqualand in the northwestern corner of South Africa where it can become locally quite common. This species is closely related to O. variegata and mainly differs in the internal flower structures and the annulus. In the sterile state, the stems are indistinguishable from those of O. ciliata (which has a different distribution and is florally very different).

O. paradoxa (I.Verd.) L.C.Leach is a stoloniferous, dwarf perennial with angled branches, with prominent teeth along the angles. The small, yellowish, purple-mottled flowers are solitary or in pairs. The flower has a shallow tube and the margins of the corolla lobes have a distinct fringe of purple vibratile hairs. It usually flowers from November to January, but it also depends on rain. It has been recorded from arid bushveld around the Pongola area, extending further into KwaZulu-Natal .

 

O. tapscottii O. tapscottii (I.Verd.) L.C.Leach grows in large tufts of up to 100 mm in diameter, containing up to 60 fleshy branches and can reach a height of 120 mm. It also has prominent teeth that turn white with age. The solitary flowers develop successively from near the base of the branches. The corolla has a tube with a distinct annulus and is greenish yellow, mottled with dark brown, giving it an almost reddish appearance. It has a strong, foetid odour. The species flowers from December to March, again depending on the rainfall. It is restricted to drier regions of the central to northern parts of Limpopo Province and extends into Botswana . This species is closely related to O. cooperi . This orbea was named in honour of Sydney Tapscott who was a well-known plant-collector in Botswana and Zambia in the early twentieth centaury.
O. ubomboensis O. ubomboensis (I.Verd.) Bruyns has the smallest flowers in the genus. It has rather slender, small tuft-forming stems and grows up to 40 mm high. A number of conical teeth are scattered along the angles. The flowers are dark maroon, up to 9 mm in diameter. They are borne in groups of two or three near the tips of the stems, from September to March. The plants are found from the Lebombo Mountains in the south along the eastern escarpment to the Soutpansberg and as far north as Zimbabwe . This well-known species was recently transferred to the genus Australluma .
O. variegata O. variegata (L.) Haw. is probably the most well known of all the species, having been in cultivation the longest. As it was one of the first South African stapeliads to be cultivated in Europe , many hybrids have been grown, most of them described as distinct species at some stage. Many of these hybrids are still in cultivation world-wide. The species grows under a variety of conditions. It originally comes from the winter rainfall area of the Western Cape mainly along the coastal belt. It grows actively during winter and spring. Growers should let the soil dry out between waterings and should feed the plants during the active growing season. Soil has to be well drained and rich in organic matter. This species is regarded as an alien invader in the southern parts of Australia .

In the Garden
All species make attractive groundcovers, even when sterile. They can be used to enhance the surrounding areas of larger focal plants. They will definitely attract attention when in full flower, if not for the beautiful flowers then for their odour or the pollinators they may attract to the garden. Most species will do best as container plants if room to spread is allowed-shallow containers usually give the best results. On a patio, hanging baskets can also be used, exposing the stems and clusters of flowers as they grow down the sides.

Most species prefer a well-drained, sandy medium consisting of equal parts of washed river sand, potting soil and topsoil. Watering should be adapted according to the natural habitat of the plants. Plants from the wetter regions should be watered more than those that originate from the drier regions. Water rather sparingly than excessively. Plants can survive long periods without water, but be sure to water them before they shrink too much and will not be able to recover. In nature, most of these plants are naturally sheltered from the hot rays of the sun by the shade of thickets and shrubs, but some can grow in full sun. Plants prefer early morning or late afternoon sun.

Orbeas are easily propagated by stem cuttings, which should be taken during the active growing stage to ensure good rooting, before the plants enter their dormant phase. Cuttings can flower in their first year, depending on the size of the cutting. Seed also germinate easily-be sure to use fresh seed and treat seedlings for damping off. Generally plants grow fast and most will flower within two to three years when grown from seed.

ScaleThe most common pests include scale (Figure 8) on the stems and mealy bugs on the roots. The latter can cause fungal infections that may devastate plants within days.

Stem rotWhen stem rot is seen, the affected parts should be immediately cut away and disposed of and the plant should be placed in quarantine. Various commercial pesticides or environmentally friendly, homemade concoctions can be used to fight pests. The stapeliad snout beetle can also attack plants, consuming the fleshy insides of the stems.

Species like Orbea gerstneri , O. longidens , O. verrucosa and O. ubomboensis are good ones with which to start. After successfully growing these plants as mother stock in containers, cuttings can be established in a rock garden. Species like O. hardyi and O. conjuncta do well in hanging baskets and shady areas and O. lute, O. melanantha and O. variegata will thrive in the dry garden straight away.

References and further reading

  • Albers, F. & Meve, U. 2002. Illustrated handbook of succulent plants . Asclepiadaceae. Springer Verlag, Berlin .
  • Barkhuizen, B.P. 1978. Succulents of southern Africa with special reference to the succulent families found in the Republic of South Africa and South West Africa . Prunell. Cape Town .
  • Bruyns, P.V. 2002. Monograph of Orbea and Ballyanthus (Apocynaceae-Asclepiadoidiae-Ceropegieae). Systematic Botany Monographs 63: 1-195.
  • Bruyns, P.V. 2003. A note on the genus Orbea Haw. Asklepios 87: 3-7.
  • Bruyns, P.V. 2005. Stapeliads of southern Africa and Madagascar, vol. 1. Umdaus Press, Hatfield, Pretoria .
  • Court, D. 2000. Succulent flora of southern Africa , revised edn. Balkema, Rotterdam .
  • Cullen, J. et al. 2000. The European garden flora: A manual for identification of plants cultivated in Europe , both out-of-doors and under glass, vol. 6, part IV. Cambridge University Press, Cambridge .
  • Gildenhuys, S. 2005. Southern African stapeliads for the beginner. Aloe 42: 24-27.
  • Hardy, D. & Fabian, A. 1992. Succulents of the Transvaal . Southern Book Publishers, Halfway House , South Africa .
  • Leach, L.C. 1978. A contribution towards a new classification of Stapelieae (Asclepiadaceae) with a preliminary review of Orbea Haw., and descriptions of three new genera. Excelsa Taxonomic Series, No. 1. Aloe, Cactus and Succulent Society , Rhodesia .
  • Leistner, O.A. (ed.). 2000. Seeds plants of southern Africa : families and genera. Strelitzia 10. National Botanical Institute, Pretoria .
  • Leistner, O.A. 2005. Seed plants of southern tropical Africa : families and genera . South African Botanical Diversity Network Report No. 26. SABONET, Pretoria .
  • Nichols, G. 2004. The smelly succulent: Orbea variegata ; carrion flower; aasblom. Farmer's Weekly , 3 September 2004 : 68, 69.
  • Oliver, I.B. 1998. Grow succulents. Kirstenbosch Gardening Series. National Botanical Institute, Cape Town .
  • Pilbeam, J. 1994. There's more to Orbea than variegata. Cactus File 2, 3: 11 , 12.
  • Plowes, D.C.H. 2002. Orbea doldii Plowes-a striking new stapeliad from South Africa . Asklepios 86: 8-10.
  • Plowes, D.C.H. 2004. Orbea elegans Plowes an attractive new stapeliad from northern South Africa (Apocynaceae-Asclepiadoidea-Ceropegieae). Asklepios 90: 14-17.
  • Smith , C.A. 1966. Common names of South African plants. Memoirs of the Botanical Survey of South Africa No. 35.
  • Victor, J.E., Nicholas, A., Bruyns, P.V., Venter, H.T.J. & Glen, H.F. 2003. Apocynaceae. In G. Germishuizen & N.L. Meyer, Plants of southern Africa : an annotated checklist. Strelitzia 14. National Botanical Institute, Pretoria .

 

If you enjoyed this webpage, please record your vote.

Excellent - I learnt a lot
Good - I learnt something new

Author

Stoffel Petrus Bester
National Herbarium
January 2006

 


This page forms part of the South African National Biodiversity Institute's plant information website www.plantzafrica.com.

SANBI Home