Orbea melanantha
Introduction
A mat-forming succulent that can spread on the surface of the soil with no trace of underground rhizomes, this water-wise plant will be an excellent addition to any rockery or succulent garden and is ideal to be used as a ground cover.
Figure 1. Close-up of inflorescence of Orbea melanantha. (Photo S.P. Bester)
Description
Description
Orbea melanantha is a perennial with fleshy succulent stems that form mats up to 1 m in diameter. The stems are pale green, speckled with red or brown to purplish, depending on the amount of sun it receives – in shady situations they tend to be homogenous pale green and in sunny situations blotched on a green background. The stems generally grow up to about 100 mm long and about 40 mm wide and contain 4 flanks that carry a series of triangular teeth (also known as tubercules), about 15 mm long. The tip of each tubercule is hardened into a corky layer.
Figure 2. Different parts of the stems of Orbea melanantha. (Photo S.P. Bester)
One to two inflorescences may be on a single stem, each containing 3–15 flowers, with the peduncle that carries the inflorescence, 10–15 mm long. The flowers are shallowly bowl-shaped, black to purple or very dark purple to red, with a paler centre. Individual flowers are carried on a flower stalk (pedicel) of 15–45 mm long.
Figure 3. Stems and full inflorescence of Orbea melanantha. (Photo S.P. Bester)
The corolla (fused petals) is 35–65 mm in diameter and contains a very shallow tube of 2.5–4.0 mm deep, which contains the gynostegium; ovate to deltoid to triangular lobes of ± 10–18 mm long and 9–14 at their widest. The corolla is smooth and glabrous outside and densely rugose and hairy inside, with the margins purple and carrying dense, clavate, vibratile hairs.
Figure 4. Vibratile hairs in the sinus of the corolla lobes of Orbea melanantha. (Photo S.P. Bester)
The corona is brown to purple, ± 3 mm high, 8–9 mm in diameter and raised on a ± 1 mm tall stipe; interstaminal corona lobe shallowly bowl-shaped, with a yellow patch in the middle, the lobes have 2 weak keels on the sides and the tips are 3-lobed. The central lobe is dome-shaped to acute, lateral lobes acuminate and divergent. The staminal corona lobes are ovate to lanceolate, laterally compressed, the apex bifid and ascending and sometimes meeting above the style head.
Figure 5. Some parts of the corona and central part of the flower of Orbea melanantha. (Photo S.P. Bester)
Conservation Status
Status
During the latest assessment done in 2005, the species was assessed as Least Concern (LC).
Distribution and habitat
Distribution description
Orbea melanantha plants mostly grow among Acacia trees, on stony ground with short grasses just like with many other members of succulents or stapeliads, in Mozambique, South Africa (Free State, Gauteng, Limpopo and Mpumalanga Provinces) and Zimbabwe.
Figure 6. Distribution records of Orbea melanantha from the National Herbarium.
Derivation of name and historical aspects
History
The genus Orbea consists of about 56 species, mainly from Africa, but some reaching the Arabian Peninsula. The genus name is derived from the Latin word orbis, which refers to the central raised disc or annulus, found in the flowers of most species, but in O. melanantha it is not very distinct. Orbea melanantha was discovered by Rudolf Schlechter in April 1894 near Pietersburg, now Polokwane in the Limpopo Province and at the time named it Stapelia melanantha. It was later then transferred to Caralluma by N.E. Brown in 1909. The genus Orbea has probably had the most taxonomic changes amongst the stapeliads. It was originally described by Harworth (1812), then combined with Stapelia (Schultes 1820), and later again reinstated by Leach (1975), who recognised 13 species initially. Later Leach (1978) increased the genus to 20 species and simultaneously also created the genera Orbeopsis, Orbeanthus and Pachycymbium as closely related to Orbea. All these genera were then sunk by Bruyns (2002) and moved back to Orbea. Eventually Bruyns (2017) moved all the stapeliads to Ceropegia. Orbea melanantha is probably closest related to O. longidens.
Figure 7. The annulus of Orbea ciliata, typical in most Orbea species, but not as pronounced in Orbea melanantha. (Photo S.P. Bester)
Ecology
Ecology
A number of Orbea melanantha were planted at the Pretoria National Botanical Garden in 2018, but before plants could even establish, they were all consumed by wild dassies. What was interesting is that close by plants of Stapelia gigantea were totally avoided and not browsed at all.
The plants have complex flowers where male and female parts are fused and an interesting but complex method of pollination has evolved in these plants (and the family as a whole). The open flowers have a strong unpleasant odour reminiscent of rotting flesh and this attracts flies from a distance to assist in pollination. The movement of the vibratile hairs, scent and colouring of the flowers apparently are all mechanisms to draw the pollinators to the flowers, in order to achieve pollination. It has often been seen that the plants so cleverly mislead the flies in mimicking rotting flesh, that they actually lay their eggs on the flowers, which could potentially be a source of food for their hatched maggots.
Pollination is very specialized with the pollen aggregated in sack-like structure, known as a pollinium. Two pollinia are held together by a clip and this 2-bagged structure known as a pollinarium, is the pollination unit in stapeliads. When a pollinarium gets stuck on the feet or mouth parts of the insect (in this case mostly flies), it can be carried to another flower where it is deposited. After successful pollination, 1 or 2 follicles or ‘fruit horns’ develop from a flower, with the tightly packed seeds inside.
Figure 8. Inflorescence of Orbea melanantha in close up. (Photo Karin van der Walt)
Uses
Use
Orbea melanantha is not used medicinally, but like most other species of Orbea, they are used for their horticultural value. 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 apparently taste like lettuce. Orbea namaquensis is also eaten occasionally, but is very bitter.
Figure 9. Root nodules forming at the base of the stem of Orbea melanantha where in contact with the ground. Plants easily reproduce vegetatively. (Photo S.P. Bester)
Growing Orbea melanantha
Grow
All species of Orbea, including O. melanantha, make attractive ground covers, 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.
Figure 10. Stems of Orbea melanantha, on the left those exposed to direct sunlight very purple-blotched; those to the right in more shady situations being homogenously green. (Photos S.P. Bester)
Orbea melanantha is an easy blooming plant that requires moderate watering through the growing season, but also needs a lot of water and some fertilizers in hot conditions for them to flower freely, especially when grown as container plants. Water more sparingly in winter according to temperatures. In cold weather it is unwise to leave plants wet, as with most stapeliads, they will rot easily. Winter care presents no problems at 5°C with plenty of light. Since roots are quite shallow, use a cactus mix or add extra perlite or pumice to regular soil potting soil. A gritty, very free-draining compost is suitable, and clay pots help the plants to dry out between watering.
Similar other species like Orbea gerstneri, O. longidens, O. verrucosa and Australluma ubomboensis are good companion plants and can be used together with O. melanantha. These species are not difficult and can easily be grown from cuttings as the plants naturally eventually die off and spread vegetatively through their stems. 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. lutea, O. melanantha and O. variegata will thrive in the dry garden straight away.
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 plants rather sparingly, than excessively. Plants can survive long periods without water, but be sure to water them before they shrink too much and not 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 2 to 3 years when grown from seed.
Figure 11. Pests on Orbea melanantha stems. Left, scaly aphids; right fungus infected stem base starting to rot. (Photo S.P. Bester)
The most common pests include scale on the stems and mealy bugs on the roots. The latter can cause fungal infections that may devastate plants within days.
References
- Bester, S.P. & Condy, G. 2007. Orbea elegans. Flowering Plants of Africa 60: 104–110.
- Bruyns, P.V. 2002. A monograph of Orbea and Ballyanthus (Apocynaceae-Asclepiadoideae-Ceropegieae). Systematic Botany Monographs 63: 1–196.
- Bruyns, P.V. 2005. Stapeliads of southern Africa and Madagascar. Umdaus press, Hatfield, Pretoria.
- Court, D. 2000. A revised Succulent Flora of southern Africa. Balkema, Rotterdam.
- Foden, W. & Potter, L. 2005. Orbea melanantha (Schltr.) Bruyns. National Assessment: Red List of South African plants version 2017.1. Accessed on 2019/04/26.
- 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 1: 4–74.
- Leistner, O.A. (ed.). 2000. Seed plants of southern Africa: families and genera. Strelitzia 10. National Botanical Institute, Pretoria.
- Müller, B., Kiel, J., Albers, F. & Meve, U. 2002. Orbea in F.Albers & U.Meve, Illustrated handbook of succulent plants. Springer, Berlin.
- Oliver, I.B. 1998. Grow succulents. Kirstenbosch Gardening Series. National Botanical Institute, Cape Town.
Credits
L.E. Makwarela and S.P. Bester
National Herbarium, Pretoria
May 2019
Plant Attributes:
Plant Type: Ground Cover, Succulent
SA Distribution: Free State, Gauteng, Limpopo, Mpumalanga
Soil type: Sandy, Clay, Loam
Flowering season: Spring, Early Summer, Late Summer
PH:
Flower colour: Black, Brown, Purple
Aspect: Full Sun, Morning Sun (Semi Shade), Afternoon Sun (Semi Shade)
Gardening skill: Average
Special Features:
Feature plant
Fragrant
Good potplant
Horticultural zones
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