Unlike most crassulas which are relatively small succulent plants, Crassula arborescens is an outstanding, squat, succulent tree that is easily recognizable by its unmistakable blue-grey foliage that contrasts well with the other plants on hillsides in the karroid scrub. It has a very attractive appearance in both its natural habitat and cultivated gardens.
Crassula arborescens is a large and impressive looking single-stemmed, many branched shrub or small tree, easily reaching a height of up to 3 m. The trunk is thick and fleshy and has a smooth, green-grey bark.
Two subspecies are found, namely Crassula arborescens subsp. arborescens and subsp. undulatifolia. The latter has elliptic, undulate, and erect leaves.
The leaves show very little variation and are thick and fleshy, with a blue-grey, waxy bloom, apex (tip) rounded with an obscure, sharp tip, the base may be tapering (narrowing towards one end), the margins are entire often with a reddish rim, and the petiole is very short or absent.
The flowers are very showy and carried in dense branches, are star-like, white to pink with almost spherical heads and carried slightly above the foliage. When flowering in spring to summer, the plant is almost completely covered with flowers. After pollination, the flowers turn to a papery brown which in itself is quite decorative.
Fruits are small, 6 mm long, with 3-5 separate, oval, sharply tipped follicles (simple dry fruits splitting along one suture) and remaining among the dried remains of persistent petals. Fruits ripen from Nov.-Jan. Seeds are very small and are dispersed by wind.
Crassula arborescens is not endangered and due to its free seeding and easy rooting nature, it generally occurs in abundance in its natural habitat.
Distribution and habitat
There are two subspecies found. Crassula arborescens subsp. arborescens occurs from the Hex River Valley to the Little Karoo and then further north as far as KwaZulu-Natal and Swaziland. Something rather unusual is that one population of this subspecies occurs on some hills north of Vanrhynsdorp and thus shows a gap in the main distribution which is in the Little Karoo. No indications of human influence could be detected nor could the northern plants be distinguished in a single character from the southern ones (Toelken 1997).
Crassula arborescens subsp. undulatifolia is found in the southern parts of the Klein Winterhoek Mountains of the Eastern Cape. Plants typically grow in sandstone and shale-derived soils in rocky to gravel-like conditions. They are associated with elements of fynbos and renosterveld in both the Western and Eastern Cape habitats. Plants often occupy large areas on hills, slopes and sometimes cliffs but are also found in valleys but with a preference for sunny and exposed situations.
Derivation of name and historical aspects
As with a number of other succulent plant families, the essentially cosmopolitan (absent from Australia and much of South America), Crassulaceae is centred in South Africa where under a great variety of conditions it has diversified and adapted to form what is now one of the most represented families of the South African succulent flora. It is in fact the world's third largest succulent family following Cactaceae and Aizoaceae. Crassulaceae are dicotyledonous (the embryo of the plant has two cotyledons/seed leaves) plants belonging to the order Rosales. The family consists of ± 34 genera with ± 1 500 species.
World-wide, the genus Sedum is the largest in the family with some 428 species (Van Jaarsveld & Koutnik 2004). In southern Africa there are five genera, namely Adromischus, Crassula, Tylecodon, Cotyledon and Kalanchoe. The largest genus here is Crassula with about 150 species.
According to Toelken (1977), Hermann was the first to illustrate any South African species of Crassula in 1687. Linnaeus recognized 28 species by the end of his life in 1753. Thereafter the numbers of species increased rapidly. The majority were added by Thunberg in1778. During the early part of the nineteenth century, Haworth not only described new species but also subdivided the genus into several sections.
Practically all are succulents, in fact, the family name Crassulaceae and genus Crassula are derived from the Latin, crassus which means fat or thick and mostly refers to the succulent leaves and stems associated with most members of the family. The species name arborescens is the Latin name for tree-like, alluding to the relatively large, tree-like appearance of this particular plant.
Crassula arborescens has a shallow root system and is evergreen to deciduous (mostly old leaves fall off, and uses the succulent leaves and stems to store water. Shallow roots permit the plant to fully utilize light rain showers.
The leaves are carried in a slightly erect and upward manner to minimize surface exposure, therefore reducing desiccation. In addition to this, the leaf colour, which is grey to blue-green and has a powdery, waxy layer, also reflects the sun and helps the plant to stay cool during very hot days.
The colourful white to pink flowers produces nectar which is usually an indication of pollination by birds. The stamens and styles are positioned at or slightly beyond the corolla opening, another feature of bird-pollinating plants (Van Jaarsveld & Koutnik 2004). Bees and other flying insects are also attracted to the flowers.
After pollination, the flowers turn brown with a papery texture. Mature fruits dehisce to release the numerous, very small seeds which are easily carried away by the wind. Flowers and fruits of C. arborescens are carried slightly above or at foliage level to simplify firstly pollination by means of visibility and secondly seed dispersal by means of wind.
Plants that occur on slopes or cliffs easily lose parts of their branches and these can root freely and develop into separate colonies. It is not uncommon therefore to see dense populations of Crassula arborescens along its distribution range.
Uses and cultural aspects
As with some other members of Crassulaceae, the tree crassula is used in the treatment of epilepsy. The roots of Crassula arborescens subsp. arborescens are eaten in Swaziland where it is known as Umchobozovithi. Horticulturally the plants are very popular in rock gardens, on rocky embankments, slopes and as container plants. Their easy cultivation has lead to them being used extensively in gardens in southern Africa. Besides their attractive flower clusters, they look delightful with their contrasting blue-green foliage, especially when used in combination with other dark green shrubbery.
Growing C. arborescens
It is easy to grow plants vegetatively as well as sexually. For quick results stem cuttings can be made from mature plants. Cuttings can be made throughout the year and root readily in a well-drained medium. Many rooted plants will form from the mother plant in the garden when they fall to the ground, provided conditions are suitable. The seeds are very fine and must be harvested as soon as the inflorescence turns brown as the fruit ripens. Seeds should be sown in a damp, sandy medium and be kept shaded and moist while cuttings are inserted into coarse river sand and later transplanted into ordinary garden soil. When considering a rock or semi-desert garden, it is well worth the effort of combining Crassula arborescens with some of its related members. Good choices include Tylecodon paniculatus, Cotyledon orbiculata, C. tomentosa, C. velutina, Crassula perfoliata, C. coccinea, C. rupestris and C. ovata. Other complimentary species include Lampranthus multiradiatus, L. hoerleinianus, Kalanchoe crundallii, K. thyrsiflora, Cyphostemma juttae, Aloe arborescens, A. ferox, A. microstigma, A. perfoliata, A. striata and A. glauca.
References and further reading
- Coates Palgrave, K. 2002. Trees of southern Africa. Struik, Cape Town.
- Court, D. 1981. Succulent flora of southern Africa. Balkema, Cape Town.
- Germishuizen, G. & Meyer, N.L. (eds). 2003. Plants of southern Africa : an annotated checklist. Strelitzia 14.
- Jackson, W.P.U. 1990. Origins and meanings of South African plant genera. University of Cape Town Ecolab.
- Wild Flowers of South Africa, 1980. Cape Town, Struik
- Stearn, W.T. 2003. Stearn's dictionary of plant names for the gardener. Cassel, UK.
- Toelken, H.R. 1997. A revision of the genus Crassula in southern Africa. Annals of the Bolus Herbarium 8,1-595.
- Van Jaarsveld, E. & Koutnik, D. 2004. Cotyledon and Tylecodon. Umdaus Press, Hatfield, Pretoria.
- Van Jaarsveld, E., Van Wyk, B-E. & Smith, G. 2000. Succulents of South Africa. Tafelberg, Cape Town.
- Van Wyk, B-E. & Gericke, N. 2000. People's plants. A guide to useful plants of southern Africa. Briza Publications, Pretoria.