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Adenium is a genus of spectacular succulents from tropical Africa and ArabiA.The species range from shrublets with subterranean or above-ground caudexes to small trees with swollen trunks and stems to 15 feet tall. Their striking forms are further enhanced by some of the showiest flowers of all succulents, often borne in masses over a long season. Last but not least, they adapt readily to container culture in the warmer parts of the world (Dimmitt and Hanson, in press).
Some authors recognize only one species in the genus, Adenium obesum (Rowley, undated); others split it into six or more (Plaizier, 1980). Each taxon, whether recognized as a species or variety, is a distinct entity from a horticultural viewpoint, exhibiting a unique combination of plant habit, growth cycle, flower form, and blooming season. We regard most of the taxa as species, based on easily recognized forms in cultivation which seem to overlap little or not at all in nature.
Despite their beauty and ease of culture, adeniums are not nearly as popular as one might expect. Perhaps they simply haven't received the exposure they deserve. This series of articles will describe the nine recognized taxa plus several selected clones and hybrids from a horticultural perspective.
Adenium obesum (A.obesum var. obesum Roem. & Schult.) (Figs. 1, 2) even as narrowly defined here is a highly variable taxon in growth and flowering habits. It occurs nearly all the way across Africa in a broad band south of the Sahara, from Senegal to Sudan and Kenya (Plaizier, 1980; Rowley, undated). Most plants in cultivation are of unknown origin, so it is not known how much of the natural variation of this wide-ranging taxon is represented. The few documented plants known to the authors are from southeastern Kenya, except for A.o. var. coetanum (='Singapore'), which is reportedly from Arabia (Albert Chan, pers. comm.).
Adenium obesum is a shrubby plant. The thickened stems taper gradually upwards and may be rigid and upright or, less commonly, rather weak and spreading. Young plants have a small, ovoid caudex, and old specimens in habitat have large caudexes. Mature plants in cultivation, however, usually lack a distinct one (Fig. 5).
Therefore this species is not, strictly speaking, a caudiciform in cultivation. (See Rowley, 1987 for an excellent definition.) The leaves range from narrow-linear to quite broad (but never as broad as those of A.multiflorum), and from bright, shiny green to light, dull green.
Adenium obesum is potentially semi-evergreen: if kept warm and well watered, plants will grow and often flower through the winter. Under such conditions they undergo only a brief leaf-drop and dormancy, usually a few weeks in spring or early summer. They can also endure a drought or cold-induced dormancy of several months, which is the normal condition in nature.
The flowers (Fig. 2) are pale pink to deep red on the petal margins, always fading to near white toward the throat. The throat (floral tube) is white, sometimes with faint red nectar guides. The anther appendages are long, equaling or exceeding the throat. Flower size averages about 6-7 cm (2 inches) in diameter, but this is quite variable among clones.
Flowering habit is extremely variable and is influenced by both cultural and genetic factors. When grown under ideal conditions of ample heat and water, some clones flower for two to four months; some clones are nearly everblooming. Most plants slow their growth and stop flowering when temperatures exceed about 100 degrees F, which occurs from about mid June to early August in Tucson (Dimmitt and Hanson, in press).
Seed-grown plants are typically vigorous and can be flowered in as little as 8 to 12 months. Cutting-grown plants are equally vigorous; their roots become greatly enlarged in a couple of years and can be exposed when the plant is repotted to make a more interesting specimen. After several years the stems also will have thickened such that cuttings are indistinguishable from seed-grown plants. This is by far the commonest taxon of the genus in cultivation.
Adenium multiflorum [A.obesum var. multifiorum (Klotsch) Codd] (Figs. 3, 4) is very different from A.obesum both horticulturally and geographically. It occurs on the east side of southern Africa, in Mozambique and the countries bordering it on the west and south. Neither Plaizier (1980) nor Rowley (undated) show any geographic overlap with A.obesum on their distribution maps, though Rowley says that intergrades occur. (They easily cross in cultivation.)
In nature the plants are tall, multiple-stemmed shrubs (Plaizier, 1980). In cultivation they are always thick, sturdy, and upright. As in A.obesum, there is no distinct caudex in mature cultivated plants, but they have very enlarged stems and roots. Seedlings have a prominent, ovoid caudex for the first several years. The leaves are large and very broad.
In distinct contrast with A.obesum, this species has an obligatory long winter dormancy. Regardless of growing conditions, the leaves fall in autumn; growth will not resume for at least four months, longer in cool or dry conditions.
Adenium multiflorum flowers profusely for two to four months in winter while leafless, never at other times. The petals are white with sharply defined red edges (Fig. 4). There are always 15 prominent red nectar guides in the white throat (three per petal). The anther appendages equal or exceed the throat. Flowers average about 6-7 cm (2 1/2 inches) in diameter, and the petals are more pointed than those of A.obesum.
Adenium multiflorum grow vigorously from seed, but mature at a larger size than plants of A.obesum. Plants rarely flower until they are 4 to 5 years old. Cuttings of A.muitiflorum develop thick roots and stems and make good specimens after a few years. This taxon is in cultivation but is a distant second to A.obesum in availability, probably because of its slower growth and shorter blooming season.
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