Adenium is a genus of spectacular succulents from tropical Africa and Arabia. They range from shrublets with subterranean or above-ground caudexes to small trees with succulent stems to 15 feet tall. Their striking forms are further enhanced by some of the most beautiful 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).
Adenium swazicum [A.boehmianum Schinz var. swazicum (Stapf) Rowl.] (Figs. 6-8) occurs on the east coast of southern Africa, in Swaziland and adjacent parts of South Africa and Mozambique (Plaizier, 1980). It is a shrubby species, lower growing and more spreading than A.obesum or A.multiflorum (Dimmitt and Hanson, 1991). The stems of most clones are weak (decumbent), spreading horizontally or even drooping over a pot (Fig. 8). Mature plants have massive roots and thick stems, but a caudex is evident only in young ones. Cuttings develop the same characteristics in a few years. The long, narrow leaves are lighter green than in most adeniums, widest near the tips, and the margins are usually slightly crisped (wavy). In full sun the leaves tend to be folded upward along the midrib. Like A.obesum it is nearly evergreen if kept warm and watered, or can be forced into a long winter dormancy. Under warm greenhouse or tropical conditions growth ceases in autumn, but the leaves fall gradually throughout the winter; new growth begins early in spring.
Adenium swazicum usually flowers for a few months in late summer and fall. Some clones, however, are in almost continuous bloom, experiencing only a few weeks of rest in late winter. The broad-petaled flowers are uniform in color from the petal margins to the edge of the darker, unmarked throat and average 6-7 cm (2.5 inches) in diameter. The color is typically medium pink, but is deep purple in some clones (Fig. 8). Plaizier (1980) says that the flowers may be crimson or white, but such plants do not seem to be in cultivation. The anther appendages are short and hidden deep in the floral tube. Adenium swazicum is fairly easily found in cultivation, and is easy to grow.
Adenium boehmianum (A.boehmianum var. boehmianum) is quite different from A.swazicum in a number of traits. It is an upright, profusely-branched shrub 10 eight feet tall in nature (Plaizier, 1980), where it is always found in rocks (Chuck Hanson, pers. obs.). Its leaves are the largest in the genus, several inches long and broad, widest near the tip. The plant has a very short (summer) growing season. It is in leaf only about three months a year, and this cannot be extended by cultural conditions. The caudex is poorly developed. It occurs in Namibia and Angola, on the other side of the continent from A.swazicum.
The flowers are very similar to those of A.swazicum, though usually smaller, less than five cm (two inches) in diameter. They are produced for only a few weeks while the plant is in active growth.
Adenium boehmianum is only occasionally offered in cultivation, probably because of its short growing season and consequent slow growth.
Adenium oleifolium Stapf (=Adenium lugardii N.E.Br.) occurs in the interior of southem Africa, in the Kalahari Desert of southern Botswana, South Africa, and eastern Namibia. It is a small, slow-growing species with a subterranean caudex rarely more than a foot in diameter (and this only with great age). Both roots and stems rise toward the surface; the above-ground stems are not thickened noticeably and are seldom as much as two feet tall (Plaizier, 1980). The leaves are very long and narrow with nearly parallel sides, but do not tend to fold upwards along the midrib as do those of Adenium swazicum.
The flowers are small, about 2-5 cm (1 inch) in diameter with a wide floral tube. The petals are pink and the tube white or gold with faint nectar guides. Plaizier (1980) describes the corollas as, "bright scarlet or red to pink," but we have seen only pink in cultivation. Plants bloom for a couple of months in summer. Pollinated follicles often take more than a year to mature; the seeds are much larger than those of other Adenium. Adenium oleifolium is only occasionally offered in the trade. It is easy to grow, but is quite slow to become a specimen.
Fig.6. Adenium swazicum 'Perpetual Pink', a twelve-year old seed-grown plant in a twenty-four inch pot. This clone has more upright stems than is typical of the species. It also has a very long blooming season. Note the absence of caudex.
Fig.8. Adenium swazicum 'Boyce Thompson'. This clone has the weak stems typical of the species, but the flowers are an unusually deep color. This is a three-year old graft onto a seedling of the same species.
Fig.7. Adenium swazicum 'Perpetual Pink'. The flowers of this species are of a uniform color on the petal blades and have dark throats.
Fig.9. Adenium oleifolium. This eight-year-old plant has not outgrown a six-inch pot.
Fig.10. Adenium boehmianum in cultivation at the Huntington Botanic Gardens
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