As would be expected in a genus that ranges from southern Arabia to southern Africa, Adenium exhibits a great deal of natural variation among and within the taxa. Stems vary from sturdy and erect to thin and decumbent, forming plants from a few decimeters to several meters tall. Flower size and form vary from round disks of broad, overlapping petals to star-shaped ones with petals that are narrow, pointed, and even curled (quilled). Flower color ranges from white through pink to deep purple and crimson; some are bicolored. Duration of bloom varies among taxa and between clones within a taxon from as little as a few weeks to nearly year-round. Variability among taxa has been described in parts one through four of this series; this article details some of the infrataxon variations.
Despite all this variation, very little effort has been expended at selecting or breeding superior horticultural forms until recently. Most adeniums are still offered with no data as to their geographical or horticultural origin. Furthermore, 'Singapore' is the only named clone that is well-known in the trade. A distinctive form of A.obesum from near Mombasa is also offered. During the past decade, interest in adeniums has been growing, and several breeders in the USA, India, Australia, and Kenya are actively selecting superior variations.
Several variables can be selected toward the goal of better adeniums for the horticultural trade. Which ones are most important depends on the breeder: flowering season, flower color, flower size and abundance, flower shape, plant form, growth rate, temperature tolerance, and disease resistance.
Adenium arabicum 'Singapore' (also known as A.obesum var. coetanum) was until 1993 the only well-known, named cultivar. According to Professor Holttum it was introduced in 1933 (Frank Horwood, written comm.). Its most distinguishing characteristic is its abundance of very large flowers (Fig.23). The deep pink flowers are up to 10 cm (over 4 in.) in diameter, twice the average size. Grown warm and watered all year, it flowers up to 10 months per year, far longer than most clones. If given a dry winter rest, it breaks dormancy with a massive bloom (see Rowley, 1987, p.184).
'Singapore' seems to be a selection of A.arabicum. Its characteristics fit those of the Yemeni A.arabicum more than those of typical A.obesum: large flowers of bright pink fading toward the throat, faint nectar-guides, large though not gigantic leaves, and a tendency to form a caudex. However, the throat is glabrous, not hairy. Nursery people in Singapore claim that the original plant was imported there from Arabia (Albert Chan, written comm.).
Adenium obesum 'Mombasa' was introduced from seed collected from a plant near that town on the Kenyan coast. It is distinguished by its small stature and profuse basal branching at an early age. It also forms a caudex, unlike most other A.obesum's in cultivation.
Adenium obesum 'Red Everbloomer' bears 7 cm (2 3/4 in.) round, bright red flowers for 7 to 11 months a year. In some years a large plant has been in continuous bloom for twelve months. This clone has a sturdy, upright branching habit, and cuttings form swollen stems and roots quickly. See Dimmitt and Hanson (1991) for photos.
'Red Everbloomer' was selected from a grex of second-generation seedlings grown by me in 1978. It has not been marketed because it has a significant flaw: much of the year the flowers are deformed and poorly colored. However, I continue to maintain it because it is a superb pollen and seed-parent that transmits its vigor, long blooming season and deep flower color to its off- spring. (Many adenium plants seem to be either male- or female-sterile, while others like 'Singapore' seldom transmit their superior traits to their offspring.) 'Red Everbloomer' is a parent of most of my hybrids.
Adenium obesum (unnamed white-flowered cultivars) are known with pure white flowers (Fig.24). Several clones have been selected by growers in Kenya, Singapore, and the United States. Plants should be available soon.
Adenium obesum (or A.arabicum) 'Fritz Dederer' is distinctive in forming a thick, corky white bark, even on small branches. It could be useful in breeding more sunburn-resistant plants. Unfortunately it has been infertile to date.
Adenium swazicum 'Perpetual Pink' is superior to the species in several characteristics. The growth form is much more sturdy than is typical of the species. Its branches are ascending and thicker than any other clone the author has seen. The flowers are of excellent, round form and much larger than average, 9.5 cm (3.7 in.) in diameter (see Dimmitt and Hanson, 1992, for photos). They are also borne nearly year-round (8 to 12 months) on mature plants, compared to a few months for most clones.
'Perpetual Pink' produces copious viable pollen and is the parent of several clones of the cultivar group Arizona (Adenium obesum X Adenium swazicum, to be described in the next installment of this series). It is a nearly infertile seed-parent.
Adenium swazicum 'Boyce Thompson' is typical of the species in growth habit: weak-stemmed, nearly pendent when grown on its own roots (see Dimmitt and Hanson, 1992, for photo). It is outstanding for its flowers, which are deep purple (Fig.25). This clone was found in the collection of the Boyce Thompson Southwestern Arboretum in Superior, Arizona; its geographic origin is unknown. The flowering season is better than average, up to 7 months a year.
'Boyce Thompson' produces copious viable pollen and is also a fertile seed parent. It contributes intense flower-color to its hybrid offspring but usually not its weak stems.
To my knowledge there are no named cultivars of any other species to date, though the need exists. For example, plants sold as A.somalense vary from squatty shrubs to tall trees with massive trunks. These growth habits are more genetically than culturally determined and should be identified so collectors know what they are buying.
Breeders in India and Florida are growing thousands of adeniums in fields, and many out-standing variants have appeared. There is a seedling from Florida with velvety, blackish-red flowers. Several forms with very ruffled petals have been selected in India. Many hybrids have also been produced in recent years and will be described in future installments.
Fig.23. Flower of Adenium 'Singapore' (right) with an average Adenium obesum flower beside it for size comparison.
Fig.24. Tom Grumbley's clone of white-flowered A.obesum
Fig.25. Deep purple flower of Adenium swazicum 'Boyce Thompson
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