The Rose that Isn't

For one thing, it has no thorns. But beyond that, it is totally unrelated to the rose family and doesn’t really even look like one. So much for common names. The desert rose is scientifically known as Adenium obesum, or the fat adenium, referring to its grossly thickened trunk. It is in the Apocynaceae, or periwinkle family, which, besides the common garden periwinkle, includes oleander (frequently used as floriferous landscape shrubs in mild climates such as Florida and southern California), the spiny Madagascar palm (which, or course, isn’t a palm at all), and the fragrant frangipani, or Plumeria which is grown worldwide in tropical climates.

Adenium is a small group of plants known from dry climates in sub-Saharan Africa and the very southern part of the Arabian Peninsula. Scientists debate how many wild species there are of Adenium. The most conservative view is that there is only one species and a handful of subspecies or varieties. But wild adeniums vary dramatically from location to location, from short fat trees that resemble a small baobab to quite small shrubs arising from huge tuberous roots. One thing that they all have in common is that they live in relatively dry climates (or at least areas that have extended annual dry seasons) and therefore they have adopted the succulent way of life. That is, they store water in their soft, swollen roots and stems to allow them to survive through periods of drought. Another contingent of taxonomic experts conclude that the subspecies warrant species status, and further subdivide these into additional named varieties. A complete list can be found below; all are in cultivation and available through nurseries that specialize in cacti and other succulent plants.

But the most commonly available forms of the desert rose are Adenium obesum (using the name in its narrow sense) and its various cultivars and hybrids. These are showing up in garden centers and also in large chain hardware and discount stores here in Wisconsin and throughout the country. The most commonly available plants currently are grown from seed and are very similar to the true species found in nature. Young plants have an inflated trunk, sometimes called the caudex. From this fattened caudex arise several slender but soft and succulent stems which are sparsely branched in youth. Even young plants 2-3 years old and 6-8 inches tall can put on a beautiful floral display, with the pink to red flowers arising from the tips of the stems.

There has been considerable work done in selecting horticultural cultivars, primarily for flowering characteristics. Other cultivars have been developed through hybridization, using other species to cross with A. obesum. These named cultivars must be propagated from cuttings, which do not have a caudex, but which do eventually form thickened roots and trunk. Flower colors range through various shades of pink and red, to bicolor flowers with white, and, recently, pure white forms. Some have yellow throats. True Adenium obesum normally blooms in spring and summer continuously for many weeks. Improved cultivars bloom almost continuously if given proper care.

In nature, Adenium obesum is quite variable but can form a small, thick-trunked tree or large shrub. However, its size can be restricted by pot culture, and 20-year-old plants can be quite happy in a 10" pot, being only a foot or two tall. Because their size can be restricted, and because of the unusual shapes, they are becoming increasingly popular subjects for tropical or succulent bonsai. Given the right conditions, they can be fast-growing and rewarding houseplants in most any climate.