August 13, 2018
Gardeners in South Florida are succulent-adverse. Some seem to think that all succulents come from deserts and won't grow here, or have undesirable spines, or are basically uninteresting. We are attracted to flashy colors and often don't understand or appreciate the forms and survival strategies of the plants that we grow. Consequently, nurseries and home centers meet the lack of demand by offering few succulents, so the feedback loop continues. In fact, many succulents are well suited to our seasonally dry climate and fast-draining soils. They can have extremely showy flowers, great color, and striking forms. Above all, the survival strategies of succulents are some of the most amazing of any plants.
An interesting genus of Old World succulents, Adenium, is popularly known as Desert Rose. Unlike true roses, adeniums are thornless. Adeniums can be found from the Arabian Peninsula to Southern Africa, as well as the floristically fascinating island of Socotra. The genus is part of the larger Apocynaceae family that includes Plumeria or Frangipani. Indeed, the relationship between Desert Roses and Frangipani is easily seen in the similarity of their flowers. Like Frangipani, Adeniums are the subjects of intensive selection and breeding programs to produce all sorts of horticultural cultivars. Flower color, patterns, and size have been greatly modified from the wild ancestors, often resulting in additional secondary changes to the overall form of the plants.
Adenium obesum is the most widespread species of the genus. In the past, its many geographic forms have generated much taxonomic confusion. Previous assigned species names such as arabicum and somalense have all been subsumed within obesum, despite the morphological differences. Local forms of the species can be found in Kenya, Somalia, Saudi Arabia, Yemen, and Oman. Some are small shrubs, others large, and others tend to grow as small trees. No matter the shape, all have similar horticultural needs. All Adenium obesum will thrive in South Florida.
Grow adeniums outdoors in full sun, in very well drained soil. Adding some none-limestone rough gravel, such as lava rock or expanded shale to the mix is a good idea. Coarse silica or granite sand also helps with drainage without raising the soil alkalinity to an unacceptable level. Adeniums tend to be quite frost sensitive, so preventing them from turning to mush on the coldest South Florida nights will require some protection. They are great in pots, especially those made of terra-cotta that offer quick evaporation and gas exchange for their roots. Use a very porous mix that incorporates much perlite and/or gravel to speed drainage. If the wet potting mix won’t hold together when squeezed in the hand, it’s the correct porosity. Whether planted or potted, mulching adeniums with gravel is a natural and very attractive way of displaying them. Give them a slow or controlled-release fertilizer that contains some microelements and is fairly low in nitrogen, such as 8-2-12. Even 10-10-10 or 7-7-7- will work, as long as the nitrogen isn’t too quickly available for the plants. Water these heat and light loving plants regularly during their active summer growing period. To prevent rot, they must be allowed to dry out completely between watering.
Adeniums tend to go semi-dormant in the winter, losing most or all of their leaves. They don’t need much water during those times, but also don’t appreciate being completely bone dry for months on end. They like an occasional soaking drink, as long as they can dry quickly. Even though the plants might be leafless, they are still actively using sunlight. Lightly scratch a Desert Rose’s silvery-grey stem. Underneath is a thin green layer that continues the job of photosynthesis, even without the presence of leaves.
Horticultural varieties of Desert Roses, especially grafted plants, don’t always produce the same thickening at their base (caudex) that wild forms are famous for. Some greatly resemble their frangipani relatives by not thickening at all. To produce a thick base for display, gradually remove some soil and raise the level of the plant in its pot or replant it in its bed with a portion of its roots exposed. Doing so will not cause harm, as long as any freshly severed roots are kept dry to prevent rot.