Grown for its intriguing trunk form and branching habit, in addition to because of its showy pink to crimson flowers, desert rose (Adenium obesum) is native to Africa. It rises at U.S. Department of Agriculture plant hardiness zones 10a through 12. Desert rose is damaged by temperatures below 35 degrees Fahrenheit but above 32 F, but if the cold-damaged places are removed, it may recover. It’s usually killed by temperatures of 32 F or lower.
Dormant Cold Tolerance
Desert rose is most cold-tolerant if it’s dormant and maintained completely dry. It enters dormancy when night temperatures fall below 50 F. The plant stops growing and loses its leaves. Reduce watering as the plant enters dormancy and stop watering once it’s dormant. Within this state, it may tolerate near-freezing temperatures. Consider maintaining desert rose in a container so that you may transfer it to colder places to pull on dormancy and protect it from cold. If you do that, choose a container with drainage holes.
Light Cold Damage
Cold damage affects the thinnest branches on a desert rose first. Thicker branches become killed since the temperature drops or lasts more. The damaged regions become discolored and lose stiffness, and may become mushy-looking. Eliminate cold-damaged branches with a sharp pruners that were disinfected by wiping the blades with a cloth soaked in rubbing alcohol. Clean the pruners between cuts to prevent the spread of infection. The moment you can identify the damaged regions, cut back the branches until the branches demonstrate just clean, healthy tissue that is not discolored. When the dead tissue is left on the desert rose, rot spreads to the rest of the plant, eventually killing it. When growth resumes in spring, then check to find out whether you’ve missed any dead tissue and eliminate it.
Heavy Frost Damage
If bigger branches or back areas have been killed, then you may have to drastically prune back the desert rose into the thickest portion of the back. It will usually resprout from the base after growth resumes in the spring. If damage occurred to the enlarged basal caudex or under, there may not be places left that can regenerate new branches. Cold can also penetrate the root zone, killing the roots. It is difficult to assess cold-damaged roots, and typically bacterial or fungal pathogens move in and the plant does not recover.
Preventing Frost Damage
The best prevention against cold damage is to grow desert rose in a container so it is possible to move the plant if cold weather threatens. Even massive plants grow well in pots. Container growing also will help prevent overwatering desert rose, which may lead to root rot. This plant does not have to undergo dormancy, and if you keep it in a greenhouse or in which temperatures are above 50 F at night, it will grow year-round. If the plant needs to endure low winter temperatures, then let it go dormant so that it may defy the natural weather patterns of sunlight. Where it grows outside in the garden, cover the plant if temperatures under 40 F threaten. Use 2 layers of covering, a frost blanket next to the plastic and plant outside. Hold the covering off the plant branches with a wooden framework.