Descent into Dormancy
A desert rose that drops its leaves in autumn is probably just entering dormancy, a natural part of its life cycle. The plant must be kept quite dry during that period, so it’s best to grow it in a container rather than in the ground where winters are wet. That way, you can move it under an overhang or take it to a sunny spot indoors to protect it from rain while it remains leafless. A large plant, such as one in a 20-inch pot, shouldn’t require any water at all for its three- to four-month dormant period. If indoors, a smaller specimen in a 6-inch pot should be watered only about once a month from October to February. In a region where temperatures seldom drop below 60 F, desert rose may not go dormant at all. If you keep your desert rose outdoors, bring inside in the fall when temperatures start dropping below 55 F.
Drought or Drowning
Because the desert rose does require more water during the months when it is actively growing, it may drop all its leaves and slip into dormancy again if it isn’t getting enough moisture. Overwatering, however, can cause root rot. To prevent either problem, grow your desert rose in sandy and fast-draining soil. During its growing season, water it any time its soil is dry two inches below the surface.
A Deficient Diet
Nutritional deficiencies can cause leaves to yellow, redden, or develop burnt brown edges or tips before they fall. To prevent such problems, feed the plant once a week in spring and summer with a 20-20-20 plant food at half strength, which should mean about 1/2 teaspoon of crystals per gallon of water for a potted plant or 1/2 tablespoon per gallon for an in-ground one. Don't feed it during its dormant period.