Evergreen tropical vines, mandevillas aren’t shy about blowing their own trumpets -- trumpet-shaped flowers, that is. With blooms in shades of red, pink or white, the plants are often sold as potted annuals, since most species are only perennial in U.S. Department of Agriculture plant hardiness zones 10 to 11. The exception is the Chilean jasmine (Mandevilla laxa), which can survive outdoors in USDA zones 7 to 11, but will die back to the ground during winter at the colder end of that range. Although they churn out lots of buds, mandevillas sometimes shed them too, for a variety of reasons.
The Light of Day
Mandevillas need sunlight to bloom well, but full sun all day, every day may be too much of a good thing for some plants. According to GrowerTalks magazine, "high light during the summer can cause bud abortion." If your plant looks a bit bleached, shift it to an area where it receives full sun only in the morning and not during the brightest hours of midday.
A Long Drink of Water
Too much water or too little may also cause your mandevilla to drop buds. It has tuberous roots which rot easily when forced to endure constantly soggy conditions. If your mandevilla is growing in a pot, make sure that container has drainage holes and is filled with a light and porous potting soil rather than heavy clay, so that excess water doesn’t linger. On the other hand, you should never allow the soil to become parched either, or the plant will cast off buds that it can no longer sustain.
A Chunk of Change
If you decide to bring your mandevilla indoors in the fall to preserve it for the following summer, it will almost inevitably lose buds due to the shock of the transition. The environment in most homes is much lower in both light and humidity than what the plant will have experienced outdoors. Rather than trying to keep it growing under those conditions, you can force the plant into semi-dormancy over the winter. To do so, cut it back to about 10 inches, place it in a cool, dark place with temperatures in the 50s or 60s Fahrenheit, and water it only enough to keep the soil barely damp until spring. Make sure to disinfect your pruning tool blades by wiping them off with alcohol before pruning. This will lessen the chances of infecting the mandevilla with any diseases still clinging to the blades from a previous pruning job.
Left Out in the Cold
Despite the shock your plant will have to endure when brought indoors, it isn’t a good idea to leave it out too late in the fall, either. A sudden freeze will kill most mandevillas, with the exception of Mandevilla laxa, and they aren't fond of temperatures in the 40's F, either. Most types will begun to sulk -- and possibly cast buds -- once night-time temperatures drop below 50 degrees F., and they should be brought indoors at that time.
- GrowerTalks: Tips for Sun Parasol Mandevilla
- University of Illinois Extension: Mandevilla Requirements
- National Gardening Association: What Is Happening to My Mandevilla?
- National Gardening Association: Mandevilla
- The New Sunset Western Garden Book; Kathleen Norris Brenzel
- The Plant Book; Susan Page and Margaret Olds
- Photos.com/Photos.com/Getty Images