Mandevilla is a genus of flowering vines with showy, trumpet-shaped blossoms in various vibrant colors, making them highly popular for patios and decks. In cold climates, however, they must be brought indoors for winter. Fortunately, a greenhouse is an ideal winter haven for Mandevillas as long as the temperature there stays above 50˚F.
Before bringing Mandevillas into your greenhouse, spritz them with bug spray to avoid indoor insect infestations. Try the spray on one leaf before spraying the entire plant. Don’t be concerned if the plants drop quite a few leaves when first set inside the greenhouse. They are usually just acclimatizing to their new home.
Mandevilla plants may need repotting after the summer growing season. To check if they are root-bound, remove them from their current pots and see if bare roots are curling around the pot’s bottom. If so, untangle the roots using a three-pronged hand hoe or your fingers. Then, spreading out the freed roots, set each plant in a larger pot with several inches of planting medium in it. This medium should be 2/3 potting soil and 1/3 sand for good drainage. Fill in around the plant using the same soil/sand mixture and tamp down the surface well so that it’s at least an inch below the rim. Keep the soil on the dry side. These plants don’t need lots of water during winter.
Now is also the time to prune your Mandevillas. For some of the newer varieties that have mounded growth habits, simply trim off any straggly vines, creating a nicely bush-shaped plant. Don’t worry if you think you’ve cut off too much. The plant will grow back. The traditional climbing varieties of Mandevilla can be cut back even more. Some people prune them for the winter to only 8-10 inches, but I leave mine about 3 feet tall, making sure to remove any vines that are leafless or appear damaged. I use a teepee of bamboo canes to give these plants something to grow up while they’re in the greenhouse.
In spring, Mandevillas will show signs of renewed growth. Feed them now with a general-purpose fertilizer to get them going and then switch to a high potash fertilizer (such as Miracle-Gro “Bloom”) to encourage flowering. Keep the soil moist but not too wet. When outdoor temperatures are above 55˚F, the plants can be set outside for another summer of magnificent flowers.