I bought a Alice Dupont Mandevilla last year…by season end it was absolutely beautiful. I was told by a fellow gardener to spray it with a pesticide before bringing it in. I did. As the days went by, the leaves dropped off – totally. I kept it at the east window -a couple months ago green little leaves began to appear, a new vine about an inch grew, wilted and turned brown and the same thing just happened all over again. Is it done for? Thanks!
The mandevilla is another tricky tropical plant. Like the candy corn vine the mandevilla will appreciate a warm, though not sunny, spot for the next few weeks. Keep your mandevilla out of direct sun (that sunny window will burn tender new leaves) until it has at least 6 inches on the vine and more than one set of leaves. If you are seeing sprouts the plant is probably still viable. I suggest repotting the roots in new potting soil, watering it lightly and seeing what happens. When you repot it, check the roots; healthy roots will be firm, white or light tan, and flexible. If the roots of your mandevilla are black or brown, mushy or stringy the plant isn’t likely to grow. If there are spots of black mushiness, cut away that part of the root and replant only the healthy root. Healthy roots, attached to the crown of the plant should grow a plant. Most importantly, no direct sunlight until the leaves have expanded, moderate amounts of water and no fertilizer until you are ready to put it outside. Remember the mandevilla likes it hot; so don’t put it, or any tropical plant, outdoors until temperatures get above 50 degrees at night.
Good luck with your mandevilla!