Mandevilla is a beautiful plant and beautiful flowers grow more when the plant is in hot weather. It is important to prune this plant at least once a year in order for the plant not to grow out of control. The best time to prune is in late winter or early spring. If you choose to cut it in summer where the flowers bloom the most but be caution to prune new growth excessively.
Key Takeaways:
- Cutting back a mandevilla vine is best done in late winter or early spring, before the plant starts to produce new growth
- You should never prune new growth vigorously, because this will result in fewer flowers.
- A single stem of old growth that’s cut back should sprout several shoots of new growth.
“This is why pruning mandevilla vines at least once per year is recommended.”
http://www.gardeningknowhow.com/ornamental/vines/mandevilla/prune-mandevilla-vines.htm