I bought a mandevilla last summer at the Hampton Court RHS show. It flowered beautifully and had lush foliage. It is on the window sill in the conservatory – light but not direct sunlight. Since autumn it has continued to grow and produce new foliage which drops off after a week or two. I now have a plant that is all stem and no foliage. Please, can you tell me what I am doing wrong? The temperature in the conservatory has been a constant 18 degrees C. and I have watered the plant regularly.
With the lower light conditions going into Autumn, your plant is trying to go into a dormancy period.
Your plant will lose leaves and will require less water.
Here is an article that will help you.