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. Thank you
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.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-mandevilla-indoors.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/fertilizing-mandevilla-plants.htm
Can I leave it outside for the winter in Jackson, MS?
Here is an article to help with wintering a Mandevilla.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm
Should I very carefully take the trellis out of the pot that the plant is on and train it onto a larger trellis or just leave it on the small one in the pot and let the plant continue onto a larger trellis?
You can take the trellis out of the pot if you want. Just do it carefully. You didn't say whether or not you live in a tropical area. If you do, you can train your mandevilla onto any trellis you want. If you live in the temperate zone, any trellis you use will have to be portable because you will have to bring the plant indoors in the winter and early spring.
We have ours in two big pots. One has tendrils on it and the other does not.
All healthy mandevillas develop new shoots and tendrils. Does the plant in question look healthy? A plant with compromised health may not grow as well.
Your one plant is in active stage of growth, the other one is probably just being a late bloomer. The best thing I can prescribe is patience and ensuring that your mandevillas receive the care outlined in the following article:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
What makes the leaves turn yellow?
There are a number of causes for this; some of the most common ones can be found here:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm
Will I kill this year's flowers if I cut back in spring? I brought my mandevilla inside in my zone 6 garden. Its leaves all dropped and it was kept in a 55-60 degree room with minimal watering. I now want to start putting it outside during warm days to acclimate, but I've got tons of bare vines all over. Do I cut it back now or leave it? Next time I guess it should be cut back in the fall.
Mandevillas can be pruned in late winter or early spring before growth begins - you can even prune it to the ground. It will grow back quickly (the same summer) and reward you with the astounding blooms on new vines which will develop from the base of the plants.
For more information on the care of mandevilla, please visit the following link:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
Is this a good plant for hummingbirds and can I leave it out in the winter?
Mandevilla needs to be protected from temperatures below 60 degrees F.
Here are some links that will help you with care information and how to winter over this plant.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm
Here is an article on attracting Hummingbirds to your garden.
https://www.gardeningknowhow.com/garden-how-to/beneficial/attracting-hummingbirds.htm