Why are the leaves of my mandevilla plant turning yellow?
There are a number of reasons why the leaves may turn yellow. One might be overwatering. Too much water causes yellowing of the leaves and mandevilla do not like overly wet feet. Another possibility could be too little water. If this is the case, the yellow leaves would be curling up as well. Mealy bugs can occasionally affect these plants, causing them to become weak and leading to yellow foliage. Finally, plant stress due to a change in location, temp., etc. can lead to yellow leaves.
Can Mandevilla stay outside for winter or does it need to be dug up in Falmouth, Mass. ?
In areas such as yours, you may want to bring it indoors over winter. In warmer regions, it can be left outside. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
I have had a Giant Crimson Mandevilla on the deck all summer. It is growing rapidily as cooler nights begin. Can this plant be pruned or should a larger trellis be added to accomadate the new growth when it comes indoors?
You can prune it before bringing it in and the plant will actually be happier if you do. You can cut it back by up to 1/3 before bringing it indoors.
I live in Michigan and didn't bring my Mandevillas in before the first frost and they got frost bite. I have cut it back, but the leaves look terrible and all flowers died. Will my plants survive?? I don't want to go to the trouble of wintering them inside if they are not going to make it.
Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm
Kept the plant covered in the winter. I live in southern Alabama. Now it has profuse leaves and new vining only at the top. The bottom half is just the old vines. Can it be cut back to force it to bush out again?
Yes, you can cut it back some. In the future, you may want to pinch your mandevilla. This method of pruning will create a bushier and fuller plant. To pinch your mandevilla vine, simply use your fingers to pinch off 1/4 – 1/2 inch off the end of each stem.
You want to watch for transplant shock. This article will help with preventing that:
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm
"Bougainvillea is a thorny, wiry vine that is native to Brazil and Peru and grown in tropical and subtropical areas. Mandevilla is a flowering vine that is native to South America. It is often grown in the United States as an annual flowering vine."
Thanks for the quick answer, Nikki.