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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. Mandevilla favor humid conditions. If your plant is exposed to the sun, the soil may be too hot. Try misting/spraying the leaves with water in the mornings and keep the soil moist (NOT wet). 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.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

If your temperatures fall below 45 to 50 degrees F. in the winter you will need to bring your plant in to an indoor or sheltered location.
Here is a link with more information.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm

ASPCA does not consider Mandevilla toxic to pets.

You don't have to but keep in mind that if you do not bring them indoors in your region, the plant will likely not return. For this reason, these plants are normally treated as annuals when planted outdoors where winters drop below 50 degrees F. That said, if you don't mind replacing your mandevilla each year, they can remain outside. Otherwise, I would recommend bringing the plant inside over winter.

In your region, the plant will likely not return. These plants are normally treated as annuals when planted outdoors where winters drop below 50 degrees F. That said, if you don't mind replacing your mandevilla each year, they can remain outside. Otherwise, I would recommend bringing the plant inside over winter as opposed to planting them outdoors.

If your temperatures drop below 50 degrees F you will need to bring indoors.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm