I have brought my mandevilla plants into the garage for years as i live in Saskatchewan Canada.This year the garage door got left open for an extended period of time.The vines themselves have now dried up. Was just wandering is it a tubour root system they have and will the vine still grow from there as it is very nice plump root system?
There are many species of the Mandevilla vine (Mandevilla sp.), and most are true tropical plants. Some grow in semi-tropical, and even a few in drier ecosystems. Interestingly, they are of the Apocynaceae or dogbane family. This family includes many common garden plants here, including the lovely Amsonia, Vinca, Stephanotis and Milkweed. Most have a milky sap, and many, including the Mandevilla, are toxic.
You have been fortunate to be able to keep your mandevilla in your garage all these years – as a tropical, they don’t survive below 45 F or 7C.
Even so, I think your instincts are correct. These fleshy organs supply the food and water needed to get the vine growing when warm, moist conditions return. If they are not wrinkly, and are still plump, then they are likely still alive. At this stage, leave the plant as it is, and make sure that the soil isn’t bone dry. It isn’t necessary to soak them. Because of the latex sap noted above, don’t cut them, as they may bleed and cause a reaction. If the vine is completely dessicated, it can be cut. Either way, if it’s a rangy plant, cut to about 12 inches or 30 cm above the pot. Whatever you do, don’t let them refreeze. It may be a good idea to bring the plant into the house, and in a dark area of the cellar to prevent this. Keep it very lightly moist, off the floor for thorough air circulation, and away from windows and drafts.
If new growth happens, it is unclear if it would come from the roots, or the old wood. If you see evidence of growth, water it more often, and bring it into the household light. When frost damage is past, or when you would normally put it outside, keep it in shade for a few weeks until the plant re-aclimatizes to sunlight. Good luck – it’s worth a try!
For more information about growing Mandevillas: https://www.clemson.edu/extension/hgic/plants/landscape/groundcovers/hgic1109.html