We purchased 2 Dipladenia Rio plants last year that were a riot of color and very beautiful. They were in the frost-free conservatory (we live in Torbay, Devon so winters are mild with hardly any snow or frost) but they died with the branches hollow. When taking them out of their pots, there are a number of tubers, looking like long potatoes. Should these be pot up and, if so, do we separate them? Would love more of these plants.
The purpose of tubers on mandevilla roots is to hold water reserves, they can't be split off and used for reproduction. When it comes to propagation, Mandevilla vines can only be propagated from seeds or cuttings. However, you can extend the life of your existing plants by overwintering them in pots - this extension article gives you two different methods on how to do so:
When overwintering mandevillas, however, you need to water them only occasionally to ensure they don't dry out. Since yours appeared to be dried out and dead, I don't think you will be able to resuscitate it.
For more information on the care of dipladenia, please visit the following link: