Well I would think they would not survive. First of all, mums come in two basic types: florist mums and hardy mums. Florist mums are those you get as gifts and see for sale at grocery stores, etc. They are not hardy and are mainly grown for the flower market and should be tossed after blooming. Hardy mums, are garden plants that will come back every year, BUT they are not always truly hardy and often don't come back or will come back for several years and then disappear. I doubt any of them would survive in a pot in an unheated greenhouse mainly because the soil would freeze in a pot that's not protected in some way. Hardy mums are just not that hardy to withstand that kind of treatment. At this time of year, the best thing to do is to get them in the ground as soon as possible, clip them back after they go dormant later in the fall, and mulch them with leaves to provide some protection during the winter. Hardiness also seems to vary between different hardy mum varieties. Some are just tougher than others. And, of course, it depends on how tough of a winter you have. If nothing else, I'd sink your pots in the ground and provide some protection over the winter. Being exposed to the elements in an unheated garage will probably kill them.