If you are going to grow hardy mums as a fall display in containers you probably should consider them temporary color. It's very difficult, if not impossible, to keep them alive over the winter in pots. Outside, the key to mums is to get them planted early, keep them well watered and offer them a bit of a mulch to get them through their first winter. Hardy mums are not has hardy as many other perennials and some of them will come back for years and others will not. They are more iffy on overwintering than other perennials. If you truly want mums to winter over the best time to plant them is in the spring when they are offered as small plants. That way, they have a full summer to get established and don't die so easily. But, it is possible to overwinter them, you just have to plant early and be very kind to them water-wise.