Glads aren't considered hardy here (they're zone 7 hardy I believe), but they usually do return for me. I have noticed some varieties seem to be hardier than others, but given some protection, most will typically survive the winter. Planting deep usually provides enough protection, but they do produce a new corm on top of the old one each year, gradually working themselves to a shallower depth, so a couple of years is about as long as they last for me without digging and replanting. If you have them near a foundation they will probably last longer, I remember my mother had some next to the house for many years.
