Mums require at least six hours of sunlight each day. If they're in pots, move them to a brighter area. If they're in the ground, transplant them in the late summer or early fall.
To revive your flowers, ensure the soil is always moist — if it's not, water the plant immediately. Be careful not to make the soil soggy, as overwatering causes its own problems.
Repot your mum immediately after bringing it home. Use a container that's a couple of inches bigger to give the roots room, and use a well-draining potting mix.
Mums in the ground may suffer if the soil has clay or is too dense, so backfill the holes with quality soil or amend the dirt with compost, mixing it in 8-12 inches deep.
Mums can survive lows in the 20-30 degree range, but frost is more than just cold temperatures. Signs of frost damage include black leaves or wilted flowers.
When they're in the ground, cover them with burlap, sheets, or towels whenever there's a chance of frost. Bring containers inside for the greatest amount of sheltering.
Wait until the spring, when you can see at least 6 inches of growth. To prune, snip an inch off the stem — it's safe to do this a couple more times throughout the summer.