If your mums (Chrysanthemum x morifolium) survive the winter, you'll see new growth developing around the base of the plant in early spring. The old, dead growth from last year can be clipped away. If nothing develops at the base of the plant, it's a sign that the plant did not survive the winter. Although garden mums are often called hardy mums, they may not survive the winter if drainage is poor or if you live in an extremely cold climate. If numerous shoots have overwintered, divide and separate the clump. Three to five vigorous shoots are enough to make a showy clump. Once new shoots start to develop, give them a little slow-release granular flower fertilizer and leave them alone. When they are about 6 inches tall, pinch back the tops of each stem by 1-2 inches or so. This promotes compact, bushy growth later on.
To prevent delaying the bloom, stop pinching after late June. Normally garden chrysanthemums bloom only in fall when nights are long and days are short. (Flower bud development is a response to day length.) The mums you find blooming in spring in garden centers have been grown in a greenhouse, where they get short days to force early bloom. A few cultivars are day-neutral, flowering at any day length. So yours may develop some blooms by midsummer if not pinched. Also heat can delay bloom.