Hardy mums, which are those that can survive a winter, should be cut back in the spring as new growth emerges. Optionally, mums can also be cut back spring through midsummer to encourage fuller blooming and a better shape.
After fall bloom is completed, allow the buds and foliage to die naturally. Some gardeners choose to prune in the fall, but pruning in spring increases the chance of winter survival. Add a layer of mulch on top of dead foliage for the winter and then remove it in early spring.
In the spring, once new growth is seen, cut back dead foliage to 3 to 4 inches above ground. From spring until the beginning of July, mums can be cut back 2 to 3 inches once they reach 6 to 8 inches tall.