If you planted mums in the fall, and just left them in the ground through the winter, you might yourself with fresh blooming greens. Start pinching them back and trimming them now, through about mid-August. Don't worry if they seem short, you have plenty of time for flowers to grow and bloom. This will help create full beautiful flowers with lower branches thus preventing your mums from blooming too early and being top heavy.
These pink lilies are surrounded by other plants in bloom, such as Clematis 'Henryetta' on the trellis, lady's mantle (Alchemilla mollis), and Lavender 'Munstead Wood'. Fescue 'Elijah Blue' is in the lower right corner, catching lily petals