The best time to prune all spring-flowering shrubs is right after they stop blooming and the flowers fade. The reason for that is that the plants will be developing flower buds over the summer. Those will produce next year’s flowers. So if you trim the plant back in summer, fall, or winter, you’ll be depriving yourself of next year’s blossoms.Rather than drastically cutting back an overgrown azalea, it’s best to cut back to about 12 inches one-third of the oldest stems. Do this again next year and the year after. Then your plant will be a good size and renjuvenated.