A vigorous shrub that can mature at 6 to 10 feet tall and 5 to 8 feet wide, Limelight hydrangea (Hydrangea paniculata 'Limelight') produces massive flower head clusters that make branches arch from the weight. The flowers appear in summer, first a light greenish cream and aging to creamy white. This and other panicle hydrangeas bloom on "new wood" -- from the new branches that sprout each spring. Schedule pruning in late winter or very early spring.
The Limelight hydrangea may be pruned in fall, winter or early spring, but never in late spring or summer while the plant is developing flowers. Fall pruning is best done only in mild winter climates, such as in U.S. Department of Agriculture plant hardiness zones 7 and 8. In regions with long, cold winters, withhold pruning until late winter or early spring after the coldest weather has passed. Dead or broken branches may be pruned off any time of year.
Need for Pruning
The Limelight cultivar, like other panicle hydrangeas, doesn't need pruning every year necessarily. Besides promptly removing any dead or damaged branches, it's more important to worry about removing any branches that rub against each other, causing a bark wound. If your Limelight hydrangea grows quite large, a reductive pruning each spring may help control the size by summer's end. Some gardeners prune out one-third of all branches in a shrub each late winter to continually rejuvenate the plant.
Effects of a Harsh Pruning
If you harshly prune back all branches of the Limelight hydrangea to stubs 4 to 6 inches above the ground, expect vigorous but slender regrowth to follow. This results in weaker branches that produce much larger flower heads in summer. However, expect the flowers to bend the stems and flop to the ground. To avoid a floppy hydrangea, cut back the Limelight hydrangeas so that 18- to 24-inch branch stubs remain. The flowers that summer may be slightly smaller, but the shrub's structure tends to hold the flowers more upright for a more attractive display in the garden bed.
Utilizing the Flowers
Pruning in summer during or after flowering to harvest flower heads may also be done on Limelight hydrangea shrubs. Do not worry about stem length or location when you cut branches for flower bouquets. You can thin out or correct the pruning imbalance at the end of winter before the shrub starts growth again. Limelight hydrangea's flowers dry well and turn creamy beige, making them long-lasting additions to dried flower arrangements, too.