Pink Diamond hydrangea (Hydrangea paniculata "Interhydia") provides long-stemmed, large blooms after pruning in spring. Growing up to 15 feet tall and wide, Pink Diamond is hardy U.S. Department of Agriculture plant hardiness zones 3 through 8. Before and after pruning this shrub, sterilize pruning shear and pruning saw blades by wiping them with a cloth that was dipped in rubbing alcohol. Use a pruning saw to cut stems 1/2 inch in diameter or wider. In some areas of the United States, Pink Diamond is invasive.
Pruning for Large Blooms
Pink Diamond hydrangea produces large flowers on long stems when most of its stems are pruned in late spring. Dense, cone-shaped clusters of white flowers up to 12 inches long and 8 inches wide appear on Pink Diamond hydrangea in summer. Over time, the flowers mature to pink, and fade to tan.
Prune the shrub in late spring when most of its leaves have expanded. Select five to 10 of the thickest stems to keep, evenly spaced around the shrub, and prune all the other stems at ground level. Prune the remaining stems at half their original length, just above an outward-facing leaf. This also produces a smaller shrub. Pink Diamond is a tough shrub that soon recovers from hard pruning. If it isn't pruned, the shrub bears more but smaller flowers.
Don't prune a young Pink Diamond hydrangea for large blooms. Wait until the shrub is three or four years old.
Pruning After Flowering
Pruning after flowering encourages Pink Diamond hydrangea to produce a second flush of blooms. This shrub flowers on this year's wood. Pruning after flowering doesn't affect next year's display, but sometimes the shrub blooms again in early fall.
Prune Pink Diamond hydrangea when the flowers are pink and they begin to fade to tan. Prune crowded and crossing branches where they join the rest of the shrub, and prune one-third of the length of the remaining stems.
Pruning to Control Size
Late winter or early spring pruning controls the size of Pink Diamond hydrangea and removes branches damaged by severe winter weather. Pink Diamond can grow up to 15 feet tall and wide, but usually grows 5 to 10 feet tall and wide, and tolerates pruning to a smaller size.
Before green leaf buds appear on Pink Diamond, prune dead and damaged stems. Prune dead stems where they join the rest of the shrub, and prune damaged stems 2 to 3 inches below the damaged area. Prune the remaining stems by half to control Pink Diamond's size, just above leaf nodes that face away from the center of the shrub. Leaf nodes are small bumps on the stem.
Pruning for Cut Flowers
Pink Diamond hydrangea blooms make attractive cut flowers, fresh or dried. Prune Pink Diamond blooms at the stem bases when more than half the individual flowers have opened for fresh blooms; or wait until the flowers have faded to tan for blooms to make dried flower arrangements. Tie the stems of faded blooms with twine and leave them to hang indoors in a cool, airy spot. When the petals are crispy and dry, the blooms are ready to use.
Some gardeners prefer to leave faded Pink Diamond flowers on the shrubs over winter, where they look attractive when frosted.