How should I prune my hydrangeas?
Three species of hydrangea are commonly grown in Iowa. Pruning practices are based on the flowering characteristics of each species.
The smooth hydrangea (Hydrangea arborescens) grows 4 to 5 feet tall. Smooth hydrangeas bloom on new growth. Plants produce white flowers in flat to roundish clusters in late spring and summer. ‘Annabelle’ is a widely grown cultivar. Smooth hydrangeas can be pruned back to the ground in March or early April.
The panicle hydrangea (Hydrangea paniculata) is a large shrub that grows to a height of 10 to 12 feet. Panicle hydrangeas bloom on new growth. Plants produce cone-shaped flower clusters in late summer. Flowers are initially creamy white, but gradually change to pink or dark rose. Several cultivars, such as ‘Grandiflora’ (commonly known as PeeGee hydrangea) and ‘Tardiva,’ are available. The size of panicle hydrangeas can be reduced by selectively removing a few branches in March or early April. Rejuvenation pruning (removal of several of the largest stems near ground level) of large, old shrubs can also be done in late winter/early spring.
Numerous cultivars of bigleaf hydrangea (Hydrangea macrophylla) are available. Flower color is determined by soil pH. In acidic soils, flowers are blue. Flowers are pink in alkaline soils. Most older cultivars, such as ‘Forever Pink’ and ‘Nikko Blue,’ bloom strictly on the previous year’s growth. Unfortunately, ‘Forever Pink,’ ‘Nikko Blue,’ and most older cultivars do not bloom well in Iowa as plants often die back to the ground in winter.
Many newer cultivars, such as Endless Summer® and ‘Blushing Bride,’ bloom on the previous year’s growth and new growth. After a mild winter, these cultivars bloom in early summer on the previous year’s growth and late summer on the current year’s growth. After a cold winter, Endless Summer®, ‘Blushing Bride,’ and similar cultivars die back to ground level but bloom in late summer on the current year’s growth.
For maximum bloom, wait until the shrubs begin to leaf out before pruning bigleaf hydrangeas. When growth appears, prune out all dead wood.