For shrubs to perform well in the landscape, home gardeners must prune them properly. Proper pruning helps maintain plant health, control or shape plant growth, and stimulate flower development.
Four species of hydrangea are commonly grown in Iowa. Pruning practices are based on the growth and 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' (introduced in the early 1960s) is the most widely grown cultivar. Newer cultivars include Incrediball® and Invincibelle® Spirit. Smooth hydrangeas can be pruned back to the ground in late winter/early spring.
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. Numerous cultivars, such as 'Grandiflora' (commonly known as PeeGee hydrangea), 'Tardiva,' 'Limelight,' and Vanilla Strawberry™ 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.
Oakleaf Hydrangea (Hydrangea quercifolia) has distinctive coarse, lobed leaves resembling oak leaves with white clusters of flowers appearing in June and fading to pink as they age. Plants bloom on previous year's (old) growth and reach 6 to 8 feet tall and wide with time. Oakleaf hydrangea is hardy to USDA Hardiness Zone 5 making it a good plant for southern Iowa. This species performs best with minimal pruning. When needed, prune back stems after flowering. Winter damaged stems can be pruned out when noticed in spring after the new leaves emerge.
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.
Newer cultivars, such as Endless Summer®, 'Blushing Bride,' and BloomStruck®, 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,' BloomStruck®, 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.