Hydrangea macrophylla 'Fasan'
Hydrangea macrophylla 'Purple Majesty'
Hydrangea macrophylla 'Twist n Shout'
Hydrangea macrophylla 'Decatur Blue'
Hydrangea Pruning - Mal Condon, Vice-President, The Cape Cod Hydrangea Society & Curator-Hydrangea Collection, Heritage Museums & Gardens
To prune hydrangeas effectively it’s important to know the plant’s species - macrophylla, serrata, arborescens, paniculata, quercifolia (the oakleafs), and anomala (the climbers). The reason species is so important to pruning is to know whether the plant flowers on new or old wood. ‘New’ wood is growth of the current season with flowers developing immediately from new growth. Arborescens and paniculatas produce blooms entirely on new wood. ‘Old’ wood is growth from the previous growing season; flowers form in early summer from buds developed the previous autumn. Importantly, these buds must survive the winter before developing into flowers during the following late spring. Macrophyllas, serratas, quercifolias, and the anomalas all flower on old wood.
The new wood arborescens and paniculata species are truly ‘bulletproof’ - extremely cold hardy (zone 4) and reliable annual bloomers. Best pruning time is late fall - November through early December - in our zones 6 & 7.
The old wood quercifolias and anomalas require relatively little pruning. They are both hardy to zone 5 and are best treated as ‘free growers’ left largely to their own growth development. Pruning is a containment exercise - keeping them within ‘bounds’ and removing occasional winter-kill tip sections. Pruning Quercifolias can be left to last - late April - as they are the last species to break bud. For petiolaris, its best to prune after flowering in late June, simply removing spent blooms and errant stems.
Macrophyllas and serratas species have similar flowers with their distinctive lacecap and mophead bloom forms. Mopheads are identified by their full, roundish heads of large petals; lacecaps have tiny fertile flowers in the centre of the bloom with an outer border of larger sterile petals. Both species respond to the same pruning timing and techniques. Macrophyllas are the most popular hydrangea, but they are the least cold-hardy of the species and often require some supplementary winter protection to insure bud survival over a long cold winter. The serratas, though typically more delicate in physical stature, are more cold hardy.
Dead or ‘sun-burned’ blooms can be removed just after flowering. This actually encourages successive blooming on selected varieties that possess this repeat blooming characteristic. The primary pruning time is April; spent blooms and winter-killed tip stem sections can be cut back to the first strong, healthy pair of buds lower down the stem. Aging wood, with exfoliating (peeling) bark, should be removed to encourage the production of new and more floriferous replacement growth. These stems can be cut back cleanly to the ground in late winter when it’s easier to get inside the plant’s base. For newly established plants - 3 gallon and under container size - only minor pruning is necessary for the first year or two. Late spring frosts can be particularly damaging as the tender unfurling leaves of new bud growth is especially susceptible. Prune back damaged stems to just above the first live pair of buds on healthy wood.