By Andrew Bunting
PHS Vice President, Public Horticulture
Hydrangea Pruning Demystified
Over the years I have gotten more inquiries about how to prune hydrangeas than perhaps any other subject. With so many species of hydrangeas, many require different pruning approaches.
Most people are familiar with the large blue or pink mophead hydrangeas, Hydrangea macrophylla, big leaf hydrangea. For most Hydrangea macrophylla cultivars, the pruning should be minimal -- minimal meaning only removing dead or damaged stems. Another question I often get is “Why is my hydrangea not blooming?,” and I would learn it was because they cut the stems to the ground, which greatly impedes flowering the subsequent year.
There are other species, too, that require minimal pruning like the oakleaf hydrangea, Hydrangea quercifolia; Japanese Mountain Hydrangea, Hydrangea serrata, and any of the hydrangeas with fuzzy leaves like H. involucrata, H. villosa or H. sargentiana.
Two species of hydrangea can be pruned severely -- in fact, relatively hard pruning will result in exceptional flowering. Both Hydrangea paniculata, panicle hydrangea and Hydrangea arborescens, smooth hydrangea will bloom on “new wood,” which means that stems that grow during that growing season will produce the flower buds and hence flower.
Hydrangea paniculata is one of the most common of the summer flowering shrubs, and rightly so. Ounce for ounce there is not another summer flowering shrub that is its equal. In the summer, this medium to large-sized shrub is covered in large panicles of white flowers. ‘Limelight’ has flowers that start lime, turn pure white and fade to lime. ‘Little Lime’ is its diminutive version. ‘Phantom’ has very large flower heads, and the commonly called PG hydrangea is an old-fashioned cultivar, Hydrangea paniculata ‘Grandiflora’.
You’ll want to prune H. paniculata in mid-March to early April and prune it to the point where you want to keep it a certain size. I often just prune back the previous season’s growth, but you can be even more aggressive with your pruning if you desire -- or not prune at all! There is an incredible 25 feet tall Hydrangea paniculata ‘Grandiflora’ in Ridley Park, Delaware County, that I marvel at every summer when it is in full flower in July.
Perhaps, one of the most common hydrangea on the planet is Hydrangea arborescens ‘Annabelle’. Like H. paniculata it blooms on “new wood.” If you choose, you can cut it back to 8 to 10 inches every spring, and then it will only reach four feet tall. It is adorned with large mop-head white flowers in mid-Summer. Invincibelle Spirit™ is a pink form and Incrediball™ has massive clusters of flowers. I think both H. paniculata and H. arborecens are two of the best summer-flowering shrubs to plant in our region.