In the realm of gardening questions, answering “When can I prune my hydrangea?” can be the most complicated. This is because of the number of hydrangea types that grow in this region, requiring an understanding on our part before the pruning saw is even employed.
It’s important to note that pruning most woody plants in April, May or June is best in order to minimize wound closure time and maintain plant health. While this may be the ideal time from a plant health perspective, it may mean not pruning at all is key to having reliable blooms. This is where an understanding of whether the plant blooms on new (current season’s growth) or old wood (stems that developed last year) makes all the difference in having flowers or not.
Hydrangea arborescens, sometimes called smooth hydrangea or snowball hydrangea, produces white flowers on current season’s wood. So no matter how hard the winter or how severe the pruning (in April, May or June), the smooth hydrangea will still reliably flower. This old-fashioned favorite has well-known newer introductions such as ‘Annabelle’ and Incrediball™.
Hydrangea paniculata is a large shrub that produces huge panicles of white to blush-pink flowers. Well-known varieties include Limelight®, PeeGee, ‘Tardiva’, ‘Burgundy Lace’, and ‘PeeWee’. Flower buds are reliably winter hardy. Pruning beyond removal of dead stems, rubbing branches and spent blooms is really not necessary but can be done in April, May or June.
Hydrangea macrophylla is the most problematic because of hardiness issues in our area. It produces pink flowers (or blue flowers if aluminum sulfate is added to the soil), primarily on old wood. Rough winters and hard pruning destroy flowering wood, affecting bigleaf hydrangea’s ability to flower. Varieties that belong to this group include ‘Nikko Blue’, ‘Pia’, and Endless Summer®. Blooms can occur on both new and old wood of the Endless Summer® series, which lends some resiliency to flowering after winter dieback or severe pruning means loss of last year’s wood. But be aware that hot dry periods during the growing season will affect Endless Summer’s ability to bloom on new wood too. It’s best to site all bigleaf hydrangeas where they’re protected from winter winds and, except for dead wood removal, refrain from pruning them altogether (and hope for the best).
Hydrangea quercifolia produces creamy white flowers and oak-shaped leaves. While resources state flower buds are killed when winter temperatures drop below -10° F, I’ve observed the one in my yard consistently flowering, even when the winter has thrown -25° winds at it. Pruning the oakleaf hydrangea involves removal of dead twigs, spent flowers, and rubbing branches, and really nothing more. ‘Alice’ and ‘Snow Queen’ are two well-known varieties of the oakleaf hydrangea.