From a botanical perspective, the single most important factor determining when to prune your hydrangea is its flowering habit, specifically whether it sets its flower buds on "old wood" or "new wood." This terminology refers to the age of the stems from which the flowers emerge. "Old wood" is growth that occurred in the previous season or earlier. These stems have already undergone a growth cycle and have spent the fall and winter developing dormant flower buds at their tips or along their length. "New wood" refers to the fresh, green stems that emerge from the plant's base or from existing branches during the current growing season. Pruning at the wrong time can inadvertently remove these pre-formed buds, resulting in a complete loss of blooms for the year. Therefore, understanding your specific hydrangea's reproductive strategy is the key to proper pruning.
This group includes the classic mophead and lacecap varieties, which are typically cultivars of Hydrangea macrophylla, as well as the stunning oakleaf hydrangea (Hydrangea quercifolia) and the climbing hydrangea (Hydrangea anomala subsp. petiolaris). For these plants, the flower bud formation process is a long one. Shortly after they finish blooming in the summer, they immediately begin allocating energy to create the flower buds for the following year. These buds remain on the stems throughout the fall and winter, vulnerable to the elements and to misplaced pruning shears.
The ideal pruning window for these types is a brief period in mid-to-late summer, just as the flowers begin to fade. At this point, you can see the plant's structure clearly. The goal is to deadhead the spent blooms by cutting the flower stem back to a pair of healthy buds. If necessary, you can also remove any dead, damaged, or spindly stems at their base to improve air circulation and shape. However, any significant pruning or cutting back performed in the fall, winter, or spring will remove the upcoming season's flowers. From the plant's viewpoint, a harsh winter can also damage these tender buds, which is why these types sometimes fail to bloom after a cold season, even without any pruning.
This category is dominated by the panicle hydrangea (Hydrangea paniculata) and the smooth hydrangea (Hydrangea arborescens). These species have evolved a different reproductive strategy. They initiate flower bud development on the new growth that emerges in the spring of the same year. This makes them much more forgiving when it comes to pruning timing, as they are not carrying precious flower buds through the winter.
For these plants, the optimal time to prune is in late winter or very early spring, while the plant is still fully dormant but just before new growth begins. This timing allows you to clearly see the plant's framework. You can be quite assertive, cutting the plant back by as much as one-third to one-half its height to encourage strong, sturdy stems that will support the large flower heads. You can also remove any weak or crossing branches. From the plant's perspective, this severe pruning acts as a signal, triggering a vigorous flush of new growth from the base and lower nodes on the remaining stems. Each of these new stems has the potential to produce a flower panicle in the coming summer.
A modern development in hydrangea breeding has created a third category: reblooming varieties, such as the Endless Summer series. These are typically Hydrangea macrophylla cultivars that have been selected for their ability to flower on both old wood and new wood. This dual-flowering habit provides a significant advantage. The plant will produce an initial flush of flowers in early summer from buds on the old wood. Then, it will continue to produce blooms later in the season on the new growth that occurs after the first flowering.
For these plants, the pruning strategy is more flexible. If you need to shape the plant or remove dead stems, you can do so immediately after the first flush of flowers fades. Even if a late frost damages the old wood buds, the plant can still produce flowers on new growth later in the season. However, the most prolific blooming will still occur if you can preserve the old wood. Therefore, the best approach is often to simply deadhead spent flowers throughout the season and perform any major structural pruning only if necessary, understanding that it may slightly reduce the total number of blooms.
