Don't Kill Your Hydrangeas! Here's When It's Too Late To Cut Them Back

Hydrangeas are among the most popular perennial plants out there. Numerous different types of hydrangeas can survive in several different climates, all the way up to USDA Zone 11, which will be a joy for those wanting to grow hydrangeas in hotter climates. A big part of your hydrangea's survival is knowing when to cut them back. This can be nerve-wracking because you don't want to do anything that might kill your hydrangeas. Ultimately, it becomes too late to prune the moment each variety sets its flower buds, which is why identifying whether your hydrangea grows on old wood or new wood matters so much. That's because new wood hydrangeas should be pruned in late winter, while old wood hydrangeas should be pruned immediately after blooming in the midsummer. 

Although general pruning advice is typically to cut when plants are dormant, usually during winter, this does not apply to all varieties of hydrangeas. Instead, this tactic only works with new wood hydrangeas, as they are the species whose flower buds only emerge from brand new wood growth in the spring. These types of hydrangea, such as smooth hydrangeas (H. arborescens) or bracted hydrangeas (H. involucrata), should be cut back in late winter or early spring before any new growth arrives. Once the fresh flowers emerge in late spring or early summer, it is too late to prune new wood hydrangeas. You can also prune back new wood hydrangeas in the fall to prep them for winter. As long as their flowers have dropped and they are fully dormant, you're good to go. 

The difference between old and new wood hydrangeas is pretty straightforward. The flower buds for the new year in old wood hydrangea varieties, such as bigleaf hydrangea (H. macrophylla) and mountain hydrangea (H. serrata), actually begin to form in the old wood branches during the fall. Those buds are then protected as they overwinter inside the wood before emerging in the spring.

As such, late summer and fall are too late in the season to be pruning old wood hydrangeas. If you do, you are effectively removing next year's growth and exposing any remaining buds to potential frost damage or killing them off entirely. Old wood varieties don't really need much pruning to begin with, but if you are going to, do so right after the summer blooms have finished in early August. There is a short window where the new bud production has not begun yet, so you won't risk harming the plant when you do this.

Unless you plan on using the blooms for cut flowers, you don't need to be too concerned with keeping your hydrangeas pruned in order to properly care for them. Make sure to always remove any dead or diseased-looking branches, but then leave things at that. Too much pruning too late in the season will probably end up killing your hydrangea, so it's typically best just to leave them alone.