There is a lot of confusion about hydrangeas, and when and how to prune them. Some hydrangeas like the typical Mophead Hydrangea (Hydrangea macrophylla) set their flower buds for the next year during the previous summer. If you've ever heard the term "old wood", this is what they are referring to.
There are other hydrangea varieties such as the Endless Summer Series (Hydrangea macrophylla 'Endless Summer') that set their flowering buds on the current year's growth. This is called "new wood".
Hydrangea species that bloom on "old wood" include Mopheads and Lacecap. Both of which belong to Hydrangea macrophylla. This group also includes Oak Leaf Hydrangeas (Hydrangea quercifolia).
This group of Hydrangeas set their flower buds for the next year around September or October.
You should prune out dead wood each year. If you're unsure what's dead and what's healthy, this task can be left until mid spring. You should be able to see the leaves budding and starting to emerge. If you have a cane that has no growth on it, it's probably safe to remove it.
After the shrub has been in the ground 5 years or more, you want to remove 1/3 of the oldest canes. This is done by cutting them down to the ground. This rejuvenates the shrub. Selectively prune these back so that you're cutting a little out of the entire shrub. It's best to try to avoid cutting a lot of canes that are close together. What you cut this year will not flower next year. It is in the process of rejuvenating. It will however, flower like crazy the second year. By this time these canes are completely rejuvenated. I recommend doing this every few years. Be careful though: never cut more than 1/3 at a time or you can shock your plant. By removing only 1/3, the loss of blooms on those canes is not noticeable, especially if you have a shrub that's loaded with blooms on the other 2/3 of the canes.
If you want to prune your hydrangea for size, this is done in June or July, before the plant starts to form buds for the following summer. A better plan is to get a hydrangea that's suitable to the space that you have.
Hydrangeas don't need to be dead headed, but if you want to remove the spent flowers, this can be done at any time. Personally, I like the look of the dried flowers on the shrub, but it's a matter of preference.
Other Hydrangeas bloom on "new wood", or this seasons growth. This group includes Panicle Hydrangea (Hydrangea paniculata), Endless Summer Series (Hydrangea macrophylla 'Endless Summer') and 'Annabelle" Hydrangeas (Hydrangea arborescent 'Annabelle').
Annabelle Hydrangeas (Hydrangea arborescent 'Annabelle') can be pruned in the fall or the winter to control their size. You may have heard you can cut the entire shrub back to the ground. While this is true, it's not a method I recommend. This can create thin stems which may cause the plant to flop under the weight of the large, heavy blooms. A method I prefer is the cut the shrub back by 1/2 and remove 1/3 of the oldest canes every year. Similar to the method for pruning hydrangeas that bloom on old wood, but you don't want to cut those back by half or you won't have any blooms! By cutting the Annabelle Hydrangea back by half and removing 1/3 of the oldest canes, you allow the rejuvenation of the shrub, while at the same time allowing those existing branches to thicken up so it doesn't need staking once the flowers emerge.
Panicle Hydrangeas (Hydrangea paniculata) can be pruned as a shrub or even as a tree. This is the only hydrangea variety that can be pruned into tree form. These hydrangeas don't need pruning every year except to control their size, as they can get quite large. Dead wood should be pruned out every year. Any crossing branches should be removed every year. The rest of the branches should be pruned to encourage a pleasing, open shape.
Endless Summer Hydrangea (Hydrangea macrophylla 'Endless Summer') looks very much like your typical mophead hydrangea. This is a variety that was developed specifically due to the confusion surrounding the pruning of hydrangeas. Notice the botanical name - Hydrangea macrophylla is the same as the mophead hydrangeas mentioned above, it's just the variety, 'Endless Summer', that designates the difference. This is a hydrangea that for all intents and purposes looks just like a mophead hydrangea, the only difference is that the plant breeders ingeniously created a shrub that blooms on both old and new wood! Pruning these guys is easiest of all. Just like with the rest of the hydrangeas, any dead wood should be pruned out. Once the plant is 5 years old, 1/3 of the oldest canes should be pruned out to rejuvenate the shrub. Cut these down to ground level. Pruning to control their size can be done at any time, in the spring or the fall. However, like with most plants, I recommend getting a plant that will only grow to the appropriate size for your spot. There are so many varieties of hydrangeas, there really is one to fit every need!
If all this sounds confusing, don't worry, we understand. We are happy to help you prune your hydrangeas and take care of all your garden needs. Just give us a call at 877-421-7887 and we're happy to set up an appointment at your convenience to discuss your needs.