Hydrangeas are one of the most striking plants you can add to your landscape because they produce enormous and often colorful blooms. But when to prune them is confusing because they are complicated!
But first, let's set the record straight about pruning in general and Mike's Rule of Pruning". The only time you need to be concerned about when to prune is when you are concerned about cutting off flower buds. Cutting off the flower buds is not going to hurt the plant, it just means that you won't see any flowers this season if you prune at the wrong time.
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On the other hand, "Mike's Rule of Pruning" is; "if it needs pruning right now, then by all means prune it and worryabout flowers later. The damage from not pruning is far worse than any damage you can do with a pair of pruning shears.
I'm serious about that. Most, and I don't mean some, but most landscapes have trees or shrubs in them that should have been pruned a long time ago and weren't and now they are detracting from the landscape. So if it needs pruning, then please prune it now.
Okay, so why are Hydrangeas so complicated? Because some of them flower on new wood, and some flower on old wood. Those that flower on new wood should be pruned in the fall after the blooms are spent, but you can prune them in the fall, or early in the spring because they aren't going to make any new flower buds until the following summer.
Hydrangeas that flower on old wood should be pruned right after they bloom because as soon as they are done blooming they start making new flower buds for the next season. If you prune them after that flower bud production begins you might cut off next years flowers.
And that makes them complicated! At least for me. Mike! Whaddya mean new wood and old wood? Looking at your hydrangea in the early spring, before the leaves appear, everything you see is old wood. It's growth that appeared the previous year or even earlier than that. If your hydrangea is one that flowers on old wood, the flower buds are already formed.
Any growth that appears once the plant starts blooming is considered new wood continues to be considered new wood until fall. Then the wood is harder and will soon be considered old wood. Confusing right?
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For instance, Hydrangea arborescens 'Annabelle' starts getting new leaves and new growth in April. (zone 5) The new branches continue to grow until about mid summer. Come mid summer the branches stop growing and at the end of each new branch a flower bud starts to appear. The flower bud is being produce on the new growth, commonly called new wood, or current seasons growth. The flower bud production happens very quickly and the huge flower opens and dazzles the world.
Because hydrangeas like 'Annabelle' bloom on new wood you can prune them from the time they quit blooming in the fall until spring when they start putting on new growth. Once the new growth comes in the spring you shouldn't prune them, but even if you did you might still get blooms later in the summer.
Nikko Blue Hydrangea on the other hand is in the Macrophylla hydrangea family and it flowers on old wood. That means that as soon as it is done blooming that's when you should prune it. The pruning window is much shorter.
Most of the popular hydrangeas are in the macrophylla family and bloom on old wood, but there are exceptions like Hydrangea 'Forever and Ever' which blooms on both old wood and new wood. Most of the big white snowball hyrdrangeas like 'Annabelle and Paniculata Grandifloria' bloom on new wood.
When pruning hydrangea you can cut them back as hard as you need to. They are very easy to maintain a given size if only you are willing to prune them that hard. Really hard pruning is best done when the plant is dormant during the winter months.
I hope this helps. Watch your hydrangea and see where the blooms come from. Or take a look at this page from the United States National Arboretum. http://www.usna.usda.gov/Gardens/faqs/hydrangeafaq2.html Great photos and a ton of information!