Macrophylla hydrangea types bloom on OLD WOOD. ("Old Wood" are those stems that grew the previous season). "New wood" are stems that developed on the plant during the current season) Most of this group of hydrangeas produce flower buds on hydrangea stems around August, September or October for the following summer's blooms. If those stems are removed (pruned) in the fall, winter, or spring, the bloom buds will be removed, and there may be little or no bloom the following summer (usually June/July for the northern hemisphere).
There are exceptions to this rule. a small group of mophead hydrangea have been developed that will bloom no matter when they are pruned. ('Endless Summer' is this type of hydrangea). Check the label at the garden center when you purchase these plants. Also, if you get a hydrangea for Easter often these will not be fully hardy for colder zones. People place them outside expecting them to bloom in subsequent years, but the bloom buds are often killed each winter by the cold.
This is not to say you can never ever prune mophead (Macrphylla type)
(1) All dead stems should be removed from hydrangeas every year.
(2) As the plant ages, usually around 5 years old, about 1/3 of the older (living) stems can be removed down to the ground each summer. This will help increase the vigor of the plant.
(3) In addition, if it becomes necessary to prune a plant to reduce its size, it may be cut back in June or July without harming the next year's bloom. These plants are fast growers and you should really place the plant in an area where it can grow to its full size. You will need an area that is at least 5x5' to accommodate many of these plants.
The type of hydrangea that can be pruned each year with out too much concern are H. arborescens (Annabelle types) and H. paniculata (PeeGee types) hydrangeas. These types of hydrangeas bloom on new wood (new stems). These hydrangeas are quite easy to grow because they are determined to bloom every single year, no matter how they are treated. The only time you should not prune these types of hydrangea is during the spring or summer when they are in bloom or preparing to bloom.
Although you can cut these types of hydrangea close to ground level and they will often still bloom, the stalks will not have a change to grow thick and the plant may flop. I like to cut mine back to about 4 feet each year and then let them grow on their own during the season.
Paniculatas (PG/Limelight types) can be pruned in the fall, winter, or spring. However, it is not necessary to prune them every year. My tip is to trim out crossing branches and those that do not contribute to their form whenever necessary.
Paniculata hydrangeas can also be trained to into a tree-form. If you aretrying to grow a paniculata as a tree, the center trunk and main top branches should not be removed. You can always choose the straightest branch growing vertically to take a shrub form and turn it into a tree. Remember to cut out the lower side branches early in the season.
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