Use Method One if you have mophead hydrangeas (these are the most common hydrangeas grown) or if you have Oakleaf hydrangeas (leaves shaped like large oak leaves).

Use Method Two if you have paniculatas (PeeGees) or Annabelle.

If you do not KNOW what kind of hydrangea you have, you will be safe if you just use Method One

METHOD ONE:
For Mophead (macrophyllas) and Oakleaf hydrangeas

• The first method is for hydrangea types that bloom on OLD WOOD. Stems are called "old wood" if they have been on the hydrangea at least 9-10 months before the hydrangea blooms. This means that flowers are formed on the stems of hydrangeas around August, September or October for the following Spring. If those stems are removed (pruned) in the fall or Winter, the bloom buds for the Spring will be removed and there can be little or no bloom the next year.
• Occasionally a mophead hydrangea will defy this general rule and bloom after being pruned at the wrong time. But for the vast majority of hydrangeas, pruning after about August/September will eliminate the bloom for the next Spring.
This does not mean that hydrangeas can never be pruned. All dead wood should be removed from hydrangeas every year. After the plants are at least 4 years old, about 1/3 of the older stems can be removed down to the ground each Summer. This will revitalize the plant. In addition, if it becomes necessary to prune a plant to reduce its size (temporarily), it may be cut back in June or July without harming the next year's bloom.

• The second method is for hydrangea types that bloom on new wood. It is a joy to grow these type hydrangeas because they are determination to bloom every single year, no matter how they are pruned. The only time they cannot be pruned is immediately before they are to bloom.
• Many people grow hedges of Annabelle and cut them within a few inches of the ground each Autumn so they will not be an eyesore during the Winter. They will still bloom beautifully in the Spring/Summer.
Paniculatas can also be pruned in the Autumn or Winter. After pruning, they will grow 5-7 feet in the Spring and Summer and then put on a beautiful show. If one is attempting to grow a paniculata in a tree form, however, you would not want to cut off the developing trunk or main branches. Try trimming out crossing branches and those that do not contribute to an attractive form.