Summer

• Prune mophead, lacecap and oakleaf hydrangeas in the summer. These hydrangea types should be pruned before August to prevent removing stems that have buds that are set to bloom for the following year. These stems are referred to as "old wood" or stems that have been on the hydrangea plant before the current season. Stems that are removed after the summer reduce the blooms for the next summer.
  
  

Fall or Winter

• Smooth-type hydrangeas should be pruned in the fall or winter months. These hydrangea types bloom on new wood or stems that have set buds during the current growing season. By pruning in the spring or summer, you remove the stems that produce their blooms. Panicle hydrangea types, which have cone-shaped blooms, can be pruned in the summer, fall or winter months. This type of hydrangea rarely needs severe pruning unless it has contracted a fungal disease.
  
  

Fungal Diseases

• Prune out stems that have contracted fungal diseases any time of year regardless of the type of hydrangea, because fungal spores can spread throughout the plant through the wind or rain. By pruning out diseases such as powdery mildew or rust, you can prevent your hydrangea from declining in health. Dip a pair of bypass shears in a mixture of 70 percent denatured alcohol and 30 percent water in between cuts to prevent spreading the disease when pruning.
  
  

Pests

• Trim off pest damage from your hydrangea during the growing season. Pests such as scales, aphids and mites can cause hydrangea leaves to discolor, distort or defoliate. Spray your hydrangea plant with a blast of water to remove the pests or use an insecticidal soap. Remove leaves that have been severely damaged from the pest damage. You may have to reapply the insecticidal soap, because it is only effective 24 hours after the initial application.