Trimming season for hydrangeas that bloom on new wood runs from late fall through early spring. This group includes smooth and peegee forms, which function well whether they are pruned lightly, to the ground, or not at all. In order to maintain a compact stature and luxuriant flowering, Peegee hydrangeas are frequently pruned into trees with one or two main stems that are between 15 and 30 feet tall.
Smooth hydrangeas can be pruned all the way to the ground; however, the more frequently they are cut, the less able their stems are to withstand the weight of their huge blooms. Smooth hydrangeas grow in mounds that are 3 to 6 feet tall and wide. When the plants are pruned to leave between 18 and 24 inches of growth, a woody base forms that helps the stems support the pompom blooms more effectively.