The blooms are formed on the new wood. Pruning in the fall or early spring will produce lots of new wood. At this time you can control the height, spread and shape of the plant, by the amount that is pruned. I like to use a pruning shears not hedge shears. With pruning shears I can better control the shape and the cuts are more precise. Thinning can help reduce some of the weight off the branches. The flowers can become very heavy, especially after a rain. Can be pruned hard.
Author's Notes
I have two Tardiva in my garden. They are as tough as nails and required little work except for pruning. A very good shrub for late flower color.