Spring flowering shrubs such as forsythia, weigela, and lilac can be pruned just after their flowering is complete. Flowering shrubs do not have to be pruned but sometimes it is necessary to control their size in the landscape.
Pruning is best accomplished using hand pruners cutting one stem at a time creating a natural looking shrub while avoiding that sheared and trimmed look. Older shrubs may need a more extreme pruning to bring them back to the proper scale in the garden.
We had this situation in the Highfield Gardens with weigela, spirea and Physocarpus this past season. These shrubs were cut in March leaving just six inches of stem above the ground. Today they are bushy and only three feet tall, looking much like they did in the original plantings many years ago!