Pruning in June and July
If your deutzia needs pruning, the best time to do this is in summer after the plant has flowered. Most gardeners think it is a shame to prune before flowering as this reduces the number of flowers.
To keep the plant reasonably compact, the side branches can be cut back to just above a leaf. New buds will form next to the leaf, and these will eventually produce new shoots.
You may decide that your deutzia needs rejuvenation pruning. To do this, cut between one fifth and one third of the branches back to about 30-40 cm from the ground. The number of branches you remove depends on how quickly you want to rejuvenate the plant. You should preferably cut down to an outward facing bud. With rejuvenation pruning it is important to start on the inside of the plant and gradually work outwards.
Repeat this step in subsequent years so that the entire plant is rejuvenated within a few years.