There are several fine species and varieties (cultivars) of Deutzia. They all have the same pruning requirement as they flower on wood growths made in the previous growing season (Last year). This is important to realise, for the current year's growth - such as in late spring or early summer does NOT bear flowers. When to prune Deutzia is essential knowledge for succession of flowers.
Deutzias can get a bit unkempt in habit if not pruned regularly. Prune at the right time!
All deutzia should be pruned immediately after the flowers have finished - this will be early - mid - summer. If you prune them too late after flowering, then they will not have enough time to produce their new stems - which will be the flowering stems for the next year. In this particular shrub's case, when is as important as how!
Pruning in the winter for instance, will remove all the following summer's flower-bud stems. They do NOT make new flower bud stems in the following spring.
Pruning is carried out by taking out all of the flowered stems - pruning Deutzia back by about one third of their previous length. So, each stem that has the faded flowers on, should be cut back, leaving around two thirds of the stem on the shrub.
Together with this, Deutzias respond to heavy pruning of around one in three of their stems - right back to - or near - ground level. This will encourage new basal shoots, which will soon grow to full length and provide a succession of healthy shoots for following years. Each year - at pruning time - cut back your Deutzia plant a further third of the old stems to ground level. This type of basal pruning will ensure that you will always have a vigorous Deutzia shrub that will not get too top heavy, and it will always be clad with foliage down to or near ground level.