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.
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.
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.
Deutzia gracilis Nikko
Pruning is carried out by taking out all of the flowered stems - pruning them 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 a further third of the old stems to ground level. This type of basal pruning will ensure that you will always have a vigorous shrub that will not get too top heavy, and it will always be clad with foliage down to or near ground level.
Deutzias that benefit from this include, Deutzia gracilis, D. compacta, D. Mont Rose, D. x Magnifica, and Deutzia x rosea. Any of the others that flower in late Spring early Summer.
By David Hughes - firstname.lastname@example.org