There are several varieties of Caryopteris with the Caryopteris x clandonensis cultivars being the main group.
All have grey-green aromatic foliage with the exception of the golden leaved and variegated forms. However they are grown in the main for their varied blue flowers which start to appear late summer and normally carry on right through to mid-winter. The blue flowers contrast well with other autumnal shades in the late garden.
Caryopteris Shrubs flower on growths made in the current growing season and generally are suited to pruning hard each spring. Pruning is not absolutely essential, but without pruning the shrub will grow into an untidy mess, and gradually lose flowering capabilities.
Annual pruning of Caryopteris x clandonensis will ensure a compact shrub with masses of flowers and a longer flowering period.
Basically you have to decide what height you want your Caryopteris to grow to each year - approximately - then experiment with a permanent framework - as in the image - which should be between 20 - 40cms (8-16in) high. This will mean that you have a 'stump' of branches which are left un-pruned each year, from which the new growths (branches) will sprout.
Hard pruning of your 'Blue Spiraea' (Caryopteris) should take place in late winter or early spring, and preferably before too much new growth has started after the winter. In spite of it's common name it is not related to the true Spiraea family - an anomaly of using common names!
Pruning can take place as late as end of April, but earlier is better.
This type of cutting back of your Caryopteris will ensure a good cluster of arching stems, with fresh aromatic foliage as an added attraction for this shrub.
Caryopteris pruned in this way will normally grow to around 60 - 90 cms (24-36in) each year and be densely compact in habit. Left un-pruned - the shrub will be a mass of untidy intertwined stems. (Still attractive, if you like an unruly shrub!). There will be much more vigour with the pruned version!
As with all silver or grey green foliaged plants, Caryopteris is happy in a sunny position on the dry side. Tolerates these conditions quite well. In fact, thrives!
As the regular pruning will remove substantial growth each year, the shrub will benefit from regular organic mulch and feeding with a slow release fertiliser such as bone meal or Osmocote. This will help to replenish the annual flush of new growth.
Your Caryopteris will start flowering on the new growth from mid to late summer, and continue well into the autumn or even mild winters. Additional 'snipping can be carried out after the first flower flush fades a little, but no more than removing the faded flower growth. Keep it growing well and this will be unnecessary.