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.