Posted: 29/10/2016 at 13:41
It depends on the variety. If they're mopheads or lace caps they flower on the previous season's growth and you should leave the old flowerheads on till next spring so they protect the newly forming buds from winter frost. If you want to maintain size and shape and vigours, you can prune out one third of the stems to the desired length and that will elave you 2/"rds to flower in spring. Repeat taking out the next third next autumn and so on till you've renewed your plant and contained its size.
If they're paniculata forms then these prune on new season's growth so can be trimmed now if you really feel the need but are best cut back in spring after the worst frosts. You can trim these hard back but I reckon its best to do some stems back to one or two pairs of buds and some a bit longer.
There's also quercifolia but I don't grow that kind and can't advise form experience but believe it needs the same treatment as paniculatas.
Either way, once you've had some good autumn rains and the soil is moist, mulch round the base with some good quality garden compost to retain moisture and boost fertility for better flowers. Give them a spring tonic of liquid tomato food if you like and some slow release blood, fish and bone or rose fertiliser.