One of the basic rules of thumb for pruning flowering shrubs (in the UK anyway) ties in with the Summer Solstice. For plants that have been flowering before the Solstice (which is around about 21st June) the rule of thumb is to prune them as soon as they’ve finished flowering. For those plants that haven’t bloomed by the Solstice, you leave the pruning until the following spring.
The reasoning for this is that early bloomers set their buds, including those which will develop into flowers, during the late summer/autumn. So, if you pruned too late (or too early – before they’ve flowered) you’d lose those buds holding the would-be flowers.
Having said that, I know that lots of people are scornful of simple rules of thumb. I know there are always exceptions to rules and caveats to complicate things, but I think a simple guideline is always a useful starting point.
When I first began gardening for other people (as opposed to my lackadaisical own gardening) I was often angst-ridden: worrying how to prune, when to prune, how to propagate and where to plant… Without some rules of thumb I’d have been paralysed into inaction.
As with all ‘rules’, do remember they’re “for the guidance of wise men and the stricture of fools”*: if you want or need to break the rules, you sometime just have to do it!
*One of my teachers said that quote in a lesson a long time ago. A quick Google shows that it’s actually a bit of a misquote of something attributed to Douglas Bader – an RAF WWII pilot.