Valley Gardener, you can prune them down now if you wish, but they are unlikely to re-bloom from now. Re-flowering is more likely if you prune down around late June. Beyond that point, they are more inconsistent with re-flowering. It all depends on your soil and the amount of sun it gets. They perform better in more sun.
Janebal, you don't have to cut them back at my suggested time. It's really for people that are very busy and not keen on small and often dead heading, but they do need a light prune back to encourage fresh branching and therefore more flowers in the later part of summer.
Some flower quite late in the year, so watch out for those ones, they may need a light trim back around mid July. The earlier (May) flowering ones, it is best to prune them back by half their height in early July.
Thank you, Borderline and everyone else. I have learned a lot from you. I have just cut back my salvias and potted up little cuttings from the twigs I cut off (as Hostafan1 advised) and put them in gritty compost in the greenhouse. I am interested in Valley Gardener's and JennyJ's comment about 'East Friesland' because I bought one called 'Caradonna' and was wondering if I could get cuttings from it. It seems I can. I didn't realize that there are so many different types of salvia.
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Some flower quite late in the year, so watch out for those ones, they may need a light trim back around mid July. The earlier (May) flowering ones, it is best to prune them back by half their height in early July.