Agree with all the above comments. I treat salvias as short lived plants. They are very easy from cuttings so I always overwinter a few in the glasshouse.
I've just checked mine and it's looking pretty much the same despite not being cut back at all last Autumn or Winter. Give it time. You do need to be brutal with them, just be careful with timing. Mine will be getting cut hard back as soon as it shows signs of life.
Frost knackered the top half of my salvia hedge and I'm itching to cut off the scrag, but will wait. I've lost plants in the hedge before now and we are getting regular frosts.
This year the street trees will be in full leaf (no pollarding) so it will be interesting to see how the hedge copes with much more shade. The garden is south facing sunny some years and shady other years.
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I treat salvias as short lived plants. They are very easy from cuttings so I always overwinter a few in the glasshouse.