This probably should have been brought up back in the fall when lots of us were spending our free time raking fall leaves, but what the heck.
For those of us who live in zones where many salvia species are marginal at best or not hardy at all here is something you might want to try. In late fall when the ground begins to freeze you cut your salvias down to the ground and cover the crown with a bag or bags full of leaves - preferably black plastic bags. This does two things. It keeps water out of the crown and also prevents the ground from freezing. My own experiments indicate that you pick up roughly two zones. In other words a zone 6 garden is now able to preserve in-ground zone 8 plants.

This doesn't work well for all plants, woody plants like microphylla/greggii can be tricky, but for dieback perennials like guaranitica, Indigo Spires, and other herbaceous salvia it works very well.

@WardDas

Great thread starter! How do you keep the leaf bags from blowing away all winter? When do you remove the bags in the spring? Does it work ok for putting a bag of leaves on a large container for the winter? When you say cut to the ground, do you mean all the way at soil level?

@WardDas
Thank you once again for sharing your experience!! Since the Guaranitica species and hybrids are my favorites (and the hummingbirds') I will be thrilled to try this!! @Marilyn great questions- I am interested in the answers to those. My guess on cutting to the ground (purely conjecture) would be that one would want to cut them nearly if not clear to ground level. But would rather know for sure what has worked as I have no experience.

No, it probably wouldn't work for plants in containers, but perhaps it would if the container were buried down to ground level.
When I say cut to ground level I mean right to the ground. An example of a plant to do that with is Salvia guaranitica. Another rule I follow is to not do it too soon, I put on the bags in December after the plants are dormant but the round hasn't frozen yet.
Yes, the bags sometimes do need to be held down so as not to be blown away by the wind.
I usually pull the bags off by about mid April here in zone 7a. If the plant is showing green growth I usually leave the bag nearby just in case there is more freezing weather.
The final use for the leaves is mulch. It is free other than the work it takes to bag them up.

Ward

Thanks for sharing your info! What do you use to hold the bags down?

Sometimes I leave the salvias alone in containers and hope they'll overwinter and be green and flowering. Sometimes they do and sometimes they don't.