Have you ever heard of micro climate? Where is it planted? I've had a lot of success with plants not necessarily rated for my zone 5/6 in Indiana by providing a micro climate for them. Meaning the plant is protected from the elements on 2-3 sides....planted next to the house or other large structure. Top dress the soil with a thick layer of mulch. Do not cut back the plant till early spring.

For years I'd dig up Canna bulbs and store in my garage. Then one fall/winter I was sick and didn't dig bulbs. To my surprise they not only survived our harsh winter but grew in the spring bigger and better than ever due to the micro climate provided by the house and other large perennials.

Do a search on micro climates.....should make it more clear what I'm saying.

Best of luck.

Whether it works probably depends on how much warmer your garage stays than the outdoor temperatures. I overwinter Salvia 'Black and Blue' in a large pot in an unheated foyer which gets some heat from the house through an uninsulated door and some solar gain. It's only supposed to be hardy to zone 7, but apparently the combination of sun and house warmth keeps things warm enough even though the foyer is surrounded on 3 sides by the outdoors. If you really don't want to lose this plant, you could take a few cuttings as well as storing it in the warmest part of your garage.

Most salvias transplant well so why not dig the whole plant, pot it up, trim back the top and try to winter it over in the garage since its not hardy anyway, there is nothing to loose. Sounds like you might have waited too long on cuttings. Are there seeds? Salvia seed germinates very easily, I often start new ones indoors in late winter by a sunny southern window or you could use lights.

The concern is that your garage temps would fall below freezing. I winter salvia 'Blue Angel' and salvia 'Black and Blue' by potting up and cutting to a short stem and storing in my dark cool basement room. Once a month I give them a bit of water as they have small tubers. If they break dormancy the new growth gets pinched off until closer to spring.

The year that I grew them under lights during the winter there were white fly problems. Glad they store dormant, also less work. Have also collected seed and started them inside.

I suspect "Mystic Spires" would overwinter nicely in an unheated garage that didn't get below the low 20s, ditto for "Black and Blue" (which survived -19F here over the past winter in an exposed bed).

I have found S. guaranitica varieties to be a pain to overwinter from cuttings under lights. As far as tender Salvias, the only one I'm growing on as a potted specimen indoors is S. macrophylla (while blue, flowers aren't much on this one, but the foliage is special).