Perennials: does a SFG freeze solid in zone 5?

I need some advice from zone 5 gardeners, please! Where is the best location for plants that are perennial or will overwinter such as sorrel, sage, perennial leeks, and garlic?

Here is my setup: My wee garden consists of a concrete condo patio with a couple foot wide soil garden border. I built 3 long SFG boxes that sit right on top of the concrete, but two of them overlap partially over the soil border. This gives me 3 planting options: 1) the SFG space that sits on top of concrete (I think this will be likely to freeze solid, yes?), 2) the SFG space that sits on top of soil (I expect this will freeze on top but the bottom that's in contact with the earth may not freeze if mulched), and 3) traditional garden border.

Where would you plant the overwintering plants to best protect their roots from winter freezing?

The SFG boxes will be lightly mulched and I'm thinking about adding a thin layer of mulch over the concrete surrounding the SFG boxes, too.

Thanks very much, I'm grateful for this forum and the willingness folks have to share with newbies.

The mix in my garden boxes freeze solid in the winter time, whether they have solid bottoms or are set upon the earth.

This is my understanding, basically there will be frost to at least a foot and as deep a three feet. I think plumbers around here bury water lines to at least 4 feet deep to prevent freezing.

The mulch will stabilize the plants after they have frozen into place, reducing the potential for heaving.

Thanks, Camp! This helps a lot. I love mulch.