Anywhere there's frost at all, hardiness isn't about air temp once there's been a frost, but about soil temp & whether or not the ground freezes, depth & length of freeze, all combined with drainage & fertility.
Then there are cheats like putting a big pile of leaves, pine needles, or straw over marginally hardy plants (to keep the soil warmer,) &/or putting questionables next to hardscape features like a patio or path, large rock, or adjacent to a south or west facing basement wall, strings of lights...
