I wouldn't do the fertilizer. It'll all wash through the soil this winter without the plant using it.
I'd cover with a thick layer of mulch - at least that's what I do with mine. 3-4" of ground up leaves/grass clippings, or something similar. This will also keep winter weeds under control.
Also, if you move them closer to the house, (assuming they are moveable), or onto the ground at least, they'll stay a tad warmer than just being surrounded by air, and probably won't freeze. This is why bridges freeze before the rest of the roads - the ground acts as an insulator.
NG line for outdoor burner