Thanks for the tag Lynn.
Greg, I'm in zone 4 here. That's happened to me before too. I usually just put my potting mix in a big aluminum pan (with holes punched in the bottom) and pot them up until ready to plant. Nothing fancy, but it'll help them survive until you're ready to plant them in your troughs. I keep them in a unattached, unheated garage, by a window if possible. If you're still getting freezing temps I'd just moisten the soil a bit before planting - keep them on the dry side.
