Coldframes

As you say they are to give protection for over wintering plants or early showings of things like salads. I have two which are directly on the ground. I built them before I had a Poly tunnel. Mine are 1 meter x two meters and built of wood with double skinned plastic tops. They are also very useful for hardening off plants before planting out, saves all that okey cokey with plants in and out of the green house. In winter when I have plants in pots to protect I cover the soil with land scape membrane. Hope that helps.