Thank you Rodger, excellent idea and very doable in my yard. <Instead of a lidded frame, however, I'd suggest a small version of a lath house. You might not think so, but even such a frail structure will significantly even out diurnal variations in temperature.> No, I can see how this would work. I've got 3 cold frames now, 2 above ground and one sunken and they've been wonderful in exactly that, evening out diurnal variations in temperature. Linda Foulis Zone 3 Okotoks, AB