"An unheated greenhouse without excellent insulation will only stay a couple of degrees above the outside temperature after the sun goes down (and on cloudy days), and I don't see how bottom-heated beds will keep the plants (above ground) alive. The part above ground will freeze."
Um, that's the point. I want my landscape cuttings to be dormant and wake up naturally with the season... just with a bit of better protection and a bit of bottom heat to get the roots going earlier. This is not meant to be kept significantly warmer than outdoors at any time of the year.
I understand this idea is properly called a cold frame, only this is a much larger version of it. Structures like this are commonly used by commercial operations to keep their potted nursery plants in over the winter. I was just focusing more on the propagation of those plants rather than catering to selling off the grown out versions.
