Have built an underground
greenhouse here in the high desert of southern oregon. (just north of Klamath falls at about 4300 feet altitude) As seasons change, it's not only temperature that effects growth but hours of light.
The greenhouse was very simple construction with a combination of lumber and t-posts for the walls and a slanted 2x4 roof with plastic on both sides of the 2x4's to create an extra barrier to the cold. Inside, after we dug down to the local
hard layer of rock(hardpan) we then built the floor back up using hugelkultur
. Large pine logs were put in rows leaving the middle of the greenhouse open. The middle was vertical pallets
covered with a mesh fabric to prevent dirt from falling through the pallets. Halfway up the pallets a simple wooden walkway was nailed in leaving an open space below the walkway about a foot wide and a foot and a half high. The idea here was to use this as a cold sink like is used in igloos in the far north. The cold air will sit there instead of on the plants which are a bit higher on the hugelkultur
bed. After the logs for the bed we added a mixture of goat poop
(we have a goat dairy
and this is what we clean out of their living area).
I built this in my nephews place and he has since moved on so I only have input from the first winter in which kale and cabbage survived the winter where temperatures dropped to -30F.
As in any underground greenhouse the idea is that the steady temperature in the surrounding earth keep the temperature inside the greenhouse higher than the frigid winter air.
Am still experimenting up here now with an above-ground hugelkultur bed but can't report on that yet.
For a few pics of the above-ground bed and the underground greenhouse construction with center walkway you can check out this facebook