The walipini / pit greenhouse
The physics is real and the same as a root cellar's. Drainage decides whether you built a greenhouse or a swimming pool.
What it is
A greenhouse dug several feet into the ground, with a glazed south-facing roof. Soil temperature below the frost line is stable year round, so the earth walls flatten both extremes — cool in summer, far milder in winter.
The numbers
- Below about 4 ft, soil temperature settles near the local annual average and barely moves — that's the same fact the root cellar runs on.
- Glazing wants to face the winter sun at roughly latitude + 10–15° to collect anything useful in December.
- Never dig straight sides over 4 ft without shoring. Earth walls fail without warning, onto whoever is standing in the hole. This is the most dangerous single sentence on this entire site.
Why it's here
The heat physics is sound and well understood. People do grow citrus in Nebraska this way. It earns a ◇ purely on the two things that sink most builds.
The automation angle
Soil and air temperature at several heights, humidity, and — this one matters — a water level sensor in the sump. You want to know the pit is filling while it's still an inconvenience.
The catch
Drainage decides everything. A pit in ground with a high water table or slow-draining soil is a pond with plants in it, and no amount of clever glazing fixes that. Dig your test hole in the wettest month of the year, before you commit to anything, and plan the drain and the sump as part of the design — not as a repair. Then re-read the shoring line above.