You want high humidity, low temps and some ventilation. Different crops need different conditions though. Potatoes like it humid and cold. Sweet potatoes like it much warmer and drier. I keep potatoes, beets, carrots and onions in my root cellar. Onions like it a bit drier but my cellar isn't humid enough so it works out for me. I keep squash and garlic outside the cellar in the basement where it's closer to 55 degrees and normal basement humidity. I keep sweet potatoes on the warm side of the basement where it's closer to 60 degrees.
My mind always goes to a tornado shelter and using it as a root cellar. I see those available installed. I never see "root cellars" available. Is there anything that makes it NOT work as a root cellar? I'm thinking the door/entry would need to be shaded.
Fyi, no basements here in the south. I am starting with nothing.
I'm not familiar with prefabbed or installed tornado shelters. Could you post a pic or link to what you are thinking about? Looks like there are at least a couple types, one is a steel box that you bolt down in your garage, the other is a buried box with a staircase. I'm assuming you're talking about the second.
I think those would be a good start. Maintaining high humidity in them and some ventilation would be the possible challenges. Another general challenge with root cellars is getting them cold enough in the fall. Mine is in our basement and insulated well from the house interior. I have a little fan blowing outside air into it at night all autumn to get it colder sooner. It takes well into October to get cold enough and usually it's barely in time for the harvest. That's with 40F or colder air blowing into it for 10 hours a day. If your ground temperature in the winter is 50 degrees, it could be hard to get a cellar much colder than that.
But I'm sure many other folks in warmer climates have figured this out, hopefully they chime in.
Hmm, I'm not sure how warm a root cellar can be and still give you appreciable benefits. I'm guessing it needs to be below 45 or 50F.
Maybe an underground box/cellar/shelter that is insulated from the ground around it would work? Then bring in cold air at night to further cool the interior. Or do the air conditioning thing if needed...
you want temp in root cellar around 35-40 degrees F (optimum) most are warmer. my fave was an old one under a farmhouse I rented - the date scratched into the wall was 1896. It had stone walls, axe marks on the ceiling joists, and a tamped earth floor. The landlord along the way had used it for the water heater & well pump (when they put in the indoor plumbing) so it was too warm for traditional root cellaring, but excellent for long term pantry storage as it stayed 50 degrees year around (this was zone 4, winters -20 at times). To compensate for the extreme humidity I coated (metal) canned goods with a very light coating of mineral oil (get at pharmacy at wal mart, it comes food grade). Without oil they rusted in 3 yrs, with oil good for 6 yrs (maybe longer, I moved at the 6 year point). Pails with mylar bags inside also did great. Tho I could not store taters, apples, and other cold loving things in that cellar I loved having it available as an extra "pantry". Unfortunately where I live now, (sone it's not cold enough for root cellar - however it is offest by being able to garden thru the winter. My garden IS my root cellar now, LOL.