Root Cellar Temperature and Humidity
A root cellar is aiming at cold and moist: 32 to 40°F at about 95 percent relative humidity, which is what carrots, beets and potatoes want. That is one of several published conditions, not the only one — onions want dry, squash wants warmer — so a single space cannot serve everything, and a heated basement serves almost none of it.
Sources last checked 2026-08-31 · next review due 2027-08-31 · reviewed by HomesteadingGear Editorial
The published conditions
| Condition | Temperature | Relative humidity | Typical crops |
|---|---|---|---|
| Cold and moist | 32 to 40°F | 95% | Carrots, potatoes, beets |
| Cold and dry | 32 to 40°F | 65% | Onions, garlic |
| Cool and dry | 50 to 60°F | 60% | Winter squash, pumpkins |
| Room temperature | Ambient | Ambient | Tomatoes, which refrigeration damages |
Temperature and humidity figures are University of Minnesota Extension's. The crop groupings follow Colorado State University Extension Fact Sheet 7.601, which places carrots and potatoes in cool and moist, onions in cool and dry, and tomatoes at room temperature.
The third variable nobody measures
Colorado State University Extension names three critical aspects of storing vegetables: temperature, humidity and ventilation. The first two get thermometers and hygrometers. The third gets ignored, and it is the one that turns a cellar sour.
Ventilation carries away the ethylene that ripening produce gives off, and it keeps the air from stagnating at 95 percent humidity — which is a mould-friendly condition maintained deliberately. CSU also pairs ventilation with rodent protection in the same breath, because the openings that provide one are the openings that admit the other.
Why a basement usually is not a root cellar
CSU Extension states it directly: basements are generally cool and dry, while root cellars provide cold and moist conditions. A basement in a heated house sits well above 40°F and nowhere near 95 percent humidity. That is not a small gap to close — it is the difference between carrots lasting eight months and carrots going limp by Christmas. An unheated cellar, an outdoor pit or a storage mound gets there; a utility room next to a furnace does not.
Common mistakes
- One space, every crop. Onions in a 95 percent humidity cellar rot. If you only have one space, decide which condition it holds and store the rest elsewhere.
- Fruit stored with vegetables. Ethylene from the fruit accelerates deterioration in the vegetables. Separate them.
- Sealing it up for stability. Ventilation is a named requirement, not an optional extra.
- Treating shelf life as a guarantee. University of Minnesota Extension says plainly that expected shelf-life times are only estimates. Eight months for carrots describes good conditions and sound roots, not a promise.
- Ignoring the freeze line. 32°F is the bottom of the range for a reason. Insulating with straw, hay or wood shavings is CSU's suggestion for stabilising temperature, and for outdoor mounds they specify six to ten inches of soil for the coldest months.
The equipment decision this leads to
- Root cellar guide — building or converting a space that actually holds these conditions
Related reading
Where these numbers come from
- University of Minnesota Extension — Harvesting and storing home garden vegetables
Primary source · read 2026-08-31
- Cool and dry storage is 50 to 60 degrees Fahrenheit and 60 percent relative humidity
- Cold and dry storage is 32 to 40 degrees Fahrenheit and 65 percent relative humidity
- Cold and moist storage is 32 to 40 degrees Fahrenheit and 95 percent relative humidity
- Carrots stored in cold and moist conditions without their tops have an expected shelf life of about eight months
- Expected shelf-life times are only estimates
- Colorado State University Extension — Storage of Home-Grown Vegetables, Fact Sheet 7.601
Primary source · read 2026-08-31
- There are three critical aspects to storing vegetables: temperature, humidity and ventilation
- Basements are generally cool and dry, while root cellars provide cold and moist conditions
- Separate fruits from vegetables to minimise the detrimental effects of ethylene produced by the fruits
- Provide ventilation and protection from rodents when storing vegetables in cellars and basements
- Straw, hay or wood shavings can be used as insulation to stabilise storage temperature
- For the coldest months of December, January and February the soil layer over a storage mound should be six to ten inches thick
Common questions
What temperature and humidity should a root cellar be?
University of Minnesota Extension defines cold and moist storage as 32 to 40 degrees Fahrenheit at 95 percent relative humidity, which is the condition most root vegetables want. Cold and dry is the same temperature at 65 percent humidity, and cool and dry is 50 to 60 degrees at 60 percent.
What humidity do carrots and potatoes need?
Both want cold and moist conditions: 32 to 40 degrees Fahrenheit at around 95 percent relative humidity. University of Minnesota Extension gives carrots stored that way, without their tops, an expected shelf life of about eight months, while noting that expected shelf-life times are only estimates.
Why can't I store onions and carrots together?
They need different humidity. Carrots want cold and moist at about 95 percent; onions want dry conditions and rot in the humidity carrots need. Colorado State University Extension lists onions under cool and dry while carrots and potatoes sit under cool and moist.
Is a basement the same as a root cellar?
No. Colorado State University Extension puts it plainly: basements are generally cool and dry, while root cellars provide cold and moist conditions. A heated house basement is usually too warm and far too dry for the crops people want to keep there.
Why should fruits and vegetables be separated in storage?
Fruits give off ethylene, which speeds ripening and deterioration in nearby vegetables. Colorado State University Extension advises separating fruits from vegetables specifically to minimise those effects.