One thing that I think we all take for granted these days is the ubiquity of food preservation technology.
Whatever you bring in, whatever is left over, toss it in the fridge or in the freezer, and forget about it until it’s time to eat. But food preservation was a very different thing prior to the advent of these miraculous things!
Nonetheless, it was still possible, and one ancient bit of technology that is making a big comeback today with homesteaders and all folks who want to be a little more self-sufficient is the root cellar.
They are as functional now as they were way back when, but just what is the ideal temperature of a root cellar?
The ideal temperature of a root cellar is between 32° F and 40° F (between 0°C and 4.5° C. This is about the temperature of a modern refrigerator, but depending on the design of your root cellar and the climate where you are, it can be difficult to maintain this target temp.
If you want to preserve your food for the long haul in your root cellar, it needs to be downright chilly. That being said, it doesn’t mean that your efforts are fruitless or that you can’t keep something longer than you would normally if your root cellar is a little warmer than that.
Trust me, there is a whole lot more that you’ll need to know whether you’ve already got your own root cellar or are thinking of installing one. Keep reading and I’ll tell you everything.
How Can You Keep a Root Cellar Cold Enough?
Probably the biggest challenge, and the single most common question I get about root cellars, is how to keep them cold enough. Like I said above, if your root cellar is optimized it will be quite chilly, as chilly as the refrigerator in your kitchen.
How are you supposed to keep what is basically an underground or partially underground room that cold? Well, it isn’t easy assuming you haven’t installed refrigeration technology which basically turns it into a walk-in chiller!
The great thing about a root cellar is that it will naturally be quite a bit colder than the air outside or above ground simply by the benefit of being subterranean.
Soil is a tremendous insulator, and the ground itself is basically one gargantuan heatsink. Properly constructed and properly insulated, your root cellar will be anywhere from 10° to 20° colder than the ambient air temperature, and potentially colder yet.
But ultimately that isn’t cold enough, not in most places. Root cellars tend to be at their best if you live somewhere in a northern or southern latitude when it is naturally much colder year-round.
Good location, combined with proper siting and construction, will make a root cellar extremely effective. Speaking of…
Site Your Root Cellar Properly for Best Results
For starters, if you live in the northern hemisphere, you should likely position your cellar on the north side of your home or on the north side of a hill or slope. That will give it maximum protection from the sun.
Also, speaking of slopes, one of the most effective ways to attain the desired depth and do so easily is by digging into the side of a hill.
Depending on your build plan, this might be slightly more complicated considering you won’t be excavating the covering off of the root cellar room itself so much as you’ll be tunneling under it, but the advantages are significant.
It’s also entirely worthwhile to do some advance work before you start digging it all…
Draw up plans for multiple sites that will work on your property and for your purposes, and then invest in or rent sensors that you can place in a small hole dug at each site to the appropriate depth.
Record readings throughout the day for a few days and, if you have the time and patience, in different seasons to get a feel for what the temperature and humidity are like at that location.
That’s the true tale of the tape, and you might be surprised to find out that a site that seemed to be suboptimal could be ideal, or vice versa.
What Happens if Your Cellar is a Little Warmer?
If it’s warmer than the prescribed temperature range your food will obviously not stay fresh for as long, and some can’t be reliably stored at all. Fruits, especially, don’t do well if the cellar is a little bit warmer than a truly cold 32° to 40° F.
Now, chances are you don’t live in a region or experience a climate that’s truly ideal for root cellar usage. Even with ideal construction techniques, your root cellar is probably going to be a fair bit warmer, anywhere from 41° to 55° F, and hopefully no warmer than that.
In that case, there are still many foods that will benefit from root cellar storage, and that somewhat higher range is actually ideal for certain vegetables like potatoes, sweet potatoes, and onions. It’s all in the name, root cellar!
Can You Still Store Food in a Cellar Warmer than 32°F?
Yes, you can, but you need to know that the ideal temperature range of various fruits and veggies varies considerably, and some items won’t do well at all unless kept truly cool.
For instance, apples and pears need to be very cold, preferably no higher than 31° F if you want them to keep for a few months. Broccoli, likewise needs to be quite cold at 30° to 32° F and even then, that will only get you a couple of weeks.
Most other types of fruit, particularly citrus fruits, need to be kept very cool for maximum longevity, and even then you’re looking at about 2 months on the outside.
If your root cellar is above these temperatures, it’s still worth keeping these foods in there compared to just sitting on your counter or in a nice warm, stuffy pantry, but understand that you’re not going to get as long a storage life as you would normally.
On the other hand, quite a few veggies do wonderfully in slightly warmer cellars, like the aforementioned potatoes, sweet potatoes, and onions.
Sweet potatoes are actually best if kept anywhere between 55° and 60° F which is relatively balmy for a root cellar, whereas potatoes do well a little bit cooler between 40° and 45° F.
Onions are quite adaptable and can be kept for a very long time as long as they are cool and dry.
Don’t Forget that Humidity is Also a Factor
Don’t forget, temperature isn’t everything when it comes to keeping food in a root cellar! Ideally, your root cellar should have a constant humidity level of between 85% and 95%. That will keep your produce from shriveling up and turning into desiccated little husks.
Learn the Temperature Map of Your Root Cellar
One worthwhile exercise to get the most from your root cellar is to understand the “temperature map”. That’s not a scientific term, that’s my term for learning where your root cellar is a little bit warmer, a little bit cooler, and which spots experience an influx of more humidity or less.
This knowledge will help you effectively place different kinds of produce for maximum shelf life and stability.
For instance, most folks know that cold air sinks, and accordingly the lower shelves and the floor will have a somewhat cooler temperature compared to the uppermost shells near the ceiling- warmer air rises.
Likewise, if you live in a really cold area, cool air will seep in around windows and doors, chilling these zones a little more than the average.
Humidity is more or less a constant unless you do something to significantly impact it, but even the movement of air currents will cause certain spots to be noticeably drier.
Keep all of this in mind and use a good thermometer or two to figure this out before you stock your root cellar.
Controlling and Stabilizing Your Root Cellar’s Temp?
Assuming you aren’t equipping your cellar with any sort of refrigeration technology or improving the insulation factor somehow, your only options for changing the temperature are to let air in, keep air out, keep light out, or let light in.
Also, consider installing an airlock or double door to help prevent a major influx of significantly warmer or cooler air from outside the cellar whenever you enter and exit.
And definitely don’t underestimate the effects of light! Sunlight streaming in through any windows can significantly warm the space, and even a single incandescent light bulb can make a big difference, so you never want to leave that on when you aren’t inside.
And if you’re planning on installing, remember that the deeper you go the cooler it will be, all things being equal. So do what you can!
Don’t Forget About Ventilation
Ventilation isn’t just to prevent the growth of mold. It also helps keep the temperature lower, although how you make it as well as your climate have a say in exactly how low a temperature would something like a vent be able to keep.
There are various methods folks have used over the centuries to ventilate their root cellar, but a couple of vents, one at the top and one at the bottom are the most common choice. Place these on opposite walls for better airflow circulation.
Make sure you can control if and how much you close these for even more accurate temperature control.
Depth
Ok, so one of the most important factors is how deep the root cellar will be underground. So, just how deep should it be?
Ideally, a root cellar will be at least 10 feet (3 meters) underground. However, due to climate, soil conditions, and other site factors, you might need to go deeper to ensure a low and consistent interior temperature.
Chances are that unless you live in a place that’s very, very cold, your root cellar will need to be truly underground if you want to stay cool inside. However, it might not have to go as deep as you think depending on many factors…
As a Rule of Thumb, Deeper is Better
One thing to keep in mind when planning the installation is that deeper is better as a rule of thumb. You might not always have to go to a full 10 feet deep to get adequate performance, defined as a temperature between 32° and 40° F (0° C and 4.5° C).
However, if you live in a warmer region, don’t have an ideal spot or if the soil isn’t great at insulating the structure you might need to go deeper than that!
If you want to save yourself some grief, plan on digging your root cellar as deep as you can go safely and finances permitting, and, of course, as long as it is still easily accessible. Nobody wants to descend a mine shaft just to get some potatoes!
Always Try to Get the Cellar Partially Below the Frost Line
Another important thing to remember when installing your root cellar is that you should aim to get it below the frost line. This is the depth at which the ground will freeze. Below this line, the ground will be very cold, but won’t freeze.
If you don’t know what this line is, you can consult the local geology department in your county, or you’ll usually have some success looking it up online.
If you’re dealing with special circumstances or a peculiar soil type, you might need to obtain the use of a probe that can be inserted into the ground to give you an accurate temperature reading at the coldest part of the day in order to determine precisely where the frost line is.
The Type of Soil Dictates How Deep Your Root Cellar Needs to Be
A thorough discussion of different soils and their characteristics is beyond the confines of this article, but the most important parts that you need to know are the thermal properties of the soil, specifically its conductivity and heat capacity.
Conductivity is a metric that measures how readily any given kind of soil will allow the transfer of heat, whereas heat capacity is the amount of heat that the soil can absorb before it changes its temperature. Basically, you want a soil type that has a high capacity and conductivity.
Looser, lighter soils and particularly soils of lots of loam or sand, rate very low on both, and so consequently if you’re installing a cellar in this type of soil, you’ll probably need to go deeper than 10 feet.
Dry Soils Don’t Insulate as Well as Wet Soils
Another consideration is how much moisture the soil tends to retain; trade degree, weather is better in this case because what are soils are more resistant to fluctuating temperatures compared to very dry soils.
Obviously, this doesn’t mean you should run off and build your root cellar next to a stream, pond or lake: it will be likely to flood! But moist soil is definitely an advantage elsewhere, and in some cases, it is possible to build a root cellar very near a water source to maximize cooling.
Tom has lived and worked on farms and homesteads from the Carolinas to Kentucky and beyond. He is passionate about helping people prepare for tough times by embracing lifestyles of self-sufficiency.