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Aug 122014
This lovely large root cellar dates back to the mid 1800s and is underneath a farmhouse in Lancaster, PA.

This lovely large root cellar dates back to the mid 1800s and is underneath a farmhouse in Lancaster, PA.

Many people add a root cellar to their retreat.  This is good, but if you are not careful with what you store in your root cellar, the gases (notably ethylene) given off by some stored fruit and vegetables may interfere with the longevity of other stored fruit and vegetable items.

In addition, some items give off strong odors which could contaminate other stored produce.  And some produce prefers warmer or cooler temperatures, and greater or lesser amounts of humidity, than others.

So maybe you potentially need multiple root cellars – or at least some barriers or partitions across your single root cellar.

Let’s first consider root cellars in general, then look at why you should have more than one – and/or how to avoid needing to have multiple cellars.

What is a Root Cellar

Root cellars have been used in the US pretty much from the days of the first settlers, and are thought to date back to the 1600s in Britain (in the ‘modern’ form of being a walk in cellar).  They are not experimental or innovative – they have truly withstood the test of time over many centuries.

A root cellar doesn’t actually need to be underground.   Many are actually above ground.  And the term ‘root’ doesn’t necessarily mean either something down among the tree roots (that would be a mistake, keep well away from tree roots) nor does it mean a cellar only intended for root vegetables.  So it is a bit of a misnomer.

If we had to come up with the absolute essence of what a root cellar is, the answer would probably be ‘a naturally cooled dark space with stable low temperature and high humidity for storing food in an optimum environment to enhance its storage life’.

More specifically, root cellars aim for a temperature range ideally between 32º and 40º F, and a humidity in the range of 85% – 95%.  The cool temperature and high humidity greatly reduces the moisture loss from stored food items, and the low temperature also slows down the rate of micro-organism growth and related decomposition processes.  Not all root cellars manage to get down to these temperatures (or up to these humidities), nor maintain them for much of the year, but that doesn’t completely matter.  The cooler the better, and even if you are ‘only’ in the low 50s, you are still getting longer life than if you had your produce in your main retreat at room temperature.

Root cellars went out of fashion when at-home refrigerators became widely used, and as part of a general trend to city living with nearby supermarkets that carried fresh food year-round.  In that context, there’s little need for a root cellar any more, but if the assumptions of convenient home refrigeration and ever-present fresh food in a nearby supermarket start to fail, then a low-tech way to store food becomes helpful once more.

Note that while most people associate root cellars with the storage of fruit and vegetables, there is no reason not to use your cellar to store anything else that likes a cool dark environment.  Cured meats, cheeses, fresh milk, and beverages in general could also be kept in a root cellar if space allowed, as can dried goods such as grains and nuts.

Three Types of Root Cellar

There are basically three ways to build a root cellar.  The first is the most obvious.  Dig.  Start in the basement of your current house or retreat, and just dig down and out until you’ve created sufficient cellar space.  Note that the classic size for a root cellar seems to be about 8′ x 8′ x 8′, but there’s no reason not to make a cellar larger or smaller, but note that the larger you make a cellar, the more the ratio between the volume of the cellar and the surface area of its sides will change, affecting the cellar’s ability to naturally heat/cool the cellar contents.

So, perhaps, it is best not to build a huge cavernous cellar, although the chances are you weren’t planning to do that anyway!

The second approach can sometimes be easier.  Instead of digging down vertically, you dig ‘in’ horizontally, going into the side of a hill.  The net result is the same, while the excavation process might be simpler.

The third approach involves some lateral thinking.  Instead of going down into the ground, bring the ground up to you.  Create an above ground structure, or perhaps a slightly sunken structure, then layer sod over the top of it.

If you are building an external above ground cellar, you want to have it as much as possible in the shade – ie with little direct southerly exposure, and in particular, you don’t want the doorway (which is probably the least insulated part of the structure) to be in direct view of the sun.

How to Create and Maintain the Cellar Environment Needed

Depending on where you live, you’ll probably need your cellar to do two opposite things.  In the summer, you want it to be cooler than the warm/hot outside temperatures, but in the winter, you want it to be warmer than the below-freezing temperatures outside.

The best way to do this is by either digging deep into the ground, or covering an above ground structure with a lot of sod.  Even a foot of dirt provides substantial insulation and will allow for as much as a 20º temperature differential between the cellar and the outside, but the chances are you’ll want more than this, so you need both more dirt ‘insulation’ and also the ability to ‘suck heat’ out of the cellar if too hot, and ‘pour heat’ into the cellar if too cold.  This requires a lot more dirt, and the dirt changes from merely being insulation to becoming a ‘heat sink’.

The first few feet of soil tend to seasonally vary a bit in temperature, but by the time you get down 10 ft or so (or ‘in’ a similar distance if digging into a hillside) you are then in a region where the soil temperature remains more or less unchanging, year-round and there’s no point in going any deeper.  As long as you don’t stress the soil around your cellar by introducing too much heat or cold – more than the soil can absorb/conduct away – the walls, floor and even ceiling of your cellar will all act as ‘automatic’ heat sinks, helping maintain a reasonably steady temperature inside the cellar.

Having said that, although the walls will stay much the same in temperature, it is probable there will be some variations in temperature inside the cellar itself, because the ability of the walls to soak up or give off heat is not very great, and outside factors such as the air temperature coming in can overwhelm the natural heat stabilizing of the walls.  A good cellar will keep temperatures above freezing in the winter, and perhaps 40º below outside temperatures in the summer.

The air flow in the summer will obviously have much warmer air coming in from outside than in winter.  You can moderate this a bit by having a ‘solar heater’ that you can attach to the air intake during the winter (nothing fancier than simply using a black painted inlet that the sun can shine on and warm up) and take off during the summer.  During the winter, have most of your airflow when the sun is shining on the inlet, and least during the cold of the night.  The opposite would apply for the summer, with little air flow in the hottest times of the day and more airflow in the coolest times of the evening.

You can also use evaporative cooling in the summer, with the air flow into the cellar passing over a wet cloth.  This helps to cool the air down and also increase its humidity at the same time.

In an ’emergency’ some people provide some gentle heating by simply leaving an incandescent light on in the cellar, while making sure that its light doesn’t harm any of the stored produce.  An incandescent light converts nearly all its rated power to heat, so if you wanted a mild 60 – 100 watt heating element, a light bulb would be the easiest approach.

One more thing about temperature.  By the time midsummer and the hottest temperatures come along, you’ll probably have emptied your root cellar from the last season’s stored foods, and so it won’t matter so much if it warms up a bit then, although you want to always keep temperatures as close to optimum as possible so as not to cause a gradual build-up of heat in the dirt walls.

You also want to have a high humidity.  Again, the ‘magic’ of a root cellar is that the water contained within the dirt walls and floor and ceiling will ‘automatically’ release moisture to keep a high humidity – assuming you don’t overload the ability of the cellar to maintain its humidity by creating too many air changes and therefore removals of moisture/humidity as part of that.

If you need to increase the humidity, you can simply spray water onto the walls, floor and perhaps ceiling of your cellar.  If you need to decrease the humidity, the usual solution is to increase the air flow, but that may cause other problems if the outside air is very hot or very cold, so don’t get too carried away with spraying extra water.

So as to get the most direct impact from the dirt, it is best not to line your cellar any more than might be essential, although it seems that most of the cellars we see these days are at least partially lined – perhaps because it looks ‘cleaner’ and ‘nicer’, even if it harms the cellar’s functionality!  If you are lining the cellar at all, make sure to use materials that won’t be harmed by the moisture – the moisture in the soil and the moisture within the cellar.

Shelving in the cellar is traditionally made of wood rather than metal.  The wood itself changes temperature slowly, adding further to the thermal inertia.  If you are using wood, we recommend you do not use treated wood (due to the poisonous chemicals in it) but rather choose wood that is least likely to rot in moist conditions (such as cedar).  Bricks and concrete blocks can also be used for part of your shelf construction – these are odorless and last a long time in damp conditions.

Shelving should be as open as possible, and set back from the walls, so as to allow for air flow everywhere.  This will keep down the growth of mold.  Be careful also when stacking produce so as to allow air to flow through the produce, and generally it is best not to store anything directly on the ground.

One other aspect of your cellar – you want it to be normally dark.  Light is an energy source which variously activates the sprouting of some produce and encourages the growth of undesirable organisms.  Keep the cellar dark except for when you visit it.

The cellar does need some fresh air flow, however.  There’s a trick to this to create a natural air flow without needing as much machinery.  You should have an air entry on one side of the cellar and an air exit on the other side, so air flows between them.

Now for the clever part.  Your air entry inlet should come in from outside and open at close to the floor level.  The air exit outlet should start at a point close to the ceiling.  This means the hotter air in the cellar will naturally rise up and out the exit, sucking in replacement fresh air from outside, where it will land in the cooler lower parts of the cellar, before gradually warming and then exiting again.

Of course, both the inlet and outlet need to have dampers on them so you can regulate the flow of air.  They also need screens so that rodents can’t enter your cellar through the air vents.

There is always a temperature gradient within your cellar, perhaps of 5º, maybe even 10º, as between its floor and ceiling.  You should keep that in mind when deciding where in the cellar to locate the various different produce items you’ll be storing.  Onions, garlic and shallots are probably the most temperature tolerant things you might be storing, so put them on upper shelves.

Visiting Your Cellar

We suggest you limit your visits to your cellar to no more than one a day.  If you’re struggling to keep the temperature optimized, you might even cut back on your visits to once every two or three days.  The less you stress your cellar with unwanted adverse changes of air and introduction/escape of heat and moisture, then of course the better it will perform.

This should not be a problem if you accept the discipline and requirement of moving things in/out of the cellar no more than once a day.  Surely it is easy to transfer produce from the cellar to a convenient at-hand storage facility elsewhere in your retreat on an occasional basis, and then whenever needed, take from the at-hand facility.  And, when replenishing, you can build up a pile of new produce immediately outside the cellar, and at the end of a day’s harvesting, then move everything in to the cellar all together.

If this is a problem, and if you’re struggling with maintaining a suitable cellar temperature, you might want to consider making the entrance into an ‘airlock’ type double door arrangement so as to cut down further on the environmental impact in the cellar every time you open the door.

You should carefully monitor your cellar’s temperature and humidity, and you will need to adjust the ventilation going in/out of the cellar to keep the temperature optimized.  We suggest you either have thermometers and hygrometers visible through an inspection window, or alternatively, if using electronic sensors, of course these can display remotely, anywhere in your retreat you wish.

The vent adjusters should either be routed mechanically to a point outside the cellar where you can open/close them, or else be operated by remote-controlled servo-motors.

Oh yes, please also remember to keep the light switched off in the cellar when you’re not present.

Do You Need Multiple Cellars?

There are two major concerns that some people feel can justify either the creation of multiple cellars or at least partitioning off one single cellar.

The first of these is that some things – apples, peaches, pears, plums, cabbage and tomatoes in particular – emit ethylene gas while stored.  Unfortunately, the released ethylene harms produce – even the produce that releases the ethylene in the first place!  So you need to keep the ethylene releasing produce as separate as possible from other produce, especially the root veges, and well ventilated to protect it from itself.

That’s the hint that can suggest how you could manage with one cellar instead of two.  Put the ethylene emitting items close to the exit vent so the ethylene mainly gets swept up and exhausted out of the cellar, while keeping the root vegetables in the other corner, and closer to the air inlet.  This keeps the ethylene away from other produce, and also vents it away from the emitting produce too.

The other main issue is odor control.  Some things – turnips, for example, or cabbage – give off odors that would get absorbed into other items if stored close to each other.  One solution is not to grow and store turnips and cabbage!

Another solution is again to put the smelly stuff closer to the air exhaust outlet, and to keep the more sensitive produce far away.

So you are probably correctly now sensing that managing ventilation is an essential part of having a successful root cellar.

There is another consideration as well that might influence whether you have one or two root cellars.  Different produce items are best stored at different temperatures, and if you had sufficient fine control over your root cellar temperature as to be able to ensure one cellar was (say) 10º different to the other, and if you had a range of produce items that could benefit from this temperature differential, then having multiple cellars might make sense.

But unless you’re going to be supplementing your natural heating/cooling with artificial heating/cooling, you’d probably find that two root cellars would have very close to the same temperature.  The better approach to temperature management is simply to stratify the location of your produce, keeping in mind that the higher up in your cellar, the warmer it will be.

So, for most of us, we can probably get by with ‘just’ a single root cellar, but keep these issues in mind when deciding where to locate the produce within it.

For Further Information

This article, although spanning over 3000 words, only lightly touches on the topic of root cellars.

Unless there is a reason why a root cellar would be impossible (ie, you are an apartment dweller with no plans to have any sort of land or rural retreat) you should definitely add a root cellar to your retreat and so it is an important topic to understand and get right.  A root cellar is a wonderful and energy-efficient way to store many different types of produce, giving you well-preserved food long out of season, without any need for the hassle and energy costs of boiling, blanching, bottling, canning or freezing.

To learn more, you can certainly roam via Google to other articles on root cellars, but can we modestly say that you’re not likely to find much more than you’ve already read here.  The best thing to do is to get a copy of the definitive book on the subject – Root Cellaring :  Natural Cold Storage of Fruits and Vegetables, by Mike and Nancy Bubel.  This 320 page book not only covers cellar design and construction, it also guides you in the choice of produce to store in your root cellar, and even tells you when to harvest and store the items you grow.

Amazon sells the book both as a Kindle eBook and in regular print.  It is better, if buying the regular print edition, to ensure you are getting the latest edition – not the original 1979, but the second 1991 edition.  For about $10, this is an excellent investment.


David Spero[suffusion-the-author display='description']

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