Storing the harvest: a storage map by temperature and humidity

The beds are cleared, the crates stand full on the terrace, and now comes the question that decides half your winter: where does it all go? Storing is not a last resort for the surplus; it is the real craft of growing your own food. Master it, and in February you are still pulling a carrot 'Rodelika' from your own sand, and in March you slice into a crisp apple 'Boskoop'. The key is not expensive gear but a simple map made of two values: temperature and humidity.

Between CW 38 and CW 44, roughly from mid-September to early November, most storage crops ripen and need a roof over them. These weeks are your window. In this article you get the four climate zones of your house as a map, plus concrete instructions for a sand box, cellar and earth clamp. Let us sort out the corners of your home.

Understand the two axes of your storage map

Before you haul the first crate, a moment of theory saves you a whole winter of trouble. Stored vegetables keep living even after harvest. They breathe, lose water and ripen on. Your job is to slow these life processes to a minimum without killing the cell. You have two levers for that.

Temperature controls the pace. The colder it is, the more slowly the tuber breathes and the longer it keeps. But below zero the cell water turns to ice, the structure bursts, and the carrot thaws into mush. Most storage crops therefore want it just above freezing, around 0 to 4 degrees. Potatoes are the exception: below 4 degrees they convert starch into sugar and taste sweetish.

Humidity controls the water loss. Root vegetables are more than 90 percent water. Kept too dry, they shrivel into rubber within weeks. Kept too wet without moving air, they rot. The trick is high humidity plus a gentle movement of air. Onions, by contrast, want it dry, or they start to sprout.

Once you carry these two axes in your head, every room in your house becomes a field on the map. The unheated cellar room on the north wall is cool and humid. The attic above the warm flat is cool to mild and dry. The garage is at risk of frost. Assign your crops to these fields, not the other way around.

Zone 1: cool and humid, the root cellar

This is the crown discipline and the home for the largest part of your harvest. Carrots, beetroot, celeriac, parsnips, salsify, radish, savoy, white cabbage, leek and winter apples feel at home here. The target is 0 to 4 degrees and 90 to 95 percent humidity. An old, unheated natural cellar with a sand floor hits that almost by itself.

The modern concrete-cellar reality looks different: often too warm and too dry, because the heating pipes run through it. Then you help it along. Set out a shallow bowl of water, hang up damp cloths or, most effectively, pack the vegetables in moist sand. The sand is your portable microclimate.

Build a sand box

The sand box is the simplest way to create a cool, humid climate without a root cellar. You need a wooden or plastic crate, unwashed and slightly moist sand from the builders' merchant, and the vegetables.

1. Harvest carrots, beetroot, celeriac or parsnips on a dry day between CW 40 and CW 44. Shake off coarse soil but wash nothing. Water opens wounds for rot.
2. Twist or cut the foliage down to a short stub of about 1 cm. Leave the root tip on; it is the breaking point for germs.
3. Put a hand's width of sand into the crate as a bed. The sand should be moist enough to hold its shape when squeezed, but not drip.
4. Lay the roots side by side without touching. Cover them fully with sand, then the next layer. That way you build upward layer by layer.
5. Place the crate in the coolest, darkest spot. Every two weeks check whether the sand is still clammy, and sprinkle a little water if needed.

Stored this way, carrots stay crisp into March. The sand holds moisture at the root, prevents waterlogging and separates the pieces, so a rotten spot cannot travel.

Build an earth clamp

If you lack a cellar or the harvest bursts it, the store moves outdoors. An earth clamp is an insulated mound of soil that rebuilds the cool, humid cellar climate in the garden. It suits robust roots and cabbage, not potatoes, which are better off in the dark of the cellar.

1. Find a raised, well-draining spot where no water sits. Dig a shallow hollow about 20 cm deep.
2. Line the base with a layer of sand or straw as drainage. Waterlogging is the enemy of every clamp.
3. Pile the vegetables into a cone-shaped heap, ideally with sand between the layers. Keep different types separate so you can take out what you need.
4. Cover the cone with a thick layer of straw, then the excavated soil on top, about 15 to 20 cm. Let a wisp of straw stick out at the ridge as a vent.
5. Dig a small channel all around to drain rainwater. To take produce out, open the clamp on one side only and close it carefully again.

Zone 2: cool and dry, the attic

Not everything wants it humid. Onions, garlic, shallots, squash and mature dried beans want the exact opposite: 4 to 8 degrees and only 60 to 70 percent humidity. A dry, airy attic, a cool pantry or a frost-free storeroom are ideal.

Onions and garlic keep best airily in nets, crates or as a braided plait. What matters is that they are well dried before storage: after the midsummer harvest, let them cure for two to three weeks in an airy, rain-protected spot until the skin rustles and the neck is bone dry. Only then are they fit to store.

Squash wants it a little warmer than the rest of this zone. Hokkaido and Butternut keep longest at 10 to 15 degrees in a dry spot, for example on top of a cupboard in the living room. When harvesting, always leave a long stalk of several centimetres: if the stalk breaks out, rot has an open gate.

Zone 3: frost-free and dark, the potato corner

The potato has its own demands and deserves its own corner. It wants it cool but not too cold (4 to 6 degrees), moderately humid and, above all, absolutely dark. Light is its greatest enemy in storage.

Store potatoes in airy jute or paper sacks, wooden crates or baskets, never in sealed plastic bags where condensation collects. A dark cellar room is perfect. If light gets in, cover the crates with an old sack or a piece of cardboard.

A second enemy is the neighbourhood: never store potatoes next to apples. Apples give off the ripening gas ethylene, which prompts potatoes to sprout. For how to prepare potatoes and the rest of your root vegetables in detail, read Storing potatoes and root vegetables. That leads us to the last important rule.

Apples and pears: the ethylene trap

Winter apples like apple 'Boskoop', 'Gloster' or 'Topaz' are wonderful keepers and last at 2 to 4 degrees and high humidity into spring. But they are sociable troublemakers. Ripening fruit gives off ethylene, a gas that makes everything nearby age faster. A single overripe apple can tip a whole crate and, in the same cellar, turn the carrots soft and the cabbage yellow.

So store apples separately from the vegetables, ideally in their own room or in well-closing crates. Lay the fruit out at a distance, without touching, stalk up. That way a rotten apple does not jump straight to its neighbour. Treat pears the same; they are a little more delicate and want using up promptly once they soften at the stalk.

Check fruit more closely than vegetables, every 7 to 10 days. The rotten-egg principle applies: whatever turns soft, brown or shrivelled comes out at once, before it infects the neighbours.

The golden rules that apply to everything

Whichever zone you store in, four principles decide between success and failure. Store only the flawless: every bruise and every grub is an entry point for rot, so anything damaged gets eaten straight away. Put the produce in dry but unwashed, because a film of water is the runway for mould; the thin layer of soil even protects it. Let the air circulate, so never seal crates airtight and keep them away from the wall. And check regularly, every 10 to 14 days, following the rotten-egg principle.

And if a crop simply will not keep, such as tomatoes, courgettes or beans, that is no loss but the natural handover to the next method: preserving, canning, fermenting. For how to do that safely, see Preserving and canning: the basics. The jar is the extension of the cellar.

What goes into storage and when

To close, a rough calendar for your storage weeks. The exact dates depend on variety and region, but the order holds almost everywhere.

• CW 38 to 40: onions and garlic have long since dried and move into their final nets. The first squash cure.
• CW 40 to 42: potatoes are lifted in dry weather, dried off and stored in the dark. Winter apples and pears are picked as soon as they twist easily off the branch.
• CW 42 to 44: root vegetables go into the sand box or cellar. Cabbage is harvested. The earth clamp is built once it turns reliably cool.

Give these weeks your deliberate time. A well-sorted store is a quiet larder that brings you joy all winter, every time you step into the cellar and pull a firm, cool carrot from the sand.