Storing potatoes and root vegetables: cold, dark, kept apart

Late September, and the first crate of potatoes is down in the cellar, the box of apples from the espalier right next to it. Looks tidy. Come January there is a sweetish smell in that corner, the tubers have pushed out white shoots the length of a pencil, and your fried potatoes go darker in the pan than they used to.

The mistake did not happen in January. It happened in September, in the twenty minutes you spent deciding where the crate would stand and who would lie next to whom. Storing is not putting things away. It is the last growing task of the season.

In order, then: when to lift, how the tuber hardens its skin, why the cellar must be neither too warm nor too cold, and how to spot a rotting tuber early.

When to lift, and why you take your time

With the potato (Solanum tuberosum), the haulm gives the signal. Once it yellows and collapses, the tuber has finished growing, but it is not yet fit for storage. It needs another two to three weeks in the ground to harden its skin.

The test takes five seconds. Rub your thumb firmly across the skin of a tuber you have dug up. If it peels away in shreds, wait a while longer. If it stays put, the tuber is skin-set. Early potatoes deliberately never form that skin, which is why they taste so tender in June and are mush by December.

Lift after two or three days without rain. Wet soil clings to the tubers and is exactly the damp nest in which bacteria get going. The usual window in central Europe runs from mid September to mid October, roughly CW 38 to CW 42.

Two weeks of wound healing, and only then cool them down

Lifting leaves every second tuber with a scratch from the fork. The potato repairs that itself by laying a layer of cork over the injury. For that it needs warmth and moisture, not cold.

So park the harvest in the dark for two weeks at 12 to 15 degrees and high humidity, 90 to 95 per cent. A garage, a shed or an unheated porch are about right on temperature, but usually far too dry. Lay a damp hessian sack over the crates or stand a bowl of water beside them, otherwise the tubers shrivel instead of corking over. Put freshly lifted potatoes straight into 5 degrees and you shut the wound healing down and hold the door open for every fungus. This is also your first sorting date: anything with holes, soft patches or brown marks never makes it into the storage crate in the first place.

The storage spot: 4 to 6 degrees, and genuinely so

The potato has a narrow window. Above 8 degrees it grows restless, from 10 degrees it sprouts. Downwards, something invisible happens: the colder it gets, the more starch it breaks down into glucose and fructose so that its cells do not freeze. This is called cold sweetening, and it creeps in gradually, long before the cellar is truly cold.

You only notice it in the pan. The sweetened tubers brown far faster, go almost black and taste stodgy. The same reaction produces acrylamide. This is why the EU acrylamide regulation requires industry to store potatoes destined for chips and crisps above 6 degrees. The same logic applies in your cellar: if the crate mostly ends up as fried potatoes, keep to the upper end of the range.

If the cellar did turn out too cold, nothing is lost. Cold sweetening can largely be reversed. Stand the tubers in the dark at around 15 degrees for a week or two before you fry them, and the tuber respires the sugar away again. An unheated cellar room against a north wall usually sits right in the window through winter; a garage in a long frost does not.

Then there is humidity: 85 to 95 per cent. In a dry cellar with the boiler in it, the tubers shrivel within weeks.

Why apples and potatoes belong apart

Apples breathe out ethylene, the ripening gas fruit uses to tell its neighbours that it is ready. On freshly harvested storage potatoes it acts like an alarm clock: after lifting, the tuber rests in a natural dormancy, and ethylene breaks it. Instead of sleeping until March, it starts pushing shoots in December.

The kitchen tip that “an apple in the potato crate keeps them from sprouting” is aiming at something else: at shop-bought potatoes whose resting phase is long over, where a steady, very low gas concentration really can slow sprouting. You will not achieve that in a crate at home.

Carrots react to the same gas even less pleasantly. They form bitter compounds and stop tasting of anything much after a few weeks in an apple cellar. If you only have one room, distance alone helps little, because ethylene is a gas and spreads through the whole cellar within hours. What does work is a tight barrier: put the apples in sealed boxes or perforated plastic bags, and air the room out thoroughly and often.

Carrots, beetroot and celeriac: the sand box

In store, root vegetables mostly lose water. An unprotected carrot is soft and bendy after four weeks. The old remedy is a box of slightly damp, salt-free sand, coarse builder’s sand for instance. Damp means this: a handful squeezed together holds its shape briefly, then crumbles apart again. If water drips out, it is too wet. First a layer of sand, then the roots side by side without touching, then sand again, and so on.

Temperatures here sit lower than for the potato. Carrots (Daucus carota subsp. sativus) like 0 to 2 degrees at very high humidity, beetroot (Beta vulgaris subsp. vulgaris, Conditiva group) and celeriac around 1 to 4 degrees, swedes and winter radish likewise just above zero. If one room is all you have, take the potatoes as your benchmark and stand the sand boxes on the floor, where it is coldest. How this fits together for beetroot, from sowing right through to the earth store, is in beetroot from seed to earth clamp.

What may stay in the ground

Not everything has to go into the cellar. Parsnips and salsify stand out in the bed all winter and are actually made milder by frost, because they convert starch into sugar. Put a fleece or a layer of leaves over them so you can still get at them when the ground freezes, and you harvest fresh right into spring. More on that with the forgotten winter vegetables. Potatoes emphatically cannot do this: from about minus 2 degrees in the soil the tubers go glassy and turn to mush as they thaw.

Spotting rot early

One single rotting tuber in the middle of the crate will, with bacterial soft rot, infect its immediate neighbours first, and from there it runs on through the layer. How bad the damage gets depends on one thing only: how quickly you find it. Which is why it pays to look at the commonest patterns properly, once.

The weekly round that saves the lot

1. Fix a day

   Sunday morning, light on, five minutes. Not when you happen to think of it, but always on the same day. Check every six weeks instead and you will not find a single rotting tuber any more, you will find a nest of them.

2. Smell first, then feel

   Soft rot announces itself through your nose long before you see it. If it smells sharply putrid, go through the crate layer by layer. Otherwise it is enough to run a flat hand over the top layer: a healthy tuber is rock hard, anything that gives comes out.

3. Rub off sprouts and air the room

   Rub short sprouts off so the tuber stops spending its substance on them. In the kitchen, peel those tubers and cut the eyes out generously. Once the sprouts are long, the tuber soft or shrivelled, or green patches appear, it belongs on the compost and not in the pot. Then open the window for ten minutes and feel the sand: bone dry means dampen it, clammy means leave the lid off.

You decide how well it stores back in spring

The most honest sentence on the subject is this: an early potato does not become storable because you make an effort. Early varieties such as ‘Annabelle’ or ‘Sieglinde’ have a thin skin and a short dormancy. In summer that is exactly their virtue; in winter it is their undoing.

For the cellar, choose varieties with a stated aptitude for storage: firm skin, long dormancy. ‘Ditta’ and ‘Laura’ are second early, ‘Agria’ second early to maincrop, ‘Blauer Schwede’ second early as well, and all four still keep into spring. So maturity group alone is not the criterion. Look in the seed potato catalogue for the note that says good keeper or suitable for clamping.

You sort carrots the same way. Bunching carrots are lifted early and thin and are summer produce. For the sand box, take late, thicker varieties from the Flakkee or Berlicum group. Plenty of Nantes types are expressly good keepers too; what matters is the words storage carrot on the label.

For this season the variety question was settled long ago, so all that helps now is honest sorting: anything thin-skinned goes in the kitchen crate and gets eaten first. For next spring the rule is this: note down which row holds which variety as you plant them. What you forget in April you certainly will not recall in September, and then the early potato ends up in the storage crate. How to plan the bed for it is in planting, chitting and earthing up potatoes. Speared tubers and snapped carrots are not waste, they are the processing crate: drying or fermenting.

The lovely thing about a store is that it gets better year on year, because you get to know your own cellar. Note in your garden journal when you lifted, which variety lasted how long and from which week the crate began to sprout, and after three winters you will have a body of experience no guidebook can hand you. Best of all, set the weekly visual check up as a recurring task, so it is the garden reminding you rather than your conscience.

Common questions about storing

What temperature should I store potatoes at?

Between 4 and 6 degrees, dark, at 85 to 95 per cent humidity. From about 10 degrees the tubers start sprouting. The colder it gets, the more starch they convert into sugar, so they taste sweetish and form more acrylamide when fried or deep fried. If they are mainly destined for fried potatoes or chips, stay above 6 degrees. If the cellar did run too cold, stand the tubers in the dark at around 15 degrees for a week or two and the tuber will break the sugar down again. An unheated cellar on the north side usually hits the window; a garage in a long frost does not.

Can I store potatoes and apples together in the cellar?

Better not. Apples give off ethylene, and this ripening gas breaks the natural dormancy of freshly harvested storage potatoes, so they sprout far earlier. Carrots in the same corner form bitter compounds under ethylene. If you only have one room, distance achieves little, because the gas spreads through the whole cellar. Put the apples in sealed boxes or perforated plastic bags instead, and air the room thoroughly and often.

Do I have to wash potatoes before storing them?

No. The soil clinging to them is a protective film against drying out and against microbes. Water softens the skin and runs into every scratch, and inside a crate it never dries off again. That is how you invite soft rot in. After lifting, let the tubers dry for an hour or two in the shade or on an overcast day, so the skin does not turn green. Washing happens shortly before cooking, not before.

How do I store carrots so they do not go soft?

In a box of slightly damp, salt-free sand at 0 to 2 degrees. Twist the tops off close above the crown rather than cutting them, and the break corks over by itself. Bed the carrots in layers without letting them touch, and cover them with sand. The sand should briefly hold its shape when squeezed, but give off no water.

Are green potatoes poisonous?

Green patches show that the tuber has had light, and in response it forms glycoalkaloids such as solanine. These compounds are heat stable: boiling, frying and deep frying do not destroy them. Some of it passes into the cooking water, which is why you pour that away. The German Federal Institute for Risk Assessment advises cutting green patches and eyes out generously, and not eating green, sprouting, old or shrivelled tubers at all. Small children should only be given peeled potatoes. Larger amounts cause nausea, stomach pain, vomiting and diarrhoea. If the dish tastes bitter or scratchy, do not eat it.

Which potato varieties are suitable for storing?

Varieties with a stated aptitude for storage, meaning a firm skin and a long dormancy, for example ‘Ditta’, ‘Laura’, ‘Agria’ or ‘Blauer Schwede’. They sit in the second early to maincrop range, but what counts is not the maturity group, it is the catalogue note saying good keeper. Early varieties such as ‘Annabelle’ or ‘Sieglinde’ are unsuitable: thin skin, short dormancy, and by December they either sprout or shrivel. The decision is made in spring, when you buy the seed potatoes.