Late October, the barrow is full and the sums do not add up. Fifteen kilos of carrots, a basket of beetroot, two crates of potatoes, plus celeriac and a few swedes. In the fridge there is one free salad drawer. That covers you for four days.
A hundred years ago nobody stood at this point wondering whether another chest would fit in the hallway. They dug. The earth clamp was the standard store on every farm and in every allotment: a shallow pit, a layer of straw, soil on top, a bundle of straw as a vent in the middle. It still works today, because half a metre down the soil behaves exactly as it did back then.
Here is the order of things: why soil cools better than any crate, how to build a clamp that still smells in February the way it did in October, what belongs in the sand box instead, and which vegetables you simply leave in the ground.
The short version
- The soil does the work, not the construction. It damps out the day and night swing, and through winter the ground mostly sits between 1 and 5 °C. That is exactly what root vegetables want.
- Straw separates, soil insulates, the vent breathes. Without ventilation the respiration heat builds up, the vegetables go damp and rot follows.
- Frost is the expensive mistake. Potatoes, beetroot and celeriac are killed at around minus 1 to minus 2 °C. When a long freeze is forecast, add another layer.
- Damp is good, wet is the end. 85 to 95 % humidity is right, waterlogging destroys a clamp from below.
- Lift them soily, do not wash them. The soil clinging to the root is its protection against drying out. Washed carrots go limp within weeks.
- Once opened, emptied completely. Which is why two small clamps beat one large one.
Why soil cools better than any crate
The air above the bed takes everything winter throws at it: minus 8 °C at night, plus 6 °C in the afternoon, and a fortnight later a hard frost that sits for days. Soil damps that down with every centimetre of depth. At 20 cm you still notice the daily swing, at 50 cm you barely do. Down there the ground in the lowlands sits mostly between 1 and 5 °C through the winter and keeps cooling slowly until February. What the soil evens out is the day and night swing. The run of the seasons still gets through, only slowly.
Then there is the moisture. In a dry cellar with heating pipes, stored vegetables lose water and turn fibrous, while in the ground the relative humidity is almost always above 90 %. And under the soil covering no potato turns green, because no light gets in at all. Whether it sprouts comes down to temperature. A fridge at 4 °C gives you the temperature but not the humidity, and it runs all winter. The clamp costs you one afternoon in October.
The site is where most clamps fail
Look for the highest spot in the garden that does not sit in full winter sun. That sounds obvious, but everything else follows from it. Waterlogging is the killer: if water stands in the pit after a long November rain, the contents rot from below within a fortnight. A gentle slope or a sandy soil beats the tidiest pit in heavy clay.
A north side or the shade of a hedge is better than a south-facing spot, because the February sun heats a dark soil covering considerably. And one point the old gardeners took seriously: never build where a clamp stood the year before. Rot organisms and mouse runs stay in the ground.
The clamp's big sister
Where space and a slope came together, the clamp grew into a cellar. The principle is identical, only permanent: undisturbed ground all around, a door facing north, and a clay floor that gives off moisture.
Cellars like this still stand in plenty of villages, often unused behind the barn. Anyone who inherits or rents one is spared the digging for the next twenty winters.
Air an earth cellar before you go in
Stored vegetables keep breathing and give off carbon dioxide. The gas is odourless, invisible, heavier than air, and it collects unnoticed at floor level in a low, long-closed room. Open the door and the vent at least half an hour beforehand, and in a deep cellar put a fan in the doorway to draw the air out. The old candle test is no help, the flame only goes out long after the air has turned dangerous. Never go down alone or with children, and have somebody wait at the door above. A permanent inlet and outlet vent is a requirement in a cellar like this, not a comfort.
The earth clamp step by step
The classic window is CW 40 to 46, so late September to mid November. For 2026 that means the weeks from 28 September to 15 November. You lift after the first light night frosts, usually CW 43 to 46, and whatever you lift goes into the clamp the same day. If you have to clear the bed back in September, keep the roots cool and dark in crates until then. The colder the soil and the vegetables are when you shut it, the quieter the clamp lies. Close up in late September and you seal the autumn warmth in with the crop, which then respires hard and sweats. In higher country you start earlier, in the Rhine valley or the Vienna basin you have until mid November.
Dig the pit and drain it
Roughly 100 to 120 cm wide, as long as you need, 30 to 50 cm deep. A hand's depth of coarse sand goes on the bottom against seeping water. Pile the spoil beside it. On heavy clay you do not dig at all, you build the clamp straight onto undisturbed ground. Even a 50 cm pit does not put the crop half a metre down, because the drainage sand and the straw lift it again. The covering makes up the difference: straw and soil on top stand in for the centimetres you are missing below.
Lay the mouse mesh
Close galvanised wire mesh, 6 mm gauge at most. Go by the number on the roll rather than the product name: mesh sold as aviary wire also comes in 12 or 19 mm, vole mesh is usually 13 mm, and a field mouse walks straight through that. The mesh goes over the drainage layer and is turned up at the sides so that the later covering overlaps it.
Put in the first layer of straw
15 to 20 cm of dry, clean straw, lightly pressed down. It insulates, absorbs moisture and keeps the vegetables off the bare soil. Hay is no good, it goes mouldy.
Layer up the vegetables
Only flawless, dry, unwashed produce, with the tops twisted off or trimmed to 1 to 2 cm and never cut into the crown. Set them in loose layers with a hand's depth of damp sand between, each kind of vegetable kept to itself. The heap ends up pointed at the top, otherwise water sits on it.
Set the ventilation chimney
A firm bundle of straw running vertically up through the middle, or a length of drainage pipe. The chimney carries off respiration heat and carbon dioxide, and without it the clamp sweats. Tie the mouth shut with the same 6 mm mesh, otherwise the chimney is the last open door for the mice. A little cap that leaves an air gap at the sides keeps the rain out. Never a sealed cap.
Cover up and top up against frost
Another 15 to 20 cm of straw, then 20 to 30 cm of soil, patted smooth, with a shallow channel all around to lead water away. The chimney stays clear. When a long freeze is forecast, add leaves, spruce brash or straw mats and weigh them down with boards. This is not optional: under 20 to 30 cm of soil the crop sits far closer to air temperature than it would half a metre down, and potatoes, beetroot and celeriac are killed at around minus 1 to minus 2 °C.
Mark the chimney
Push a long cane in beside the ventilation chimney and tie some bright tape to it. After the first snowfall you will otherwise find neither chimney nor clamp, and you will tread the opening shut while adding leaves.
Mice will ruin a clamp in a week
Voles and field mice find an earth clamp without fail. They do not just eat, they gnaw at many roots at once, and every gnawed root then rots. Check in December for fresh soil throws and empty the clamp early rather than leave it to the mice.
What belongs in a clamp and what does not
The choice comes down to a single question: does the crop tolerate cool and damp? Root vegetables say yes, almost everything else says no.
Crops put to the clamp test
- Carrot, beetroot, celeriac
The classics, cool and damp is their climate. With carrots and beetroot, twist the tops off or leave 1 to 2 cm of stalk, and never cut into the root itself, or it bleeds and rots at the cut. Celeriac wants the same humidity but no standing wet at the crown. Cut its leaves off just above the bulb and bed the bulb into a middle layer, with slightly drier sand around it. It does not belong right at the top: that is the coldest place, and at minus 1 to minus 2 °C the celeriac is finished.
- Swede, winter radish, late kohlrabi
They keep superbly, because their skin is thick. Late kohlrabi and winter radish go into the clamp with the root and a stub of leaf base still on.
- Potatoes, but carefully
Below 4 °C the tuber converts starch into sugar and tastes sweetish, above 8 to 10 °C it sprouts. In a clamp you sit right on that line: cover the potatoes more thickly and keep them separate, which holds them nearer 4 to 6 °C. Tubers that have turned sweetish come back to normal if you leave them at room temperature for a fortnight before cooking. At minus 1 to minus 2 °C the tuber is lost: glassy, sweet, and mush the moment it thaws, and frozen tubers never go back into the clamp. Once they come out, the dark rule applies again, because green patches are solanine and cooking does not destroy it. More in storing potatoes and root vegetables.
- Storage cabbage works, but not in company
White cabbage, red cabbage and savoy go in upside down, that is with the stalk pointing up. They smell and they need more air, so give them their own clamp. More in storing cabbage.
- Not for the clamp: pumpkin, onions, garlic
Pumpkin wants 10 to 15 °C and dry air, in damp conditions it rots at the stalk, and below 10 °C it takes chilling damage. It belongs in a dry, frost-free room at around 12 °C: an unheated hallway, a cool bedroom, a store cupboard on the north wall. Next to a radiator at 20 °C it keeps only a few weeks, and in an uninsulated loft the first frost gets it. Onions and garlic sprout in damp air and rot at the neck, so they want dry, airy and frost-free. More in harvesting and storing pumpkin.
- Better left outside
Parsnips, salsify, parsley root and Jerusalem artichoke are so hardy that they overwinter in the bed and taste better for it. The reasons are in parsnip and salsify.
Daucus carota subsp. sativus
- Familie
- Umbellifers
- Standort
- Sunny and free of stones
- Saison
- Lift maincrop carrots CW 43 to 46
- Wasserbedarf
- Even, never wet
- Frosthärte
- Light frosts fine, prolonged frost damages the roots
The sand box for indoors
If you have no ground to dig, you rebuild the clamp in miniature. A wooden box, damp sand, root vegetables in layers: the same principle, only in the cellar and without a spade. The box replaces the humidity, though, nothing else. The room has to be cold on its own: above 10 °C it will not work, and the box is better off in an unheated garage or a porch.
Beetroot (Beta vulgaris subsp. vulgaris, Conditiva group) keeps just as reliably in one as carrot does. All that matters is that the roots go into the sand dry and with their soil still on.
How to layer a sand box
Take a wooden box or a shallow fruit tray with no plastic liner and put in two fingers of slightly damp sand. The rule of thumb: the sand should hold together when you squeeze it and fall apart again when you open your hand. If it sticks, it is too wet.
Then set the roots in side by side without letting them touch, and pour sand over until none of them shows. Second layer, sand again. Three or four layers make sense, any more gets too heavy to shift. If the sand throws dust when you check it, damp it with a hand sprayer. With carrots the crown has to stay intact, because a cut into the flesh is the way in for rot. More containers in storing your harvest.
Apples belong in a different room
Apples and pears give off ethylene. The ripening gas makes carrots bitter and yellows cabbage faster. With potatoes the effect cuts both ways: short bursts break the dormancy of freshly lifted tubers, while a steady low level of ethylene suppresses sprouting and is used in commercial stores for exactly that. So rely on neither direction and keep the two stores apart. Distance alone helps little, because the gas spreads through the whole cellar within hours. If one room is all you have, put the fruit in sealed boxes or perforated plastic bags and air the place thoroughly and often.
The old natural cellar and its clay floor
A cellar built before 1950 with a rammed clay or brick floor is something you can barely have built new. That open floor is not carelessness on the builders' part, it is the reason the room works: moisture rises constantly out of the ground, the humidity stays at 85 to 95 %, and that is exactly what keeps root vegetables plump. A concreted, sealed cellar is dry and often too warm because of the heating pipes, and there carrots shrivel and potatoes sprout in January.
What ruins a natural cellar is above all the wrong kind of airing. In summer warm outside air carries moisture in, and it condenses on the cool walls. So you air in the cold half of the year, and at night, but only while it stays above 0 °C outside. In a frost the door and the flaps stay shut, or you pull the frost straight into the store. A thermo-hygrometer tells you whether your cellar will do the job: 2 to 8 °C at over 85 % humidity is storage climate.
Heeling in: the bed as a store
The simplest way of keeping a store is the one where you never take the vegetables out at all. To heel in, you lift the roots, trim the tops and push them close together at an angle into a shallow trench at the edge of the bed, all the crowns facing the same way, then draw soil over them. Leaves or straw go on top of that, and over that a fleece or a few spruce branches against the wind.
This is not a makeshift, it is a deliberate middle option: quicker to reach than a closed clamp, and no wire needed if the spot is free of mice. It only holds until about CW 8, though. Parsnip and salsify stay standing where they grew anyway. With leeks that applies only to winter varieties, and you bring summer and autumn ones in before the first long freeze. More in harvesting winter vegetables.
In a long freeze you cannot get at it
A heeled-in bed is out of reach at minus 8 °C with the ground frozen solid. Fetch a week's worth into the house before a forecast cold spell. That is precisely what gardeners used to do.
A clamp is closed once and opened once. In between you only look after the roof over it.
Old gardener's rule
Checking through winter, and when to open
Once it is closed, the clamp does not get opened again. Walk out to it after every change in the weather and look: has the soil covering sunk? Is water standing in the channel? Is the ventilation chimney clear? Are there fresh soil throws at the edge? Sunken patches get topped up, a blocked chimney gets cleared. Snow is not a problem here, it is an extra layer of insulation. A frozen chimney is a problem, because then the moisture inside climbs.
You open on a mild, dry day with no frost in the ground, and then you do it properly: you do not take out three carrots and shut it again. Opening lets warm air in, the vegetables start to respire, and the covering never sits as tightly afterwards. Whatever you take out goes into the sand box. Sort as you go, because one rotting root will infect half the layer within weeks. The last clamp gets emptied towards the end of February and into March, so around CW 8 to 10, once the ground thaws. That is when dormancy ends: leave it later and you find the first pale shoots as you empty it, and the vegetables have turned woody.
What the old method teaches you
An earth clamp forces you into something the fridge has trained out of us: planning properly once a year. If you clamp fifteen kilos of carrots and three kilos are soft by February, that is not a failure, it is a number you can improve on next year. Put clamping into the garden journal as a recurring task for CW 40 to 46, and note down what was left when you open up. After three winters you will know which variety keeps in your soil and how thick your covering needs to be. That is the knowledge that used to sit in the neighbours' heads, and it only comes back if somebody writes it down.
Common questions about earth clamps
How deep does an earth clamp need to be?
On free-draining soil a pit of 30 to 50 cm is enough. The covering matters more than the depth: 15 to 20 cm of straw over the vegetables and 20 to 30 cm of soil on top of that. On heavy clay you do not dig at all, you build the clamp as a heap on undisturbed ground and cover it just as thickly.
Which vegetables can you clamp?
Carrot, beetroot, celeriac, swede, winter radish and late kohlrabi all suit it well. Potatoes need a thicker covering, because they turn sweetish below 4 °C and are killed at around minus 1 to minus 2 °C. Pumpkin, onions and garlic do not belong in there, they need dry air. Parsnip, salsify and Jerusalem artichoke you simply leave in the bed.
When do you open an earth clamp?
On a mild, dry day with no frost in the ground, and then you empty it completely. Every opening lets warmth and air in, and the covering never sits tightly again. That is why two small clamps beat one large one. The last one should be empty by the time the ground thaws in March, otherwise dormancy ends and the vegetables sprout.
Why does an earth clamp need a ventilation chimney?
Stored vegetables carry on breathing, and that produces heat, carbon dioxide and water vapour. With no outlet it all builds up under the soil covering, the vegetables go damp, and then rot starts. A vertical bundle of straw up through the middle is enough, a drainage pipe lasts longer. Tie the mouth shut with close 6 mm mesh, or the mice will climb in through the chimney.
How do you keep mice out of an earth clamp?
With close galvanised wire mesh, 6 mm gauge at most. Go by the number rather than the product name: mesh sold as aviary wire or vole mesh also comes in 12 to 19 mm, and a field mouse gets through that. It is laid under the vegetables and turned up at the sides so the covering overlaps it. Chicken wire will not stop voles. Pick a spot where no clamp stood the year before.
Do I need to wash carrots before storing them?
No, quite the opposite. The soil clinging to them protects against drying out, and washed carrots go limp within weeks and rot more readily. Shake the soil off roughly, twist the tops off or leave 1 to 2 cm of stalk without cutting into the crown, and let the carrots dry off for a few hours.
Sources and further reading
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