What is a root clamp and how do you build one to store vegetables over winter?

A root clamp is an outdoor storage method that uses the insulating properties of straw and soil to keep root vegetables cool, humid, and frost-free through winter — without a cellar, root store, or any purpose-built structure. Used by farmers and smallholders for centuries, a clamp can store hundreds of kilograms of carrots, parsnips, beetroot, or swede in a tight outdoor pile that maintains near-ideal storage conditions through temperature extremes.

If you have a large harvest of roots but no suitable building for storage, or if your shed is too warm or too cold, a clamp is a traditional, low-cost solution worth understanding.

Why clamp storage works

The straw layer provides insulation that buffers against both frost and warm spells, keeping the temperature inside the clamp remarkably stable — typically 1–5°C through winter regardless of outside temperature swings. The soil covering prevents the straw from blowing away, adds further insulation, and creates an almost airtight environment that keeps humidity high around the stored roots. The roots themselves also produce a small amount of heat as they slowly respire, which contributes to frost protection.

How to build a root clamp

Choose a well-drained site — never build a clamp where water pools. Lay a 20–30cm layer of dry straw on the ground as a base. Pile the roots in a cone or ridge shape on top — carrots and beetroot pile neatly; parsnips need a bit more care to arrange. The pile should not be more than about 80cm high to allow heat to escape. Cover the sides and top with a thick layer of straw, at least 20–25cm deep. Then cover the straw with 10–15cm of soil dug from around the clamp, firmed down and patted smooth to prevent gaps. Leave a small tuft of straw sticking out from the top of the cone — this acts as a ventilation chimney, allowing warm air to escape without letting frost in.

In mild areas with few hard frosts, the straw layer alone may be sufficient. In colder areas or during prolonged cold snaps, a second layer of straw and soil, or a covering of old carpet or cardboard under the soil, adds extra protection.

Accessing the clamp during winter

To take vegetables from the clamp, peel back the soil and straw from one end, take what you need, and reseal carefully. Each time you open the clamp you allow cold air in and disturb the insulating layer, so plan your access to minimise how often you open it. Many growers build the clamp in sections so they can remove one section completely while leaving the rest sealed. Repack and re-cover the opening firmly each time to restore the temperature stability.

What roots are suitable for clamping?

Carrots, parsnips, beetroot, swede, turnip, and celeriac all store excellently in a clamp. Potatoes can be clamped but must be in complete darkness — the straw and soil covering achieves this, but any light exposure causes greening. Onions and garlic are not suitable for clamping because they need dry air circulation rather than the humid, enclosed conditions a clamp provides. Squash can be clamped in dry straw but prefer slightly warmer conditions than a clamp typically provides in winter.

Checking and trouble-shooting

If the outside of the clamp feels warm — warmer than the surrounding ground temperature — it may indicate rotting inside generating heat. Open and inspect if you suspect this. A clamp that has been breached by frost will smell sweet and almost fermented as frozen root cells break down. Remove any affected roots immediately and adjust the insulation before closing. A well-built clamp should be opened no more than once every two to three weeks for routine access.