2. Place straw on the ground in a circle about 4 feet in diameter and 6 inches thick.
3. Inspect the vegetables to be stored and remove any with cuts, bruises, or signs of decay. Pile the vegetables atop the straw in a conical mound.
4. Layer loose straw over the vegetables 6 to 8 inches thick.
5. Carefully add a 6-inch layer of dry soil over the straw. Leave a tuft of straw exposed at the top of the clamp for ventilation.
The clamp shown here was constructed in late fall 2011 to hold about 100 pounds of potatoes. We opened the clamp 14 weeks later, in March 2012, and found that the potatoes remained in excellent condition. The winter here in eastern Pennsylvania was milder than usual, dipping to 10°F one night and into the teens on several other occasions.
Clamps are often recommended in regions of Europe that are the climatic equivalent of USDA Plant Hardiness Zones 6 to 8. In these areas, the temperature inside the clamp is more likely to remain within the ideal range of 32°F to 40°F. In colder climates, underground clamps—straw-lined pits or barrels—offer additional protection from cold. Clamps are probably not the best storage method in areas warmer than Zone 8, colder than Zone 5, or where rodents or slugs are a problem.
In theory, vegetables can be removed a few at a time by reaching through the straw at the top of the clamp. Instead of a single large clamp, it may be easier to build several small clamps that can be opened as the vegetables are needed. Or construct a long, narrow clamp and remove roots a few at a time from one end, replacing the straw and soil to protect the remaining vegetables.