Here are some tips I picked up from Organic Gardening and The Royal Horticultural Society.
For the best flavor, let plants grow for as long as possible. During colder Fall nights they will produce more sugar. This improves taste and enables them to tolerate freezing conditions for short periods.
Bury the plants with leaves or straw just before the ground begins to freeze. A 1-foot-deep mulch should protect the crops and keep the ground from freezing in Zone 5. Adjust for your conditions. Use netting to hold the mulch down or put in plastic bags for easy clean up in Spring.
Beets,carrots and parsnips may keep up to March or April. Parsnips do particularly well stored in the ground.Turnips will keep until late winter.
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