A cold frame – also known as a hot bed – is a kind of tiny greenhouse. It can be built from wood, stone, straw bales, or other materials, with a clear lid made of glass, acrylic, or clear plastic.
Just like a full-size greenhouse, a cold frame can be used to protect plants over the winter. It can also be used to extend the growing season, allowing the gardener to put seedlings outside earlier in the spring and continue harvesting crops later into the fall.
A cold frame is simple enough that it can be built as a do-it-yourself project. After that, it’s also easy to use.
First, put the cold frame in a sunny spot protected from wind. This will allow the cold frame to do its job most effectively. Sinking the cold frame into the ground will provide additional insulation, while sitting it on top of the soil keeps it portable.
Fill the cold frame with closely-spaced plants. The plants can be growing either directly in the soil, or in buried pots. Either way, the soil between the plants should be covered with mulch or leaves, to help moderate its temperature.
A cold frame is amazingly effective at absorbing and retaining heat. If the outside air temperature will be just 35°F, the top of the frame should be left ajar, to prevent the air inside the frame from becoming too warm. The top of the frame can be opened all the way if the outside temperature will clear 45°F. This is especially important in the spring – seedlings that don’t experience some bracing temperatures will have a tough time acclimating to life without protection when they’re moved out of the cold frame.
The warmth inside a cold frame tends to have a drying effect on the soil, so it’s important to keep the plants well-watered. Be careful not to water too much in the winter, though. Plants that are not actively growing don’t take up water, and their roots can be damaged by the soggy soil.
Finally, don’t forget about your tiny greenhouse when it gets buried by snow or fallen leaves. Clear off the top so it can continue to do its job of absorbing sunlight and keeping your plants warm.