Cold Frames

Now that our gardens are at rest for the winter, we have time for additional garden-related projects, such as building some cold frames. These are transparent-topped, low enclosures that are used to protect plants from cold and wet conditions, like miniature greenhouses. The sides reduce the effects of cooling from wind, the top lets in light, and the whole thing limits heat loss through convection, especially at night.

Cold frames were traditionally built out from the walls of full greenhouses, and were used as part of the acclimatization of plants from the protected environment of the greenhouse to being fully outdoors. Seeds would be started in the greenhouse, and when big enough they would be moved to the cold frames, then finally to the garden. This gradual change improved survival rates considerably.

In addition to transitioning young plants in spring, cold boxes can also protect plants from early frosts or other weather conditions throughout the growing season.

A commercial cold frame

Construction of these handy structures can be fairly simple. Although kits that only need basic assembly are available, cold frames sit at the lower end of complexity as DIY projects. They can even be made entirely out of recovered materials, such as pallets and plastic sheeting.

The sides are slanted and joined to a higher back and a lower front,  ensuring the top is angled to help with water runoff. Any clear or translucent panel can be used as a lid, making this a great way to repurpose old windows, or glass doors for larger frames. The top can be hinged for easy opening, or simply latched in place. 

A cold frame can even be converted into a hot frame with the addition of a heat source, such as an outdoor rated heating blanket or heating coil.

A variety of crops respond well to containment in a cold frame, including lettuce, parsley, onions, spinach, radishes, turnips, and so forth. Depending on the size of the cold frame, one type of crop can fill the entire space, or multiple plants can be grown so they produce in sequence to ensure a more constant flow of vegetables.

When placing a cold frame, make sure the top is facing south to get as much sun as possible. If high winds are a concern, having a way to stake the frame to the ground is helpful; a few large U-Nails set into each side allow a plant stake to be run through them and into the ground.

Whether starting young plants or protecting mature ones, cold frames are a cool project for the home gardener.