Published on 01/05/99

Weekend Project: Cold Frames Help Stretch Gardening Season

If you don't have a cold frame, making one would be a great weekend project for any gardener.

During winter, the cold frame will offer protection to less hardy plants and newly started perennials. It's also an ideal place to overwinter some plants or grow a small patch of lettuce or other cool-season crop. It simplifies the storage of bulbs and plants, too, for midwinter forcing indoors.

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P. Sumner, UGA CAES
COLD FRAMES like this one can be built in a weekend. This bottomless box acts like a miniature greenhouse, protecting young or tender plants from temperature extremes.

A cold frame is simply a bottomless box with a clear glazing or covering to let in light. Some gardeners make cold frames lightweight so they can move them from one section of the garden to another.

You can build frames from a number of materials. Wood and cinder blocks are the most common. Never use creosote-treated wood or wood treated with pentachlorophenol or any other material that could be harmful to you or to growing plants.

Wood frames aren't hard to build. You can buy an easy-to- assemble kit. Some kits even contain automatic ventilation equipment.

Ventilation is most critical in the fall and again in late winter and early spring on clear, sunny days above 45 degrees. Partially raise the sash then to keep it from getting too hot inside the cold frame.

As the fall progresses, you may have to add extra insulation to the cold frame to reduce extreme drops in temperature. Straw, bags of leaves or foam insulation boards all help control the temperature in the cold frame. Or just stack bales of straw or hay against the frame.

Some people like to experiment by using containers of water to absorb solar heat during the day and release it slowly at night. They make the cold frames larger to include the water containers.

While this may be useful in managing temperatures, remember that it won't help protect tender plants, which will need to be moved indoors if they are not cold-hardy.

Sinking the frame into the ground somewhat will provide protection, too. It will use the earth for insulation.

To make using the frame simpler, provide a walkway to the front, leave enough space behind the frame to remove the sash, and add weights to make raising and lowering glass sashes easier.

Unheated frames are useful for much of the year because they collect heat from the sun through the panes.

To make the most of the heat and light, put the cold frame in a southern or southeastern exposure with a slight slope for good drainage. A sheltered spot with a wall or hedge to the north will protect against winter winds.

Wayne McLaurin is a professor emeritus of horticulture with the University of Georgia College of Agricultural and Environmental Sciences.