How to Make Cold Frames from Recycled Materials

Cold frames help thrifty gardeners in cool climates get an early start on the growing season, harden off seedlings, and grow cold-hardy vegetables into the fall and winter. Essentially a miniature greenhouse, a well-constructed cold frame captures the heat from the sun and raises the air and soil temperature by several degrees. Cold frames can be constructed of a variety of recycled or found materials, such as old window frames, shower doors, plexiglass, fiberglass or plastic sheeting.

Difficulty:

Moderately Easy

Instructions

Things You'll Need

• Shovel
• Tape Measure
• Saw
• Level
• Hammer and nails (or screws and drill/screwdriver)
• Cold frame roof -- old window, full-view door or shower door in glass or plastic
• Appropriately sized dimensional lumber
• Garden Soil or other plant growing medium

1. Determine the best location for the cold frame. Try to select a southern facing site, to maximize sun exposure. Convenient access to water is another factor to consider.

2. Measure the size of the window, shower door, or other recycled material being used for the cold frame roof. The wooden frame should be constructed to these dimensions, and the area excavated for the bed should also be the same size.

3. Excavate an area matching the dimensions of the cold frame with a shovel, and bank the sides with soil to conserve heat. Dig to a depth of a few inches to a few feet, depending on desired application, and ensure the bottom and sides are level. Gravel can be added to the bottom of the bed to promote drainage, if necessary.

4. Construct a wooden frame of the proper size. Frames can be constructed using a variety of methods and materials, but 1- to 2-inch-thick lumber is appropriate for most applications. Use redwood or cedar, as these woods resist rotting from water contact and prolong the life of the frame.

5. Build the wooden frame to a height of 12 inches in the front, and ensure the rear of the frame is 6 to 12 inches higher than the front. The entire frame should be inserted into the previously excavated soil bed at least a few inches.

6. Cut the lumber being used for the sides to the proper angle, so the glass or plastic being used for the frame slopes downward toward the direction of the sun. The slope between the back and front of the frame should be a minimum of 1 inch per foot.

7. Attach hinges to the rear of the frame, and set the material being used for the top(window, glass shower door, etc.) onto the frame. Fasten the hinges to the top of the enclosure, so the transparent portion may be easily opened or closed as needed.

8. Slope the soil surrounding the frame away from the structure to ensure adequate drainage, and fill the interior of the cold frame with the desired planting medium.

Tips & Warnings

• Cool season vegetables such as caulilower, lettuce, cabbage and spinach are well suited to cold frame use.

• Cut one or two additional pieces of 2-by-4 lumber to use as venting props for the lid on warmer days.

• Never use pressure treated lumber for cold frame construction, as it may contain chromated copper arsenate, creosote or pentachloro­phenol. These chemicals can leach into the soil, and are harmful for plants and humans.

References

• Purdue University Cooperative Extension; Hotbeds and Cold Frames; Michael Dana, et al.; 2005
• University of Georgia; Center for Urban Agriculture; Stephen Pettis
• University of Alaska Fairbanks Cooperative Extension: Greenhouses and Season Extenders
• University of Missouri Extension; Building and Using Hotbeds and Coldframes; Denny Schrock; 1998
• Washington State University; Snohomish County Extension; Community Horticulture Fact Sheet #14, Plant Protection Structures; 1999