Building a Cold Frame

By building a cold frame, you can plant cold-sensitive vegetables before the last spring frost.

| March/April 1981

Building a cold frame is one of the best ways for an aspiring gardener (especially one whose bankroll won't support the purchase or the construction of a full-sized greenhouse) to get a head start on a summer vegetable plot. In most parts of the country, a mini-hot house will help you beat the final spring frost by three to four weeks ... and it can even extend the growing season enough to let you get in an extra succession planting!

Cold frames aid growing plants in two ways. First, the devices admit sunlight to enable seeds to sprout into up-and-coming edibles. And, in much the same fashion as do solar collectors, they convert solar energy to heat ... in this case to maintain air and soil temperatures which are conducive to growing. Consequently, the two major concerns when designing a cold frame are to let the sun's rays in and to retain heat.

In keeping with state-of-the-art energy efficiency, we have designed a passively solar, earth-sheltered, and well-insulated cold frame. The box is built from a sheet of inch-thick, foil-backed foam insulation board. Using the technique developed while constructing MOTHER EARTH NEWS' Heat Grabber and used frequently in subsequent solar projects, we trimmed out 7" corners from a 39" X 61 1/4" section of the insulation, and carved 90° grooves in the foam so that the sides and ends could be folded up without our having to cut the exterior foil. (Note that — in order to achieve the proper angle — the slicing tool must be drawn first along one side of the half-round-molding guide, then reversed to cut along the other side.)

Once the insulation board was shaped to the proper dimensions, we further insured a good seal by lining the joints — inside and out — with metal tape.

Two triangular frames built from 2 X 2 lumber hold up the corrugated fiberglass lid, and the 18" X 18" X 25" (outside dimensions) sides describe an isosceles triangle. Once the pieces have been mitered and screwed together, tack on 18"-long strips of ripple board, and connect the two frames by butting 44" lengths of 1 X 2 between them.

The glazing adds additional stiffening to the lid assembly, and both of the 19 1/2" X 50" sheets and the 4" X 50" ridge cap can be cut from a single 10' piece of corrugated translucent fiberglass. Attach the sheets to the ripple board by drilling 1/8" holes in the material and sinking No. 6 X 1/2" sheet metal screws — fitted with rubber washers from aluminum roofing nails — into the openings. Then pop-rivet the fiberglass cap over the joint between the two sheets at the peak of the roof and seal each fastener with a dab of silicone caulk. An additional application of sealant between the ripple board and the fiberglass, and along the seam where the material contacts the horizontal 1 X 2's, will help to retain heat. To close in the ends of the cold frame's lid, cut out two 18" X 18" X 25" triangles of 3M Flexigard (or a similar material) and staple the plastic — spacing the fasteners three inches apart — to the ends of the frame.

2/10/2011 8:41:48 PM

I built a cold frame by using an old dining canopy frame and simply covering it with clear plastic sheeting. It is 10'by 10' and I have about a foot of leaves on the bottom and about a foot of leaves all around the outside. I added about 40 gallon milk jugs full of water all around the outside and around the plantings (is recycled old pots) to absorb the heat from the day and give it off at night. it's not working Do you have any suggestions for keeping it cool? It was 120 degrees in there this afternoon when I got home from work - and the high today was only about 60. help! I think I've cooked my seeds.

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