Weekend Wonders: Building A Cold Frame

Its the annual spring time dilemma: seduced by the promise of wonderful annuals that you would never have otherwise found at your local nursery, you have decided to start your own seeds. Whether you have a large greenhouse or just a windowsill, without fail there comes a time when your seedlings will need more room than you possess. Unfortunately, its equally crucial that they not yet be planted directly into the garden. Little seedlings are like fair-skinned blondes in the early spring -- they sun-burn easily, and need time to get adjusted to the higher light levels and temperature swings found outdoors. If you have only a few plants, once the weather has warmed up (meaning nights in the 50s) you can set them in a protected spot out of the direct sun in bright light for a week or two before transplanting them into the garden. If you're like me though, and have been beguiled by visions of horticultural grandeur into undertaking a substantial planting – or if you're merely looking to save some money at the nursery by buying smaller plants early in the season and growing them on yourself – you should consider building a cold frame.

Our coldframes here at the Garden, custom made by my father to fit next to the greenhouse. Each door is 4' x 8', and has two drop down legs to allow for easy movement within the frames.

Cold frames are one of the most useful of all garden structures. At their simplest, cold frames are just wooden or plastic boxes with a raisable lid. They are called “cold” because, unlike a greenhouse or conservatory, they are not actively heated, but rely on the passive warmth of the sun. Uses for cold frames are numerous -- many gardeners employ them as extended season growing beds for early spring and fall vegetables; others for forcing bulbs,  still others for starting new perennials. But the cold frame truly comes into its own when growing annuals – both flowers and vegetables – as the frames are the ideal place to “harden off”, or acclimatize your seedlings to the great outdoors. Snuggled inside the cold frames, the plants are protected from the burning effects of the harsh sunlight and wide temperature swings so common in early spring.

Here at the Garden, where we grow almost all of our own annuals, the frames begin to fill with seedlings by early April, first with the more cold tolerant plants like broccoli and leeks, then later, as the weather warms, with the more heat loving annuals and vegetables. Depending on where you live, and on the type and size of your frame, you may be able to use your cold frames even sooner, perhaps even all winter. The inside temperature of a cold frame will generally remain about 10 degrees warmer than the outside air at night. As soon as night-time lows remain in the thirties, I start placing annuals in the frames.

Remember though, this same heat conservation can work against you: on warm sunny days,  inside temperatures can soar well over 100 degrees and  roast your young plants without adequate ventilation. Be sure to partially open the frames in the morning if temperatures are predicted to be hot. (Remembering of course to always close them again at night; automated openers/closers are available through specialty catalogues if you fear forgetting.) I generally leave young annuals in the cold frame at least two weeks, and sometimes longer, depending on how big they were when they went in, and quite honestly, how much time I have had to spend in the garden planting out the seedlings. As the season progresses, you should raise the cover a bit more each day, until the plants have had a chance to become fully acclimated to the sun. And don’t forget to water; remember, it doesn’t rain under the glass, (something I have several times forgotten to my detriment) and your little charges will need fairly constant monitoring.

Building a cold frame is an easy, do-it-yourself project, and one that can be accomplished in a few hours some weekend. For this simple model you’ll need an 8 x 4 foot sheet of exterior plywood, two pressure-treated eight-foot 1 x 2’s, (or similar scrap lumber to make stakes) a box of 1 1/2 inch screws, some exterior paint or stain, two door hinges, and finally an old storm window, which if your basement is like mine, will be easy to come by. (If you’re fresh out of old storms, check with a friend who has an old house -- there’s almost sure to be an extra one! If all else fails, a heavy duty sheet of Plexiglas from the hardware store will work just fine). Just make sure the window is in good condition before you begin, repairing any broken or loose panes.

1) Mark out the dimensions shown at right on the sheet of plywood, and draw the lines for cutting. For the purposes of this example, we’ll use a 3 x 4 foot storm window, but you can use any size window you have on hand; you’ll just need to alter the 48” and 36” measurements to match the dimensions of your window.

2) Cut out the pieces from the plywood sheet using an electric saw, or have the lumber yard cut out the pieces for you. Cut the 1 x 2’s into four pieces: two, 30 inches long, and two, 24 inches long; cut the ends to make four pointed stakes. Attach the two 30 inches stakes to each end of the back of the frame using three screws on each stake. (An electric screw driver is very helpful here.) Use the other two stakes on the front. The pointed end of the stakes should protrude beyond the bottom of plywood pieces, as shown in the diagram at left.

3) Fasten the sides of the frame to the front and back by screwing into them into the sides of the stakes already mounted on the front and back. The body of the cold frame is now complete.

4) Paint the entire assemblage. White is a good choice to reflect light; equally, black will help absorb additional heat. Whatever color you choose, use at least two coats, making sure to cover all surfaces thoroughly.

5) Place the frame in its intended location, and carefully drive the four stakes into the ground with a hammer. In colder climates, you may wish to bury the frame six inches or so into the ground for increased insulation.

6) Mount the window to the back side of the frame using the door hinges. (If you are using a sheet of Plexiglas, you may need to attach a wooden strip to the plastic to allow for mounting the hinges. To build a double-windowed cold frame similar to the one shown here, simply build two individual frames and place them side by side. A single piece of plywood mounted across the front will give the illusion of a single unit.

If the idea of building your own cold frame doesn’t appeal, I've also seen an excellent temporary frame made from bales of hay with a sheet of Plexiglas laid on top. You can also purchase a variety of ready-made cold frames directly from online suppliers.

This article is an updated and expanded version of one that originally appeared in my second book, From A Victorian Garden. For more interesting projects like this one, be sure to check out my latest book, the Victory Garden Companion. Photo above courtesy of the Victory Garden Companion.

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