Use Cold Frames to Grow More Food

Image Gallery Get a jump start on your spring garden by using cold frames to sprout seeds in a warm, protected enclosure and protect tender seedlings from wind and frost. ELAYNE SEARS

Gardening guru Eliot Coleman asserts that “the basic cold frame is the most dependable, least exploited aid for the four-season harvest.” We couldn’t agree more. Last winter, my humble box built of 2-by-4s topped with an old shower door added a month to the front end of salad season, but the best part was being able to sow some of my spring seeds directly into the frame. This made more space available under lights indoors for tomatoes and other crops that don’t like chilly conditions, and eliminated the hassle and setbacks involved with hardening off seedlings and then transplanting them. Best of all, seedlings get a nice head start in real sun so they never get stretched out and leggy as they often do when started indoors. (Indoor grow lights are vastly less intense than real sunlight.)

What can you sow in a cold frame? In spring you get a boost with virtually any crop by sowing into frames. The list of “Top 12 Winter Cold Frame Crops” (below) can get you started, and as days get longer and warmer in spring you can try your hand at framing up peas, bulb onions, potatoes or even tomatoes. When a cold frame is no longer needed for a crop that is up and growing, simply move it to a new location and plant more seeds.

Traditionally, gardeners have used cold frames to harden off seedlings started indoors, and you should have a frame suited to this purpose. But one cold frame is not enough. In addition to direct seeding some vegetables right where they are to grow, you can use a cold frame to winter-sow onions, cabbage or other hardy crops that are easily lifted and transplanted into rows.

A cold frame can be a wood box with a recycled window (or shower door) top, a hay bale enclosure covered with plastic, or you might build one with bricks or concrete blocks and top it with translucent corrugated fiberglass (see “Anatomy of a Cold Frame,” below). Your frames need not be all alike, though having two of the same size makes it possible to stack them for added height. I like frames I can move around by myself without straining, so size and weight are important considerations. If you live north of Zone 6, you may want to create frames that are large enough to accommodate black, water-filled containers for solar heat storage, and insulate the sides by adding a snug berm of soil or mulch. In climates with chronic winter cloud cover, you can maximize available light by painting the interior walls of your frames bright white, or by covering them with heavy-duty aluminum foil.

How Cold Frames Work

Cold frames shelter plants from ice, snow and treacherous winter winds, and heat up the soil whenever the sun shines. The soil inside a frame will warm up much faster than open ground, and because seeds of many hardy vegetables can germinate in the 50 degree range, a three-day spell of mild weather often coaxes them to life. Weed seeds will germinate alongside the seeds you plant, but you will see far fewer weeds if you cover the soil’s surface with a 1-inch-deep blanket of potting soil (purchased or homemade). Be prepared to add water as needed to keep the soil from drying out.