One of the coolest ways to get a head start on gardening in the spring--or to extend the gardening season well into winter--is to grow things in a cold frame. A cold frame is a sort of mini-greenhouse that allows you to create near-perfect conditions for plant growth without spending a fortune. For instance, even though you may love salads, freezing temperatures force you to stop growing lettuce, spinach, carrots, and other salad crops by the middle of October or November--or delay your spring crop by several weeks. With a cold frame, you can extend the growing season at least another 45 days or so, even if there is snow on the ground.
Although you can buy a cold frame, it's fairly easy to construct one. It's not fancy, but it is functional. You'll need between four and eight bales of hay or straw, depending on how large you want your cold frame to be and you are also going to need a lid. My frame was constructed from six bales of straw and a lid made of Plexiglas, though I could have used an old window frame or an old glass door.
The soil beneath the cold frame should be dug to a depth of eight to 12 inches and amended with compost. It should slope in such a way that it is higher in the back than the front by about 10 inches. Ideally, the cold frame should be located in full sun and face the south. Place the frame against a north wall, a fence, or a row of trees or shrubs that will offer protection from howling winds.
The reason the bed needs to slope from back to front is that you want the maximum amount of light to reach inside the cold frame. During the spring and summer, the lid should slope 35 to 45 degrees, but a 55-degree slope catches the most light in the fall.
To assemble the cold frame, create a rectangular box with the bales of straw, making sure you get a tight fit between the bales, and set the lid on top. After a season or two of growing and it is time the replace the straw in the cold frame, the old straw can be used as mulch in your garden.
Cold frames are ideal for hardening off flats of tender young annuals or perennials, and for storing root cuttings. For those of you who like to force spring-flowering bulbs to bloom indoors, most of those bulbs will first have to be chilled from six to 14 weeks. Most books suggest that bulbs be placed in the refrigerator, but using a cold frame you can actually pot those bulbs, bury the pots up to their rims with soil, and leave the lid propped open so just enough cold air gets in.
Temperature control is probably the most important thing to keep in mind when growing in a cold frame because even in the middle of the winter, if the sun is bright, temperatures inside the cold frame can climb quickly to 100 degrees or more. That much heat will kill the plants inside, so a thermometer placed inside the cold frame is a must.
Monitor the temperature inside the frame daily, and when temperatures rise above 80 degrees, vent the frame by propping the lid approximately six inches. Keep checking the temperature, until it is between 45 and 75 degrees, which naturally depends on the crop you are growing. Before sundown, you should close the lid, and if temperatures are expected to drop below freezing, throw a blanket over the cold frame to hold the heat in. You may also consider placing plastic milk jugs filled with water on the north wall of the cold frame. The water absorbs heat during the day and releases it at night, thereby stabilizing temperatures overnight. The jugs will absorb even more heat if they are spray-painted black.
There is something called a hot bed that is structurally identical to a cold frame. A hot bed starts out with a four-inch layer of fresh barnyard manure in the frame, topped off with a two-inch layer of soil. As the manure rots, it generates heat that keeps the soil nice and warm and extends the growing season even more. My preference, however, is the cold frame.
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