Merriam Webster describes cold frames as “small wooden or metal frames covered with glass or plastic that are used for growing and protecting plants in cold weather.” That definition doesn’t take into account the inventiveness of gardeners, who often build such structures with bricks, cinderblocks, or even straw bales. They then top them with whatever transparent materials come to hand, including old windows, tempered glass shower doors, or greenhouse plastic tacked over old doorframes. Usually rectangular and lacking bottoms, cold frames can be set directly over plants in the ground. Their front sides generally are lower than the back ones, so that their transparent covers tilt toward the sun and rain or snow slides off easily.
Cold frames often are used to harden off seedlings that were started indoors or to protect cold-tolerant vegetables in early spring and late autumn. They also can provide shelter for seeds which require varying outdoor temperatures to germinate. In that case, the seeds either may be sown directly into the soil protected by the frame or sown in pots which are buried up to their rims in that soil. You can purchase polycarbonate cold frames ready-made if you aren’t inclined to build your own. All such season extenders should be placed in a sheltered, south-facing location in full sun. They sometimes are sunk partway into the ground for further insulation, but that may cause drainage problems. It also makes the cold frames less portable.
If you intend to use one only for plants which already are potted up, it’s a good idea to excavate a hole 6 inches deep beneath that frame, replacing the soil with gravel for drainage. Should you want to set plants directly into the ground inside the cold frame instead, you will need to position your gravel layer deep enough that plants can comfortably stretch out their roots in a layer of soil above it. To turn a cold frame into an old-fashioned hot bed, dig a hole 18 inches deep beneath it and fill that hole with 12 inches of damp fresh horse manure mixed with straw. You’ll need to pack the manure into a pile a few days before you plan to use it, until it begins to heat up.
After you add the steaming manure to the hole, cover it with 6 inches of soil to make sure that your plants don’t come into contact with the “heating element.” It’s difficult to regulate the temperature in this type of hotbed and its fuel isn’t as readily available now as it was in horse-and-buggy days. So you might want to just use a heating cable instead, though that lacks the odorously organic ingenuity of the original idea! If you can’t be home during the day to keep an eye on your cold frames, you probably should acquire automatic openers that lift the covers whenever the temperatures inside the frames climb to a certain point. Otherwise, you’ll need to prop open those covers yourself on sunny days to prevent overheating.
Provide at least a 6-inch opening when the temperature outdoors rises above freezing and lay the covers completely back once that temperature surmounts 50 degrees Fahrenheit. You then must remember to close those covers again in late afternoon to protect your plants from plunging temperatures at night. If those temperatures are forecast to nosedive, place heavy blankets or bales of straw over the cover of your cold frame during the frostiest nighttime hours. All these efforts to extend the growing season admittedly are a gamble. But, with gardening, what isn’t?