Cold Frame — Hot Box
Cold frames are a great gardening accessory, giving you a place to harden off transplants before putting them into the garden, giving seeds a head start in germination just before the last frost, and giving warm weather crops — tomatoes, peppers, eggplants — a warm boost when the days and evenings might still be cool. In general, they’re a great way to extend the growing season from either end, spring and fall. Building one is simple. Resourceful gardeners make them from scavenged wood and reclaimed window sashes. But you can also build them from scratch, allowing you to use materials that will better withstand the elements while putting your woodworking skills to use. And, of course, you can buy them as kits.
The English, because of their cool, moist summers, are great champions of cold frames, often incorporating them right into the design of their homes. The English row house often features cold frames along the south or west side of the building, often made of the same brick as the home. While most of our cold frame experience is of the salvaged wood type, we’ve also seen some pretty nice set ups built by those who are more handy than we are.
Check out these frames, some incorporated to the sides of houses (stays warmer), one featuring an attractive gazebo shape. Notice the handles for lifting the glass tops and the installation of rods designed to keep the top propped open (critical during warm weather). All design features should be considered when you’re drawing up plans for a cold frame. Another thing to consider: will your frame have a stone or gravel bed, or other type of solid floor? Or will it be open to the soil it sits on. The first method is ideal for gathering and conserving heat during cool and cold weather. The second allows you to get a head start on heat loving plants while letting them grow outside the box (heh) without transplanting come warmer weather. Of course you could do one (or more!) of each.
Adding a heating element beneath the floor of your cold frame turns it into a hot box. Not to be confused with hot beds — glass or plastic covered frames that are set over germinating or young plants to hurry the process — hot boxes speed seed germination and protect plants during the coldest nights. Naturally, you’ll want to put them close to an electrical outlet, say up against your house. The old school way of heating up a hot bed was to work a layer of manure under the growing medium, allowing decomposition to provide heat. This method, though not as consistent and dependable as electricity, had the advantage of providing a layer of composted manure that could be turned into the soil at the end of the season.
Modern cold frame kits make building and using cold frames easy. Constructed of durable materials, including transparent sides that allow in sunlight, they can be place almost anywhere and many models can be easily moved when called for. Some carry features like automatic venting, a great convenience for those not around the garden all day. Or you can purchase the vent separately and incorporate it into your own cold frame/hot box design. Find plans for cold frames here and here. A good discussion regarding cold frames and hot boxes — with plans — from Cornell University Cooperative Extension (PDF) is here. (Note: Organic gardeners will want to avoid treating the wood in their cold frame to avoid rot where it makes contact with the ground as some of these plans suggest. Instead, nail a strip of rot-resistant wood like cedar along the bottom of the frame. If it does start to rot, it can always be removed and replaced without damaging the rest of the frame.)