A cold frame offers a comfortable environment for less hardy plants through the winter months in cold regions. Relying solely on the heat and warmth of the sun, a cold frame costs nothing to use and very little to build. To give your semi-hardy plants a fighting chance, tuck them in for a long winter's nap inside a cold frame you built yourself.
Step 1 - Pick a Location
The first thing to consider before building a cold frame is location, location, location. A shady spot won't produce enough sun to keep plants warm, so choose a spot that receives an ample amount of sunlight each day. A south-facing wall works the best.
Pick a place where it butts against an existing structure: your home, garage, outbuilding, or solid fence will do nicely. Obviously, a heated home will give added warmth to the cold frame, but other buildings will work, also. By placing the cold frame in front of a building you're providing protection from winds and the elements to some degree.
Next on the list of must haves for an ideal location is a site with good drainage. It will do little good if the plants sitting inside the cold frame are stuck with their feet in water all winter. They will not survive in wet conditions. The ideal spot would be slightly sloped to allow for water to flow away from the cold frame. You may consider building a slope yourself. For added insulation, set the cold frame into the earth just a bit.
Step 2 - Build the Cold Frame
The top of most cold frames made at home are old window sashes. The panes of glass let the sunshine in. The dimensions of the frame depend on the size of the window you use. You will need to hinge the window, so purchase heavy-duty hinges. An extremely simple cold frame is nothing more than cement blocks lined up to hold a window on top.
If you opt for a wood frame, make sure the wood will not decompose. Cypress or cedar are good choices for the wood frame. Do not make the frame so wide that you cannot easily reach the plants at the back, especially if you're growing vegetables in the cold frame. Three to four feet is the maximum width for most people to reach the back for weeding or harvesting.
Build the frame to fit the windows, higher in the back than in the front. Add weights to keep the window sashes from blowing open during strong winter storms. You'll also want to include a way to keep the window open for ventilation during warm days when the temperature reaches above 45 degrees Fahrenheit. A prop comes in handy for this purpose.
Step 3 - Use Your Cold Frame
Seasoned gardeners know the advantages of a cold frame. Use it to harden off seedlings in the spring that were started indoors. You can start annual seedlings in a cold frame to get a head start on growing plants before the earth is ready to be planted. This offers various blooming or harvesting times. The plants started in the cold frame will mature sooner than those planted directly in the garden. Plant cool weather crops in the autumn — it's possible to plant, grow, and harvest fresh herbs, greens, and root crops like carrots throughout the winter if the thermometer doesn't plummet too low. Additional insulation may help in your winter growing endeavors.
Two problems may arise in your cold frame, both from the weather. If you do not open the windows on sunny days, your plants could fry in the heat of the sun and the reflection of the glass windows. On the other hand, if a cold snap hits with freezing temperatures lingering, the plants may freeze. If in doubt, cover the entire cold frame in old blankets or burlap bags. Just be sure to uncover when the sun comes out to warm the cold frame again.
A cold frame gives you the opportunity for fresh greens and herbs in the dead of winter. It protects semi-hardy plants from dying off in freezing temperatures. It extends the growing season in the spring and in the winter. Best of all, it requires little money and few materials to build and recycles old materials. Build a cold frame and discover yet another way to make nature work for you and your plants.