Cold frames are an excellent way to give your seeds a head start without the mess of starting them indoors. With a well-constructed cold frame and proper seed management you can gain as many a much as six weeks head start on the growing season.
Place the cold frame on the south side of a garage or building, this will help in trapping warm air around the cold frame at night. Make sure the location receives plenty of sun. You can construct your cold frame using scrap lumber and 2×2’s or use hem-fir. Pressure treated wood has too many chemicals and can sometimes effect ground water.
The sides should be angled up towards the back of the frame and the lid should fit tightly, covered with heavy gauge clear plastic available at your local lumberyard. A good way to prolong the cold frames life is to raise it off the ground on blocks when you aren’t using it. This will slow down any rotting that may occur. Mine usually last for ten years at a time.
Make sure to lift the lid in the morning on days that will be warm (above 50 F) or very sunny. You can prop up the lid using a 2 x 4 or later in the spring on hot days lift the lid back completely. Close the lid in mid afternoon to allow heat and humidity to build up in the cold frame, keeping the temperatures warm inside as the outside temperature falls.
If you live in an area where night temperatures are extreme, you can provide more reliable heat for your plants by digging out the area beneath the cold frame to a depth of 6-8 inches and filling it back in with fresh manure and then covering the manure with some soil. The manure will begin to compost and provide heat for your seedlings. You can also use an outdoor light with a 60-watt bulb to moderate the temperature on very cold nights.
As the days get longer and temperatures moderate you will need to expose your plants to more direct sun and wind conditions. Open the lid completely for at least 4 hours a day. Make sure your plants get enough moisture so they don’t dry out in the heat of the day. Use a diluted organic liquid fertilizer such as kelp meal every 10 days. There are a lot of hardy annuals and biennials that prefer to be started in the cool setting of the cold frame.
Below you will find a list of when it is best to start a few different plants indoors and in the cold frame. The list provides the plant name then the number of weeks to start seed before average frost-free date indoors and the second number is for the cold frame.
Plant Names: