Cold frames are considered a season extender. Place plants in the frame in the early spring to help get them used to outdoor conditions while temperatures are still too cold to introduce plants directly to the garden bed. You can also plant cool-season plants in the cold frame in late fall, protecting them in the garden long after winter temperatures have killed off other plants. Most cold frames are a wooden box-shaped frame with a glass top that allows the sun to reach the plants. Some versions are self-opening so the glass lifts when temperatures inside are too high, allowing fresh air in to the plants inside.
Set the cold frame in an area that receives full sunlight throughout the winter or early spring. Place it close to the house or another building, where the ground is often warmer.
Set plants out in the cold frame in late fall if you wish to extend your fall crops into early winter. This allows you to extend the fall harvest later into winter than you could in the regular garden bed.
For spring planting, place seedlings in the cold frame two to four weeks before you would normally move them outside. Leave the plants in their current pots when you place them inside the cold frame.
Place a thermometer inside the cold frame. Open the glass, if it isn't a self-opening cold frame, when the temperature inside reads 70 to 75 F. Even on cold days, the air inside the cold frame can become hot enough to cook the plants if it isn't vented.
Cover the cold frame with straw or old blankets if temperatures are predicted to drop below freezing at night. Place the straw over the cold frame while the temperature is still warm inside from the sun, but after the sun begins so you don't cook the plants.
Remove plants you placed in the cold frame in the fall once the plants have stopped producing or died back, usually in mid- to late-winter.
Remove spring planted seedlings from the cold frame once the proper planting time has arrived for the plant variety and after they have spent at least one week in the cold frame. This ensures they are adjusted to outdoor conditions before being placed in the regular garden bed.