Ready for winter to end so you can start growing things again? One way to extend your growing season is to use a cold frame to start your plants. In fact, a cold frame can be used in many places to have vegetables such as lettuce all year round. It can also be used to overwinter plants and harden off plants in the spring before they are transplanted to the main garden.
A cold frame is essentially a big four sided box with a dirt floor and a glass top. Many people use an old window as the top so they can raise it to vent the cold frame on warm days. Old glass shower doors also work well. The sunlight coming through the closed window raises the temperature of the soil and air inside the cold frame. You monitor the temperature, moisture, light, and wind exposure to provide the best conditions for the plants in the cold frame.
Cold frames should be larger than 2X4 to make them worth your while, but not much larger than 4X6 so you can reach every part of the soil inside the frame. Just as with a raised bed, you never want to step inside the cold frame as it packs down the dirt and makes it harder for the plants to grow. Make the back of the cold frame four to six inches taller than the front, to make sure the plants get the most light possible. This also makes it easier for the cold frame to shed snow or water during a storm.
Overwintering Dormant Plants
A sunken cold frame can be used to over winter tender plants. First, cut the foliage to the ground at first frost. Then, you should put the plant in a pot with lots of room for soil to be packed in to insulate the plant’s roots. Pack the cold tightly with pots of tender plants. Use leaves or mulch to fill the open spaces so that the plants are well insulated. Place the leaves or mulch over the surface of the potted plants to insulate them even more. Water the plants well. During their dormancy, water them enough to keep them moist but not soggy. Soggy plants rot and die.
You need to use a white transparent cover over the cold frame to keep to much sun from penetrating it and bringing the plants out of dormancy. Temperature also needs to be modulated to keep the plants dormant. If it is 35 to 40 degrees F and sunny, open the cold frame part way to vent it so it doesn’t get too hot. If the temperature is 45 to 50 degrees F, you need to open it entirely to vent it. Be sure to close the cold frame at dusk to keep the cold frame warm enough at night.
Give Seedlings An Early Start
If you have a portable cold frame, you can place it in the garden and use it to give seedlings an early start. This helps avoid transplant shock because you do not have to move them once they are growing well.
Place the cold frame where you want the seedlings to grow two weeks before planting. This gives the cold frame a chance to warm the soil so your seedlings will grow well. Focus on cool season vegetables as these have lower germination temperatures. Seedlings are more at risk during extreme temperature fluctuations than are established plants. Keep the seed bed evenly moist to help the seedlings germinate. Once they have come up, vent the cold frame more often to let air circulate and prevent damping off.
To encourage active growth, use a transparent covering of plastic or glass. Vent established plants when the temperature reaches 40 degrees F. Vent seedlings when the temperature is 45 to 50 degrees F. Be sure to keep the soil moist in the cold frame. The soil will dry out faster in it than the surrounding area. More plants die of drought and heat in cold frames than they do of cold.
Hardening Off Seedlings
When plants first go outside from the greenhouse or a house, they are very vulnerable to dying from the harsher conditions outside. One way to mitigate that is to use the cold frame to harden off the plants.
Hardening off is a process that is generally started once plants have several true leaves and are ready to be planted outside. Typically, it consists of carrying the plants outside for a few hours each day and placing them in a sheltered area. They are then brought back inside. Each day, over the course of a week, the plants are left out longer and longer to get used to the outdoors. This is a labor intensive step, especially if you have lots of seedlings. Instead of carrying the plants to and fro each day, you can place them in the cold frame. Then you raise the top of the cold frame a few hours each day to let the seedlings harden. You increase the time the top is raised over a week until it is up all day. At this point, you can transplant your seedlings without the outdoors being such a shock to them. This improves the survival rate of your transplants.
Salad In Winter
Some crops, such as greens like lettuce, spinach, and collard greens, can actually live all winter in a cold frame. Plants in the cole family, such as cabbage, broccoli, and cauliflower, will grow all year, too. You can have fresh vegetables all year with your cold frame.
Cold frames are a wonderful season extender. They can be built from salvaged lumber and windows at a very low cost, so you can have as many portable cold frames as you can fit in your garden. They store easily, too, so when gardening season is in full swing, they can be stored out of the way.