I mentioned using a cold frame in my previous posts, Take the Next Step: How to Grow From Seeds, and Transplanting Your Seedlings. A cold frame can be used in different ways and will give you more flexibility in starting your plants for a summer vegetable garden. Even if you have a long growing season, you’ll have your yummy veggies on the table earlier in the summer than if you wait to sow your seeds directly in the soil.
What is a Cold Frame?
Cold frames are a great low-tech way to create a protected mini-environment for cold sensitive plants. They can be made with wooden sides, with a glass lid using old windows, or with fiberglass or plastic (polycarbonate or acrylic) sheets to let in sunlight. Some are made entirely of plastic or glass, like a small greenhouse. The top is movable for venting, because cold frames will heat up quickly (think of how fast your car heats up with all the windows rolled up). You will need to prop the top open varying degrees, and it’s also helpful to be able to remove it. They can be set on bare ground over a planting bed, or on paved areas. They are generally small for home use, about 4 to 6 feet long, 2 to 3 feet wide, and 1-2 feet high.
Uses for a Cold Frame
Some cold frames are set over soil and seeds are sowed directly in the ground. The vegetables grow to maturity and are never transplanted, usually over the winter. Digging the planting bed down a few inches below ground level is also helpful to provide better insulation from cold temperatures. I use mine to hold my newly sown seeds in flats or containers, then to raise the seedlings until it’s warm enough to plant into garden beds. Or if I started the seeds inside my house as early as February, I’ll move the seedlings out into the cold frame after they begin to grow (second set of leaves are forming). This is crucial for the warm season vegetables, herbs, and flowers: squashes, cucumbers, tomatoes, eggplant, peppers, basil, marigolds. Even in northern-central California, our early spring nighttime temperatures aren’t at least 50° F yet, and we are still getting frost. My seedlings are doing great protected in the cold frame (a Christmas present from my husband).
Managing the Cold Frame Environment
Place your cold frame in an area that gets the most hours of direct sunlight. On sunny days you’ll have to be very careful to prop the cold frame’s top open so that your plants (especially tender young seedlings) don’t dry out or get damaged by too much heat. You can start to acclimate your seedlings to ambient temperatures by propping the lid open just an inch or two at first on cold days, then wider as it gets warmer. I leave it closed up at night, and prop the top open (or remove it) when the sun starts to hit the structure, depending on the air temperature. If it’s below 60° F and cloudy I leave the top on, but propped open. At 60° F and above I’ll remove the top until the sun sets, then put it back on for the night. For daytime temperatures on in the 40’s and lower you may just leave it closed. High humidity could cause problems fungal diseases, but if temperatures are cool, it’s less likely. Check your young plants for moisture at least every day, and more often if the cold frame is warm. If you are germinating seeds, remember that they have to remain evenly moist at all times.
Save $$ by Growing Your Own
If you like growing vegetables from seed I think you’ll discover as I did, that a cold frame is a fantastic way to have nursery-quality seedlings ready for your summer vegetable garden when planting time rolls around.
Photos: Patricia Larenas, Urban Artichoke