In areas with early frosts, it’s a challenge to grow fruits and vegetables that require a long growing season. Cold frames can make all the difference. These miniature greenhouses warm the soil and protect plants from frost. Basic cold frames have wood sides and a glass top that traps the sun’s heat during the day. These moveable heat catchers sit directly on your garden beds and can be placed wherever your plants need a little extra warmth.
Where the winters are long, cold frames can help you get a jump on seed starting in the spring. As soon as you can work the soil, place the cold frame on the bed to trap heat and help your seeds germinate. The frames will also protect delicate seedlings from late frosts and hungry rabbits until they’re ready to transplant. In the fall, cold frames extend your harvest of hardy greens and root vegetables by keeping a killing frost or hard freeze at bay.
Traditional cold frames have wooden sides and a glass top that can be raised to access plants and propped open to let out excess heat. The most popular design is a "lean-to" shape, which evolved from cold frames originally built to lean against the side of an existing greenhouse. The back of the lean-to frame is taller than the front, creating a slope for the glass lid. Portable cold frames are built with lighter-weight materials like plexiglass and aluminum.
To best hold in the heat of the day, cold frames should be placed in garden beds with full sun during the early spring and late fall. Ideally, the slope of the lid should be angled toward the south to take full advantage of the sun’s rays, but you can also get good results facing a cold frame to the west or east.
If you’re handy, all you need to build a cold frame is some lumber, galvanized screws, and some old windows. Build the wood frame with a back that’s twice as high as the front, and cut a slope on the side pieces to connect them. Once that’s screwed together, you can use an old window for the lid. You can also build a frame to hold a piece of plexiglass if you don’t have an old window available. If you don’t have tools, you can “build” a frame of straw bales and place windows on top.
As soon as you can work the soil in early spring, rake in compost and a balanced fertilizer, making sure to loosen the soil at least a foot down. If the ground is frozen or muddy, you can place the cold frame on the soil to help warm and dry it, but don’t plant anything until you can turn the soil easily.
Before sowing seeds in your cold frame, moisten the soil well and allow it to drain for several hours. The goal is to plant in moist, not wet, soil. Place seeds according to the packet directions for depth and spacing. Gently pat the seeds into place and lightly water again. Keep the cold frame cover on tightly until germination, when you can vent as needed on warmer days. Check the soil daily for dampness and water when necessary—you don’t want it to dry out.
Plants grown in a cold frame are used to warm conditions, so they need to acclimate to the outdoors as the temperature warms. A couple weeks before your last frost date, begin taking the lid off the frame for a few hours each day, gradually increasing the plants’ exposure to the elements. Once nighttime temperatures are above freezing and plants are acclimated, you can remove the cold frame entirely.
For a crop of fall veggies, plant seeds or transplants in your cold frame near the end of the summer, but keep the lid off. As nighttime temperatures drop, place the glass lid on the frame a few hours before sunset. Eventually, you’ll keep the lid on all day, venting only as needed. This will help plants survive many weeks beyond the first frost date in the fall and even into early winter for an extended harvest.
When using cold frames, it’s important to keep an eye on daily temperature fluctuations to keep your plants under optimal conditions. When temperatures rise above 60 degrees, you’ll need to remove the lid to keep them from burning in the sun—the glass acts like a magnifier. When temperatures fall below 40, close the lid again, ideally before the sun sets to trap the last heat of the day. In between 40 and 60, prop open the lid to split the difference.
You can start any kind of seeds you like in a cold frame before transplanting to elsewhere in your garden. For plants you wish to keep warm in the fall, leafy greens and root vegetables work best. Consider cold-hardy varieties of lettuce, arugula, spinach, and kale. Carrots, turnips, parsnips, and bok choy are also excellent choices for a cold frame garden. Just be sure to check plant height so they don’t hit the lid.