How to Use a Cold Frame in Your Garden

A cold frame is a box-like covering placed over plants in a garden to shield them from wet and cold weather. This simple structure consists of a transparent roof that lets sunlight inside, naturally warming the box with solar energy and creating a microclimate. A cold frame is more or less a mini-greenhouse, and its primary purpose is to prolong the growing season.

A cold frame is a multipurpose gardening tool with different uses, depending on the time of year.

1. To protect plants from inclement weather: If you live in a climate that experiences a season with strong winds and heavy rain, use your cold frame to shelter your plants.
2. To extend the growing season: By using a cold frame to provide your plants with warmer temperatures in early spring, you can sow cool-season vegetables up to a month earlier. This allows you to enjoy an earlier harvest of salad greens, kale, and spinach. In autumn, a cold frame protects vegetables from harsh cold weather for an additional month, extending your harvest even longer.
3. To harden off seedlings: Cold frames are perfect for transitioning tender seedlings from the indoors to the outdoors. The cold frame prevents transplant shock by protecting the seedlings from frost and allowing them to gradually adapt to outdoor conditions.
4. As a raised garden bed during warmer months: Once the weather becomes hot enough that your cold frame is no longer necessary, simply detach the lid and your cold frame transforms into a regular raised bed.

You can purchase easy-to-assemble cold frame kits, but making a DIY cold frame is a simple and satisfying option for the hands-on gardener.

1. Choose a good location. You want your frame to be exposed to as much sunlight as possible, so put it in a south-facing location and make sure there aren't large structures or trees blocking the sunlight. Pick a spot with good drainage so water doesn't pool around the frame after it rains.
2. Select a transparent lid. Old windows or glass shower doors are great options. If you can't find a salvaged window, fiberglass or twin-wall polycarbonate sheets are also effective. A cheaper route is to use clear plastic sheeting that you can staple to a wooden frame, but the drawback is you’ll need to replace it annually. Remember that the size of your lid also determines the dimensions of your frame—around three feet wide by four feet long is a good size to fit a diverse selection of plants.
3. Build a sturdy frame and submerge it partially in the soil. You can use different types of materials, but wooden frames are the most common. Measure the length and width of your lid, then use a jigsaw or circular saw to cut wood panels of corresponding dimensions for the front and sides of your box. Use a drill to secure your frame together with screws. It's also a good idea to submerge the frame at least eight inches into the ground to maximize heat insulation in your soil.
4. Seal and insulate your frame. Use galvanized steel hinges to attach your lid to your frame, and apply weather stripping to the lid's seal for extra insulation.
5. Purchase and install an automatic vent opener. On particularly hot and sunny days, venting your lid is necessary to allow air circulation and prevent overheating. While adding an automatic vent opener is optional, it will save you the trouble of constantly monitoring the cold frame's temperature and manually propping open the lid yourself. Automatic vent openers are don't even require electricity—they're made of a mineral that expands when heated, causing a piston to push your lid open. When the temperature cools, the mineral condenses and causes the lid to close.