Cold frames are structures that protect plants from wind and cold, while taking advantage of solar warmth.
3 Ways to Use Cold Frames
- Grow plants well past the time when gardening has usually ended in the fall.
- Harden off seedlings you’ve started indoors for early spring planting.
- Hold tender perennials that might freeze during an especially cold winter. Dig up the plants in the fall, “heel” them into the soil under the cold frame for the winter, then remove and replant them back into the garden in the spring.
Alternative Protection During Cold Snaps
Cold frames are an easy-to-use and attractive alternative to throwing plastic tarps or old bed sheets over a garden every time a freeze warning goes into effect. Cold frames provide more protection for a longer period than plastic or fabric. And, because the lid is attached by hinges, it won’t blow away in a big wind. Best of all, the lid is lifted each day, without the hassle of carefully removing tarps or sheets.
A Miniature Greenhouse
A cold frame is essentially a miniature greenhouse that fits over an individual garden bed or acts as a “holding pen” for potted plants. They take a bit of carpentry skill to build, but are a worthy weekend project. The idea is to install reclaimed glass windows, or new polycarbonate sheeting, as a lid on a wooden (or brick) frame. This creates a bottomless box where the plants can take root. Repurposed windows may need to be caulked on occasion to keep the panes intact, but can offer a more appealing look than plastic sheets.
Building a Basic Cold Frame
The frame sides, which the lid will rest on, should taper down from back to front at an angle between 35 to 55 degrees. The sides should be at least 8 inches (20 cm) tall at their shortest point. The cold frame front needs to face south, to catch the most sunlight from late fall through early spring. If you use windows for the lid, they can be directly hinged to the base frame via their existing frames. Sheets of polycarbonate need to be mounted on separate framing first, which is then hinged to the base frame.
Double Frost Protection
You can easily double the degree of frost protection by installing several incandescent light bulbs inside a cold frame. Flip them on at night and the extra heat will keep things growing right through to spring. Make sure any light fixtures you use are rated for outdoor installation.
Cold frames are perfect for growing cool weather loving vegetables in the early spring and late fall. Check out our list of veggies suitable for cold frames.