Cold frames can allow you to harvest fresh vegetables throughout much of the year, even if you live in a cold climate. Cold frames are essentially mini-greenhouses that you can purchase from a garden supply company or make yourself. The temperature inside a cold frame 10–20°F warmer than the surrounding air, protecting plants from frost and wind, making them invaluable for:
- starting plants outdoors in spring
- transitioning plants outdoors
- extending the fall garden season
- overwintering plants that are not quite hardy in your zone.
Cold frames are great protection for cool season crops and tender perennials in all parts of the country, but they aren’t recommended for protecting warm season crops like tomatoes, peppers, and beans unless you are in a fairly mild climate or you are just trying to get these plants through the first few spring or early fall light frosts.
The typical cold frame is a box made of wood, bricks, cinder blocks, or hay bales with no bottom, so you can place it directly over growing vegetables or start seedlings inside. It is fitted with an angled glass lid to let in the maximum amount of light and can be opened and closed to control temperature. You can make a simple one yourself or purchase one from a garden supply company. (Some manufactured cold frames from garden supply companies even have temperature gauges and automatic vent openers.)
Tips for Success with your Cold Frame:
- Cold frames should be oriented towards the south to allow the greatest amount of light inside.
- Put a thermometer inside to monitor temperature. When temperatures are above freezing during the day, keep the lid open. Then, be sure to close it again before nightfall. (The lid should always be opened to allow heat to escape when temperatures are 65–75°F).
- If temperatures dip below the mid 20’s, you may need to add additional protection by covering your cold frame with blankets.
- Mulch plants inside with dry leaves or straw.
- Don’t let too much moisture build up inside the cold frame which could rot plant roots or cause mold to grow.
- Watch for aphids and other pests that might survive in the cold frame.
To use a cold frame to extend your vegetable growing season into winter:
Cool-season autumn crops that you want to keep eating into winter should be planted in late summer or early fall to establish a good root system before the first fall frost. You can place a cold frame over them when frost threatens or direct seed them in the cold frame (keeping the lid off or open when temperatures are warm).
These include: Arugula, Beets, Carrots, Green Onions, Kale, Lettuce, Mache, Mesclun, Mustard, Spinach, Endive, Escarole, Parsley, *Parsnips, Radicchio, Radishes, Spinach, Swiss Chard, Turnips
(*Very winter hardy. May not need the cover of a cold frame. Harvest at any time when ground is not frozen.)
Most crops will be dormant when temperatures are below 40 degrees. But, they will stay at the peak of freshness until you are ready to harvest.
To use a cold frame to harden off plants started indoors and transition them outside in the spring:
Seedlings can be moved outdoors weeks earlier than the recommended date on the seed packet if they are protected by a cold frame. A waterproof heating cable underneath may provide additional warmth if needed. Be sure to open the lid and allow ventilation when days are warm enough to prevent damping off young seedlings and to keep plants from overheating.