A cold frame is a simple, low structure with a transparent cover that functions as a miniature greenhouse. Placed directly over the soil or a raised bed, it protects plants from excessive cold, wind, and wet conditions. The clear top admits sunlight, allowing solar energy to heat the interior and the soil beneath. This passive heating creates a localized microclimate, which significantly extends the gardening season in both the spring and the fall.
Early Spring Seed Starting
The cold frame is a tool for giving many crops a head start before the last frost has passed. By creating a warmer environment, gardeners can sow seeds up to one month earlier than planting directly into the cold ground. This allows young plants to develop strong root systems, preparing them for better production once transplanted into the main garden.
Cool-season crops, such as hardy greens like spinach, leaf lettuce, and kale, thrive when started in the cold frame. These plants prefer air temperatures between 50 and 60 degrees Fahrenheit during early growth. Starting brassicas like broccoli, cabbage, and cauliflower weeks early ensures they reach maturity before the onset of summer heat.
The cold frame is also utilized for “hardening off” seedlings started indoors. Hardening off is a transitional period that gradually acclimates young plants to outdoor weather, including direct sun exposure and stronger winds. Even ornamental plants like pansies and sweet peas can be started early, providing earlier blooms.
Harvesting Cool-Season Crops in Winter
The most compelling function of a cold frame is its ability to provide a continuous harvest of fresh produce throughout the winter months. This is achieved by planting cold-tolerant crops in late summer or early fall, allowing them to reach near-maturity before the deepest cold arrives. The structure then protects these plants from hard freezes, wind, and snow.
Leafy greens form the backbone of a winter cold frame harvest due to their natural tolerance for low light and cold temperatures. Excellent choices include corn salad (mache), claytonia (miner’s lettuce), and Asian greens like tatsoi and pac choi. While growth slows significantly once daylight hours drop below ten, the plants remain available for harvest on demand.
Certain root vegetables also perform well under the cold frame’s protection. Carrots, radishes, parsnips, and beets can be left in the ground and dug up as needed throughout the winter. Exposure to cold temperatures enhances the flavor of these root crops by concentrating their natural sugars, resulting in a sweeter taste. A perennial herb like sorrel can be cultivated for winter use, providing fresh, lemony greens when other plants are dormant.
Protecting Tender Perennials and Bulbs
Beyond active cultivation, the cold frame serves as an unheated holding area for plants that are marginally hardy for a specific region. This use focuses on storing plants in gentle dormancy rather than encouraging active growth. The enclosure shields containerized plants and bulbs from extreme temperature fluctuations and damaging freeze/thaw cycles.
Semi-hardy perennials, such as containerized rosemary, fuchsias, and geraniums (Pelargoniums), can be cut back and placed inside the frame for overwintering. This protection prevents root death and saves gardeners the expense of purchasing new plants each spring. Tender bulbs like Agapanthus can also be stored in their pots within the frame.
During this dormant period, the plants require minimal maintenance, but proper moisture control is important. Watering must be infrequent and light to prevent the soil from drying out while avoiding the risk of root rot. Utilizing the cold frame for storage ensures these tender specimens are ready to resume growth when warmer spring conditions return.