Lewis Ginter Botanical Garden makes extensive use of cold frames to extend the growing season. (Photo by Tom Hennessy courtesy Lewis Ginter Botanical Garden)
Part incubator, part greenhouse and part time machine, a cold frame is anything but cold. It’s an empty, bottomless box that protects plants from winter weather. With its hinged lid of glass or rigid plastic, a cold frame captures solar energy and converts it to radiant heat, creating a warm microclimate where plants thrive. Like the windshield of your car on a cloudless day, sunlight passing through the glass is absorbed by interior surfaces and re-radiated as heat. It makes for a snug, safe space for plants to grow when the weather is inhospitable.
Sara Barton is a big fan of a cold frame. Barton got her master’s degree in public health from the School of Public Health at City University of New York, but she likes to say she got her work experience “in the field,” on an organic farm. Since 2017, Barton has been the Learning Garden Coordinator with the VCU Office of Sustainability, where she manages three urban campus green spaces.
All three are “small-scale gardens where a cold frame is a very useful tool,” to extend the season and expand the harvest, she says. “Just like in a home garden.”
Cold frames come in a variety of shapes, sizes and materials. You can buy one ready-made, or DIY a cold frame using lumber and old windows or glass shower doors. Barton prefers a portable cold frame made of lightweight materials, which is easier to move around to different raised beds.
“There’s a cold frame for everyone,” she says, “especially those who like to experiment with new gardening strategies to make even better use of their garden space.”
Control the climate in your cold frame carefully by paying attention to the degree of sunlight, moisture and temperature. Because soil dries out more quickly in a cold frame, and plants will rot if overwatered, maintain moderately moist soil. Keep the interior temperature below 60 degrees Fahrenheit during the day. “Chilly mornings followed by full sun in midday might overheat the plants,” Barton warns. “So vent a cold frame in the sunniest part of the day. Prop the lid open partially to let heat out.” Use a transparent cover when you are encouraging active growth, and a translucent or white cover when you are wintering over. In either case, be sure to keep the top clear of leaves, ice and snow. Barton also suggests “insulating with bales of hay in extreme weather or when the frame sits directly on the ground to add a few degrees of warmth and some security to the structure.”
All it takes is an empty box and a little TLC to extend the growing season in your garden.
Because a cold frame is the warmest spot in the garden, it’s a gardener’s hedge against winter weather and serves a variety of functions.
- Get some rest: Overwinter dormant plants in a cold frame. Before the first fall frost, cut plants back, water them well, pack the cold frame with as many pots as possible and fill the gaps with mulch or leaves for insulation. Plants will go dormant and resume growth in the spring when the air temperature rises.
- Get a head start: In spring, start your garden a few weeks early by direct-sowing seeds in the shelter of a cold frame. As the days warm, open and close the lid until seedlings are acclimated.
- Get used to it: Tender seedlings that are started indoors need to acclimate to the growing conditions of their new outdoor habitat. After young plants have developed several sets of true leaves, transfer them to a cold frame to harden off.
- Get more time: Extend the growing season into the fall and early winter. In her cold frames, Barton plants “low-growing, leafy greens and hardy, frost-tolerant kale, chard, cabbage and broccoli that thrive in cool weather.”
