A humble cold frame offers a great way to produce nutritious, homegrown food outside of the prime growing season without investing in a full-sized greenhouse. Although many gardeners already use these mini-greenhouses to shelter seedlings before planting in spring, they may not realize that crops will stay alive inside a cold frame during winter, providing fresh food from the garden in December, January, and even February.
Commercial cold frames can make beautiful additions to your garden, but an inexpensive DIY setup is a great way to get started. You can quickly assemble one or more DIY cold frames with upcycled materials, and then move them around your property to find the best microclimate before you install a more elegant cold frame.
Using a Cold Frame
At its simplest, a cold frame is a box supporting a transparent top that admits sunlight. The top typically lifts to provide access to the crops living inside. The bottom is usually open to garden soil, allowing crops’ roots to dig deep. Using a cold frame is the equivalent of moving your garden one growing zone to the south. Adding a cold frame to the interior of an unheated greenhouse is the equivalent of moving your crops two zones south.
You should begin by identifying the best places on your property for growing cold-hardy crops. You’ll want to establish a cold frame in direct sunlight, atop good soil in a well-drained location that has easy access to the house — a long walk from the kitchen to the cold frame can be unpleasant in January! Ideally, you should maximize the cold frame’s effectiveness by angling its transparent top to the south, where the sun hangs low in winter.
Gardeners in cold regions — Zone 6 or less — can boost temperatures inside their frames by adding thermal mass to the interiors, or by insulating their cold frames’ sides with mounded-up soil, straw bales, or bags of leaf mulch. Growers in low-light regions can paint their cold frames’ interior white or add metal foil to reflect minimal sunlight. To prevent excess heat from wilting your plants, always vent your cold frame on sunny days when the temperature will rise above 50 degrees Fahrenheit. Just crack open the lid during the day and close it again at twilight.
Planting the right crops is important when using a cold frame successfully. Spinach, for example, will germinate and grow at temps that are only slightly above freezing. Kale has strong resistance to cold. Arugula is hardy all winter under protection, and will even re-grow — albeit slowly because of winter’s limited sunlight. Some varieties of cold-hardy crops are more resilient than others, so read the seed packets carefully.
Get Growing in a DIY Cold Frame
After you discover you can grow fresh produce year-round, you’ll probably wish for more than one cold frame. The following eight cold frame plans are all easy and inexpensive to assemble using repurposed materials, so you can build as many as your vegetable garden will hold.
1. Window-well covers. Heavy-duty plastic covers designed to prevent trash from collecting in basement window wells also make great cold frames for taller crops. The 19-inch-high pair has sheltered sage in the Mother Earth News office garden, where this herb will die back to the ground in harsh winters. To make a cold frame from two clear, plastic window-well covers, simply push the tall sides together and secure the pair at the top with clothespins or binder clips. To keep the covers from lifting off your plants, weigh them down at the base with stones or bricks.
2. Bathtubs and appliances. Britain is blessed with a mild climate, but gardeners there must still protect tender plants from frost, snow, and ice. U.K. grower Bren ffrench (sic) fashioned a mini-greenhouse by filling an old tin bathtub with soil. The top is a piece of clear, corrugated PVC roofing that had been damaged by builders at his neighbor’s house. Held in place with rocks, this top is easy to vent on warm days.
Some gardeners in colder climates have taken advantage of the insulating qualities of nonfunctioning chest freezers or refrigerators. They partially bury the inoperable appliances in soil, and then remove the lids or doors and replace them with a clear top to let in sunlight. How ironic that a chest freezer can actually prevent vegetables from freezing!
3. Shower doors. Cindy Koepke planned ahead for her DIY cold frame by purchasing a raised bed kit and installing it so its top tilted slightly to the south. When frost threatened in fall, she simply placed a pair of salvaged tempered-glass shower doors over the top of the raised bed.
Koepke later doubled the size of this garden bed. Because the old shower doors no longer covered the whole bed, she simply dropped a straw bale inside the raised bed at one end to fill in the gap. She has grown arugula, lettuces, mustard greens, radishes, and other hardy crops inside her frames in snowy Wisconsin. Learn more at http://MyYarnstead.com.
4. Straw bales. This easy DIY cold frame requires no building skills. Gardener Lisa Lombardo calls it her “redneck cold frame.” She used two straw bales and an old storm door left over from a remodeling project to assemble her protected garden on the west side of her garage. On sunny days, she tilts the glass door slightly upward so hot air can escape. When the weather warms in spring, she breaks down the bales and uses the straw to mulch her garden beds and pathways.
Many cold frame plans call for upcycling doors and windows. If you go this route, try to find shatterproof glass. Lombardo recommends avoiding very large or heavy windows because they’re difficult to move for accessing the plants inside. She also recommends protecting your building’s siding by slipping sliced lengths of old garden hose over the jagged metal edges of the window or door. You can also assemble a cold frame using straw bales for all the sides. If overnight temperatures threaten to drop significantly, cover the cold frame with a tarp or piece of plastic for protection. Learn more about this cold frame plan at www.TheSelfSufficientHomeAcre.com.
5. Windows. Inspired by a friend’s project, retired garden educator Lisa Wagner created several versions of cold frames from matching double-pane windows purchased at a local Habitat for Humanity ReStore. To form an open-bottomed box, she joined the windows with jute twine wrapped around tacks driven into the wood. The top is held in place by gravity. The resulting cold frame is easy to dismantle and store. Read more at www.NaturalGardening.Blogspot.com.
6. Pits. Although I own two commercial cold frames for winter growing, I also enjoy improvising with cheaper alternatives so I can grow even more winter-hardy crops. The past couple of years, I’ve had good luck with a shallow-dug pit on a south-facing slope outside my home. The cover of this “cold frame” is an upcycled wooden window tacked with heavy-duty plastic sheeting. I maintained the pitch of the slope on the inside of the pit. Normally, this would make watering a challenge, but cold frame crops don’t typically need much moisture during winter. The biggest problem has been remembering to vent the pit, as the interior can get too warm, even on cold days, because of its protected location. Arugula and other greens have thrived inside this simple mini-greenhouse. I’ve planted radishes in February and harvested them nearly two weeks before my area’s average last frost date.
7.Skylights. If you have access to a building site, or keep your eyes peeled on Craigslist, you may be lucky enough to score a skylight to make a quick and easy cold frame in your garden. Valinda Mullin got one from a family member who had purchased several inexpensive skylights at an auction. She positioned eight straw bales to form the base and has grown cold-hardy crops such as Swiss chard and parsley inside the frames.
8. PVC pipes. Some gardeners prefer a fancier cold frame rather than a pit in the ground. Utah grower Paul Gardener used 1⁄2-inch PVC to build a 4-by-6-foot frame that’s tall enough to shelter mature broccoli. He covers the frame with 6-mil poly sheeting, securing the plastic with clamps he made by sawing short lengths of 3⁄4-inch PVC into C shapes to pop over the 1⁄2-inch frame. Because of heavy snow in his area, Gardener used thick-walled PVC. He glued the end frames together, but not the ridgepole and top plates so the mini-greenhouse can be disassembled for storage. You can design a PVC cold frame to suit your own garden space and crops. Learn more about this cold frame plan in “Gardening Tips: A Cold Frame to Build.”