How to Build a Cold Frame and Grow Veggies in Winter
If you're living in a cold clime, grow fresh winter vegetables in your own miniature greenhouse.
By Dan Sullivan
What you can do
Use old wood and repurposed materials to build a cold frame, and then enjoy a wintertime harvest.
RODALE NEWS, EMMAUS, PA—Ever notice how warm the inside of your car can get in sunny weather, even when it's bitter cold outside? The enclosed space becomes, in effect, a greenhouse, letting solar energy in while keeping the cold out. If you miss growing your own veggies when the weather turns cold—or want to try it out—you can take advantage of that principle by building your own cold frame. Free to operate and easy to construct, a cold frame will allow you to nurture ornamental plants and culinary and medicinal herbs through the winter, and to harvest more cold-tolerant vegetable crops like hardy greens and carrots all year long! Cold frames are also an excellent place to start seedlings for planting out in the open garden as the weather warms, giving you a jump start on spring gardening, and for acclimating seedlings you start indoors to the outdoors.
Where to Put It
If possible, locate your cold frame along the south-facing wall of an existing building. This will help protect the structure from foot traffic and the elements, and will provide additional warmth (especially if the adjacent structure is heated). The southern orientation affords maximum solar-collection capacity. Clear a space that matches the intended dimensions of your cold frame. Preparing the ground so it slopes slightly away from the exterior will facilitate drainage. If you are siting your cold frame in an established garden, herb, or flower bed, you might not have to do much in the way of soil preparation, perhaps working in a half inch layer of compost before planting anything in the frame. Otherwise, dig out the top three or four inches of soil and replace it with a layer of coarse gravel. Then put six inches of topsoil back. This will ensure good drainage.
Assembling the Frame
A cold frame is essentially a bottomless box with four sides, covered on top by glass or clear plastic. The back edge of the box should be four to eight inches higher than the front, so the top of the frame slopes downwards into the sun. You can use all sorts of repurposed material in the construction. Old wooden window frames with the panes intact make excellent cold-frame covers (just make sure the windows you plan to use don't contain lead paint). Alternately, you can use an old wooden screen door with plastic sheeting stapled to it. Plexiglas or plastic sheeting tacked to the top of the frame will also work. Choose a rot-resistant wood for the walls, such as black locust, cedar, or cypress. Exterior plywood is another viable choice. Do not use pressure-treated wood, as arsenic and other harmful chemicals can leach into the soil and be taken up by plants. Concrete blocks or bales of hay are other classic choices.
The dimensions of the frame are up to you, but they'll be influenced by the materials you use. If you're using windowpanes on top, for example, you'll need to match the size of the frame to fit. As a rule of thumb, the frame should be at least two feet by four feet wide, but probably not much bigger than three by six feet. Four feet down is probably as deep as you'll want to go. Once you have settled on your dimensions, and measured and cut the walls as necessary, join the wooden walls together with wood screws; for extra sturdiness, brace the corners with metal brackets. If you're using windows on top, attach them with hinges. You can tack other materials onto a simple frame that you can then hinge to it. A hook and latch attached to the low side of the frame will help the lid stay closed in harsh weather, or you can use weights to keep the top in place. Weather stripping along the edges will help your plants stay toasty.
Using Your Cold Frame
While they offer added protection from the elements, cold frames can still get too cold to give adequate protection to plants when the mercury drops past a certain point. If nighttime temperatures are going to dip into the teens or below, you can tuck in your cold frame by covering the glass (where most of the heat escapes) with old blankets, sleeping bags, loose straw, or newspapers. If your cold frame is constructed of poured cement or concrete blocks, these will absorb heat during the day and help keep your plants warm in the evening as that stored heat dissipates. Another way to store the day's warmth for the frigid, dark night is to paint plastic milk jugs black, fill them with water, put their caps back on, then place them inside your cold frame. They will absorb heat during the day and give it off at night. J.I. Rodale dug in large boulders between some of his garden beds to serve as daytime solar-heat collectors, a technique that would also work well as a base for your cold frame.
Another, more common challenge is too much heat. The key is to keep aware of the temperature conditions in the frame, and ventilate and insulate as necessary. When outdoor temperatures are above 40°F, prop open the lid six inches during the day; when the mercury tops 50°F, open it all the way (don't forget to close the lid in the late afternoon to trap in heat for the night).
What can you plant inside your frame? Lettuce is a good choice, as it doesn't require much height. But really, any veggie or plant that will fit in the frame is worth a try. And as spring nears, you can start seedlings that can be transplanted when they outgrow the cold frame. Or just leave them where they are, lift off the frame, and store it away until next year.
Don't feel handy enough to build your own cold frame? Check your favorite garden-supply catalog or website for kits, parts, and complete frames.