Under Wraps

cold frame © Christopher Weber

Winter is the mortal enemy of gardeners and backyard farmers from Florida to Saskatchewan. No matter how well a person waters and weeds during summertime, eventually Jack Frost will assassinate even the most lovingly tended crops. So what’s a protective planter to do? Build a cold frame, that’s what. When fall’s freezes loom, this simple garden structure will turn out fresh carrots, greens, peas, radishes and more right up to the Thanksgiving feast.

Think of a cold frame as a rustic cabin for vegetables. It does not boast the steamy luxuries of a greenhouse, but it’s much better than leaving crops unsheltered in the spring and fall. A proper cold frame can extend the growing season by two to four months.

The classic cold frame is a wooden box with an angled window as its roof. The tightly sealed walls deflect fierce winds. The tilted roof allows rain and snow to roll off.

Garden supply companies sell fancy cold frames via mail order, some for as much as $400. But by far the more satisfying, economical and environmentally friendly way to acquire a cold frame is to build your own. Using old windows and scrap lumber instead of virgin materials saves trees, packaging, landfill space and carbon—which is part of the point of vegetable gardening.

Another advantage of the DIY approach: It allows a gardener to tailor the cold frame’s dimensions, making it square or rectangular, wide or narrow, as needed.

Building a Cold Frame

Many cold frame plans are available online. One of the best resources is a webpage, video and list of links compiled by Gateway Greening, a nonprofit that supports more than 200 gardens in St. Louis, Missouri.

Ryan Barker, Gateway Greening’s community educator, provides tips based on long experience. “The most basic cold frame possible is made by stacking a bunch of bales of straw and placing a window on top,” he writes

Barker recommends finding the window first, as its size dictates your cold frame’s dimensions. He finds old windows at garage sales, thrift stores and junkyards. “Old storm doors work well because they don’t have separate frames, and the glass is really strong,” Barker adds. Some gardeners avoid glass entirely for fear of breakage and top their cold frame with clear, stout plastic.

Place your cold frame in a dry spot that faces south or west so that it will receive maximum light. The bottom edge of the cold frame’s box should be flush with the ground. If you see daylight beneath the base, pile up extra soil, mulch, straw or compost until the gap disappears. Barker recommends using a roll of weather stripping (adhesive foam tape) to plug any remaining crevices.

How to Plant and Tend

Vegetables that thrive in cool weather—such as greens and root vegetables—do best in cold frames. On the other hand, sprawling summer favorites like cucumbers, tomatoes and squash will find the box too confining.

Plant small, short plants up front and taller crops towards the back. As the mercury rises, prop the window open during the warmest part of the day. As Barker explains, “The most important thing that people don’t realize is even during cold sunny days, the frame must be opened to allow hot air to escape. Plants can easily be cooked on cold sunny days as the frame, just like a greenhouse, builds up a tremendous amount of heat.”

This is especially the case as summer approaches. With the arrival of hot weather, many gardeners take the windows off their cold frames and replace them with shade cloth, a porous plastic sheeting that blocks out part of the light. Alternately, some gardeners disassemble their cold frames entirely once summer arrives and bring them out again in time for a fall crop.

Barker recommends planting seeds in the cold frame by late September. Any later than that and they will simply lie dormant in their snug “cabin” until February or so.