DIY Cold Frames

Gardening in the north is rewarding but challenging—summer days are long and warm, but there aren’t always enough of them to accommodate all our gardening aspirations. Maybe you have your eyes on a new variety you’d like to grow, but the maturity time falls into your potential frost dates. Have you considered cold frames? They can not only protect sensitive plants from the first couple of fall frosts, but they can also permit you to get things going earlier in the spring. You can even use cold frames to help potted seedlings acclimate to the outdoors. Sure, you can always purchase cold frames, but the concept is pretty simple and well within the reach of a home DIY-er who wants to customize their project, use up some scrap materials and save a little money.  

Wooden cold frames

One option is a well-constructed, permanent set of cold frames; something that can be built once and used for years to come. In a case like this, maybe you’d like to design one the size of a small garden bed—perhaps something five or six feet long and three feet wide. A basic cold frame this size can be constructed out of standard 2×6 or 2×8 lumber and then covered with repurposed window sashes fastened with hinges. Lightweight Plexiglas is another excellent choice for the transparent material. Two wedge-shaped pieces of lumber can be used on the sides to angle the windows toward the sun. A depth of about 1 foot is common, but the overall dimensions are very flexible and can be tailored to the specific situation Add a few 3-inch exterior screws and you’re all set. If you keep the footprint size reasonable, your cold frame will be semi-portable so that you can move it wherever it’s needed from season to season.

Cold frames like these can be custom-designed to fit your garden’s décor and even painted colors that reflect your garden’s theme (painting can also help prolong the life of the cold frame, since pressure-treated wood should be avoided in the garden).

The main downside to this style of cold frame is the shallow depth; it’s primarily suitable for very young plants or crops that don’t grow very tall like carrots, leafy greens, etc.

Window “boxes”

If you’re okay with very small cold frames, you could even make a small 5-sided box out of just repurposed window sashes screwed together. This gives you a small cold frame—or you could call it a large cloche! In this case, you don’t open the cold frame during the day—it’s small enough that you just remove it entirely as needed. If you don’t have window sashes in need of recycling than this method is probably not worth the effort, but if you have a source, it can be a fun option.

Straw bale cold frames

Maybe you’re looking for something really simple. Not everybody has the time or desire to take measurements, source materials, and build it. No problem! If you’re looking for something fast and easy, try straw bale cold frames. Just purchase a few straw (not hay) bales from a home/garden improvement outlet, animal feed store, or local farmer who grows oats or barley (straw bales are made from the leftover stems after harvest). Arrange the straw bales around the perimeter of the garden bed you’re looking to protect. This doesn’t have to be super precise—a small gap here and there shouldn’t make any significant difference in the insulating properties. Then place transparent plastic sheeting over the top of the straw bales and weigh it down. It’s maybe not as convenient as other methods and certainly lacks atheistic appeal, but it’s easy to do and takes very little time. And if frost is coming tonight, it’s something you can construct fast.  

Row covers

While not a cold frame per se, row covers allow you to protect a lot of garden space. You can even accommodate taller plants—say tomatoes—that need extended growing times. In these cases you can construct hoop-shaped row covers—frames that really look like mini-greenhouses. Thin, flexible PVC pipe can be used to build semi-circle hoops, which can then be covered with clear plastic sheeting. Alternatively, a competent DIYer could make the hoops out of conduit, carefully curved with a bender tool specifically for that task (these will be much more durable). You’ll need to fold back the plastic sheeting every morning for ventilation, rather than just opening the window on a typical cold frame. Still, it’s a good way to extend the season for taller crops.

DIY cold frames give you an opportunity to extend your growing season in both directions, and they don’t have to be overly complex. Making them can be a fun project to kick off your spring growing season.

Daniel Johnson is a Wisconsin-based freelance writer, professional photographer and co-author of over a dozen books. See his garden and animal photography at foxhillphoto.com.