How to Build a Cold Frame

It is easy and inexpensive to build a cold frame. Certainly a good project for frugal people like us. Basically, a cold frame is a wooden frame with windows set on top. 

We’ve seen some pretty expensive versions of cold frames that have built-in irrigation systems and self-opening windows. Cold frames need a little watering here and there, but not enough to require an irrigation system, in our opinion. The enclosure helps to retain moisture, and so does the soil as long as it is high in organic matter (compost). As for opening the windows, my method is to prop them up with a little stick. The windows need to be raised when the sun is out and temperatures are above freezing otherwise it can get warm enough in there to bake the plants.

This cold frame is an extra-long version using three windows.

Windows are often free for the taking if you know someone who is replacing theirs. Or you might try the local dump, or check around the neighborhood on large-item pick-up day. Our windows happen to be 34” wide by 31” long, and we use two side by side, making our useable soil space about 68” wide by about 27”. A little note on the windows is that ours are double-pane, giving an extra bit of insulation. People who use single-pane windows sometimes need to throw a blanket over the frame on really cold nights. We have never needed to do this with ours. 

Here are some basic instructions to make a cold frame.

Go to the lumber yard for 2x12 and 2x8 boards. You will need one 2x12 the width of the two windows, 68” in our case. Get the 2x8 the same length. Then you will need two lengths of the 2x12 for the sides of the frame, for us this is about 29-1/2” - about 1-1/2" shorter than the length of the window to allow for some overlap so the window can be lifted. You can have them cut to size at the lumber yard but it is cheaper to saw them yourself. Use a good hand saw or a circular saw. 

Attach the sides onto the ends of the front and back pieces. Use corner braces to fasten them together. These make the frame more stable than simply screwing the lumber pieces together. The corner brace packets come with the screws needed to attach the front, back and sides of the cold frames.

Work on a flat surface so all the boards stay lined up. Use an electric drill to start holes for the screws, and a good old fashioned screwdriver unless you use the drill with a screwdriver bit. 

It's a good idea to check that the windows will fit nicely before putting in the final screws (there's likely to be some variation between your windows and ours in how they will fit onto the boards). When the cold frame is all put together, the windows rest on the back and front boards of the frame. They should fit snuggly side by side with a little overlap on the front so they can be lifted easily.

We have placed our cold frames over existing raised garden beds or set them up in new spots. When using a space that was not previously gardened, prepare the soil below it with loam, peat moss and plenty of compost. Ideally, prepare the spot a month or so ahead of planting and apply manure. Set up the cold frame with the 2x8 facing toward the south. Place it directly on top of prepared soil. Now you're ready to plant seeds that will grow your late fall, winter and early spring produce. -G.H.