Building a Cold Frame

One of my gardening dreams is to have a greenhouse. It doesn’t have to be enormous, just a pretty little affair like Lynnette’s over at Chooks and Roots blog, where I can grow a few prized tropical plants all year long. (Tomatoes… lemons… limes… bananas… mangoes… coffee… okay, maybe it’s going to have to be a smidge bigger than Lynnette’s….)

Well, at present I don’t have the money to spend on a full-blown greenhouse, but I did decide that this spring I would make the effort to build a cold frame. A cold frame is basically like a miniature solar greenhouse. The sun warms the soil and air inside to a few degrees over what it is outside, but it’s called a “cold” frame to distinguish it from a “hot” frame, which has an additional, non-solar source of heat. (This could be electrical, or it could come from decomposing manure located under the soil.) A cold frame isn’t going to allow you to grow limes in Zone 7, but it might allow you to grow lettuce and other salad fixings a few weeks earlier (and later) than normal. It’s also a cinch to build.

I’m all for keeping cash outlay in the garden to a minimum. (Buying seeds is the one area in which I really splurge. But at $2 a packet, an extra variety always seems like a great deal. I know I’ll get much more enjoyment from seeing my Eva Purple Ball tomatoes ripen than I would get from, say, watching one fifth of a movie in the theater. I just take money for seeds from my entertainment budget!) So as much as possible, I try to make garden equipment from whatever materials I happen to have on hand. For a few years, we’ve had some Plexiglas panels (2’x5’) sitting around. They’re the remains of a drum set surround from years gone by. They would make an excellent top for the cold frame. What was lacking was the frame part.

My first DIY cold frame effort

A few years ago, I tried building a frame by felling some small, fairly straight ailanthus trees in the woods behind our house. I used a hatchet to cut notches out of them and then assembled them into something that looked like a very drafty log cabin (with a super-modern Plexiglas roof). I tried chinking the holes with mud, but it dried, cracked, and promptly fell out. That “cold frame” might have protected the lettuces from the harshest of the winter winds, but it wasn’t going to do any agricultural miracles. What I really needed were some actual sawn boards. Only I didn’t want to buy them. I wanted them to magically appear.

Well, a couple of weeks ago, I got lucky. My dad took apart some built-in furniture from one of our old bedrooms, and voilà: four 2”x6”s, untreated and unpainted. You don’t want to build your cold frame from pressure-treated wood (it can leach some nasty chemicals into the soil), and you don’t want paint around your vegetables either, if you can help it. Of course, untreated wood deteriorates rapidly if it’s in contact with the soil. So that meant I needed one more thing to complete the cold frame: a foundation. Rocks and concrete could do the job, but we happened to have some unused bricks on hand, so I used those.

This one's a keeper!

To assemble everything, I first measured the Plexiglas. Then I laid out the bricks to those dimensions. I decided to overlap two pieces of Plexiglas to make one large cold frame, as you can see in the pictures. The weight of the center is supported by an extra brick stood on end. Then I cut the 2”x6”s to make a box slightly smaller than the outer dimensions of the Plexiglas. I wanted the Plexiglas to overhang the outside slightly in order to keep as much rain as possible off the wood. Then (with my dad’s help), I used screws to join the corners of the boards into a box (two screws to a corner). Once the box was assembled, I lay it on top of the bricks. The bricks, sitting on garden soil, were of course not level, so I then went around digging out soil under the high parts and adding soil under the low parts, to make the bricks even and allow the box to settle tightly on top of them all the way around. (Hopefully, no drafts this time!) Serendipitously, when I was done, there was a slight slope to the entire box, which should help rainwater drain from the lid.

A few wild plants enjoying their new tropical environment

Even though I haven’t planted anything in the cold frame yet, I went ahead and put the Plexiglas panels on top. This will start warming the soil for when I am ready to plant. Since I happened to have a refrigerator thermometer sitting around (the kind you can buy at the grocery store for a few bucks), I put this in there so I could monitor the temperature and see how it differs from outside. Today, even though it’s cloudy, there seems to be a difference of at least a couple degrees.

Now that the cold frame’s all set up, I’m going to have to decide what to plant. The interior dimensions of my box are 42” by 62”, which gives me about 18 sq. ft. to work with. I figure it’ll be mostly lettuce, with maybe some spinach and radishes. But stay tuned to find out!

P.S. Many cold frames have a lid that’s slanted ten degrees or so toward the south. This increases the amount of light getting into the box, and this might be a good improvement for the future. For now, though, I’m keeping it simple. Better a simple, cheap cold frame than no cold frame at all.

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