Although you can certainly plant too early even in a cold frame, it does give you, and the seed, a chance to get an earlier start than open ground planting might allow.
    Of course, cold frames are not just spring tools, though that’s their busy time. I use them as well in the fall, and some years all summer long. In milder climates winter is their season. With the varying and unpredictable weather we have, they are a tool to reach for any time of the year.
    In our short growing season of the upper Midwest, cold frames are more than just an interesting tool, they are a necessity in my quest to grow most of the food we eat. They ease the "frost stress" part of gardening, which makes this gardener a happier camper. And they allow for more in-season-out-of-the-garden eating, which makes for healthier, happier, and richer homesteaders. Besides which, they are easy and inexpensive. I wouldn’t want to garden without them.

MAKING THE COLD FRAME -- Cold frames can be fancy or simple. Like many of the tools on our homestead, ours are straightforward and functional, using recycled and scrap materials. Our current frames are of two designs based on the windows used for the tops. I prefer glass for glazing over plastic, not only because of the non-disposable aspect (important to me) but because they are heavier and not as apt to blow off in a wind. Even with that weight, I’ve had a few go flying in a good gust, but it is rare and in over twenty years have only had two break. It’s nice to make use of something destined for the landfill rather than buy new something that will have to be landfilled and replaced many, many times over the life of the cold frame.

  INSET FRAMES: We made our first cold frame windows ourselves because we had extra glass left over from building our house. Window glass is also readily available in old windows whose frames are past use (or that had lead based paint which you don’t want in your garden). Just remove the frames and use the glass. Or look for good, used framed windows. Our homemade window frames are of simple, overlapped corners design, made of pine.
    Whether you make your own or use recycled windows, scrape off any loose paint and caulk around the glass with silicone sealant. You can use any non-toxic oil finish for the wood (remember, this will be in your garden). An inexpensive and easy solution we’ve used is two parts raw linseed oil to one part turpentine. I’ve also used a more expensive commercial pine-tar based product, thinned with raw linseed oil to make it go farther (it is a black finish). Boiled linseed oil would be fine, too. Or just leave the wood unfinished.
    You could set your windows on a plain wood box for a cold frame, and I’ve done that. It works, but if your window frame isn’t flat on the bottom, there will be a too-generous air leak on two edges. And when you tilt the window up for air circulation (which you will be doing quite often), it has a tendency to slip right off the back of the box. Our solution for this set of windows was to build the frame box large enough to set the windows down in.

    The size of your box will be determined by the size of your windows. Add a half inch to the measurement of the window (both length and width) and that will be the inside dimensions of your box. The extra half inch (1/4" on each side) is important because your box is likely to swell and warp some in the rain and sun, preventing you from opening the window if it is too tight. It does give space for some air leakage, but I’ve not found that to be a problem. I rather prefer the design because of that little bit of air circulation.
    Use whatever wood you have or is available. Our first boxes were of poplar -- probably one of the least recommended woods for outdoor use. But it was what we had, and those boxes lasted for many years (long enough I can’t remember just when we first made them). When they finally did rot enough that they were falling apart at the corners, we rebuilt out of salvaged wood from our torn-down chicken coup -- a combination of poplar, pine, and cedar -- much of which was, itself, salvaged from some other project! As with the window frames, just make sure the wood doesn’t have a toxic finish you don’t want in your garden.
    Cut your boards to size, then nail or screw them together at the corners to form a box. We’ve found deck or drywall screws to work well. Then make 2x2" blocks for the corners. Two of them, installed in opposite corners, should be as long as the width of your boards (the height of the cold frame box). The other two, installed in the other opposite corners, should be the width minus one inch. Install (with appropriate sized screws) the long ones so they stick down one inch, the other two so they are even with the bottom of the box. This way all of the corner pieces are one inch down from the top and this is what the window rests on, inside of the box. In both securing the boards to each other and installing the inside corner pieces, it helps to first drill holes for the screws. The corner pieces not only hold up the window, they add to the rigidity of the box which is necessary since you’ll be moving the boxes a lot over the years.

    So what about the two corner pieces that stick down? Well, one box is seldom tall enough for the cold frame as the plants grow so you will soon want to add one (or more) boxes, one on top of each other. These lowered corner pieces fit into the box below and help stabilize the stack, keeping you from pulling the top box off as you open and close the windows. It’s not necessary, but it helps a lot.

    The last refinement is to add some type of handles to your windows so you can easily open them. Ours are a wide variety of salvaged and homemade designs. Anything that will allow you to lift the window up for propping it open will do. Since my frames are rectangular and used sometimes across and sometimes lengthwise on a bed, I have handles on two adjacent sides of the window.

  TOP SET FRAMES -- Another simple design is the top-set frame, but with a slight enhancement to keep the window from slipping off the back. We came up with this second cold frame design when a friend gave us a batch of old aluminum clad windows he’d replaced on a job. They were pretty flimsy compared to our wood-framed windows, and not easy to attach a handle, so we made a box they could sit on. This design can, of course, be used for any window that is flat on the bottom, whether metal or wood framed.
    We had also been given some pallets which we tore apart to use for the boxes. These boxes are slightly smaller than the dimensions of the window with the back board (which should go entirely across the back) sticking up 1/2" to 3/4". The window then sits on top of the front and sides of the box, snug against the raised back board. This prevents the window from sliding off the back when propped open, and gives some stability when the boxes are stacked as the raised board of one snugs into the raised board of the other. With the aluminum windows, we sized the boxes so the window would stick out over the front edge which allows us to easily lift the window.
    These boxes also have 1 1/2" to 2" corner pieces to sturdy up the box, particularly needed since our pallet lumber was only 1/2" thick. These salvaged cold frame boxes are pretty fancy with wood of cherry, maple, and oak. They definitely weigh more than the poplar and pine ones!

USING THE COLD FRAMES -- The biggest danger of using cold frames is forgetting to open the windows on a sunny day. Plants will be damaged more by over heating than cold. So a prop is an important part of the system. I’ve found a 6" length of 2x4 to be a good prop giving you three easy options for how far you want the window open -- 2", 4", or 6". Generally my cold frames are set up to open from the south, but if there is a strong wind out of the south, I will prop them open on the north to keep a gust from flipping the window off. Since my frames are set flat on the garden beds, there is really no front or back, so they are quite flexible.
    On a warm, sunny day the windows can be simply lifted up and slid over the back edge of the boxes (or to one side), depending on what other plants are nearby. If you are going to be gone and it is too cold to leave the windows open, cover the cold frame with a blanket or rug. It is better to have the plants in the dark for a day or two that have them freeze or cook.

MINI-GREENHOUSE -- If you don’t have a greenhouse (or even if you do), you can make a large cold frame to do the job of starting seedlings. For many years we had a five window long permanent cold frame in the garden made of discarded windows with back and front walls of used unmortared cement blocks. The back was taller than the front and the sides were made of scrap boards. We also put scrap boards on top of the cement blocks nailed to blocks of wood wedged into the blocks to make a nicer top for the windows to rest on. Into this large garden cold frame, I moved my windowsill-started plants as soon as it was warm enough. I also started many crops such as the coles and cucumbers directly into the cold frame dirt, transplanting later into the garden. For squash seedlings it was great as you could use a shovel to transplant and so avoid the transplant shock usually associated with the cucurbits.
    We now have a greenhouse and that does do a better job, particularly with the warm weather crops such as tomatoes and peppers. But a large, permanent cold-frame can perform surprisingly well, either as a substitute or as a supplement to the greenhouse. The large area is warmer than individual cold frames which the early plants appreciate.

  CROPS IN THE COLD-FRAME -- Once you have a few cold frames, you’ll find many uses for them throughout your garden. You will soon be scrounging up materials to make more. In the late-summer and fall I start lettuce seedlings in a cold frame box in the garden. As the weather gets cold and frosts and freezes arrive, I add the window to protect them, propping it open on warm days. As the cold settles in, half of the seedlings get transplanted to the greenhouse for late fall and winter eating, the others continue to grow in the cold frame in the garden. As the freezes end our regular lettuce crop, we eat the cold frame protected plants until winter and deep snow arrives and our harvesting turns inside to the greenhouse (where the plants are, hopefully, well along and producing). In a milder climate this could continue throughout the winter, and for other crops.
        Spring is when the entire troupe of cold frames end up out in the garden, some going from one crop to another, others sticking with their initial inhabitants until the weather warms and stabilizes. Lettuce and greens are the first -- some to be planted in a cold frame, others transplanted from the greenhouse. These first outside-grown early greens are appreciated as no others can be. Sometimes I try some extra early carrots as well, though they don’t do as well as those planted when the weather is more suitable.

       As the weather warms, I set out frames to start cucumber and squash seed in, planting directly into the ground where they will grow. Later the greenhouse tomato and peppers seedlings are transplanted into more frames. As they grow, frames are stacked on to keep up. Hopefully, the weather warms up before I run out of frames. The early lettuce and greens are sturdy enough by then to have their frames removed and they can be used elsewhere.

   In a good year (thankfully most years), as summer arrives and frosts abate, the cold frames are removed from all of the plants. Four of the windows go on the solar food dryer, while the rest are stored in the shed until needed in the fall. The boxes are stacked out of the way not too far from the garden. If you stack them up off the ground, kitty-corner to one another in the stack, they will dry better and last longer. I’ve found old tires to be a good base, as are used cement blocks.

    There are those years, however, that some of the frames never leave the garden. Those are the cold, cloudy summers that make us worry if we will get even one ripe tomato to eat, let alone enough to can. These are the times that we really appreciate the cool weather cole and root crops as we continue to stack cold frames on the pepper plants as they grow, snug and warm in their protective boxes, producing in spite of the weather. Cold season gardeners often manage to grow melons and watermelons this way as well, and the smaller varieties of tomato plants, too. Larger plants usually soon outgrow their cold frames and take their chances with the weather with blankets to cover them on frosty nights.
    If you’ve spaced your plants right, you can replace the cold frames in the fall over the full grown plants to get them through those early frosts, and extend the harvest just a few more weeks. There is some aspect of game and some of challenge, along with fun and just plain gathering for food in gardening. No matter which, cold frames can help the gardener out, and make the gardening life a little easier. If you take your plants’ natural needs into consideration, they, too, can appreciate the cold frame as much as we do a good sweater on a cold night.

 * * * 2008 * * *

The above article was written many years (decades!) ago, but I'm still (2021) using the same or similar cold frames. Some boxes have been replaced, some windows redone, but they are still one of my most important gardening tools. The individual boxes above work fine but some years ago I envisioned a larger, plot sized cold frame for my tomatoes, with removable rack to hold semi-determinant plants upright. Steve helped me design and build it and it has been great -- four eight foot long cold frames, which fit my 4 ft x 32 ft plots. The last few years when I didn't grow so many tomatoes they served well for peppers, and squash. They are no longer fresh-wood-gold colored now but they still work great.