Cold Frames, How and Why
Before moving to the temperate climate, I’d assumed that winter was pretty much a wash for growing a decent crop. I knew soils were good and summers abundant, all of which led to lots of food storage for getting through winter. I even looked forward to the squashes and pumpkins, and I couldn’t wait for the berries and hard fruits. That all seemed doable, even exciting in a way, but the thought of shutting the garden production down still felt scary.
I always assumed some winter crops were there to be had in a glasshouse. I imagined growing enough greens for salads, but I also knew that a giant glasshouse is a bit too costly for a low-income homesteading couple, even without trying to heat it. Emma and I, as with every hopeful designer, daydreamed of a small attached glasshouse to help with passively heating our house, but there is only so much that can grow in one of those.
Cold frames were something I knew about, and even before investigating them further, they seemed a decent solution to this problem. Now, I ‘m really keen. Not only are they a way of growing a full bevy of crops in the winter (there is a lot to be grown, even in freezing weather), but also they can pretty quickly be pieced together with scrap and salvaged materials. In other words, they are effective and inexpensive, as well as easy to sustainably source.
The Basic Build
Cold frames are so simple in design: Essentially, it’s four sides of a bottomless box with a window on top. They aren’t necessarily restricted by any size specifications, so they can more or less be designed to fit whatever windows or storm doors someone happens to find. The sides can be made from many different materials, including stones, bricks, cob, straw bales, logs, or scrap wood.
The top, called appropriately a light, needs to be clear to allow the sunlight in but trap the warmth inside. Obviously, Plexiglas would work and minimize the risk of breakage, but finding glass to reuse will make for a heavy lid (so it won’t fly open in winter winds) and avoid petrochemicals. Whatever the lights are made of, they are usually framed with wood, tilted slightly towards the equator, and sized according to the material on hand.
Generally, sides are made of wood, and old pallets would probably be just perfect for them. The back is around 30 cm tall, the front roughly 20 centimeters, and the two sides slope between them. While the boxes don’t need to be air tight, the less breezes can wiggle through the better. Reinforce the corners with a scrap of two-by-four and try to seal any noticeable seams.
When designing cold frames, it’s important to keep in mind that the lid ideally will be light enough for one person to open, and there should be some sort of prop to hold the lid open when the days are hot. If the boxes are made of wood, it’s a good to let them sit on some something rather than directly the ground. Some people attach small strips of wood along the bottom edge of the box so that it can be change when it begins to rot. Other lay out a layer of stones or bricks for the boxes to sit on.
Why It Works
The basic gist of cold frames is that the reduce the stress of climate on the plants. They keep the plants a bit warmer than it is outside, quite a bit so when the sun is out. In fact, during the spring and fall, when freezing temperatures are just a nighttime concern, the boxes should be propped open to avoid overdoing it. They also help with maintaining humidity while simultaneous providing protection from cold, hammering winter rains. In the winter, they rarely need to be watered. Lastly, they remove the wind (and windchill) factor, which can be extremely damaging.
For me, the idea also works because, once constructed, they don’t require any energy inputs or even much watering. They aren’t hothouses, expected to produce tropical crops in the dead of winter; rather, cold frames are simple devices that allow us to protect cold weather crops so that they can be harvested fresh through the coldest months of the year without a heating system. Eliot Coleman, author of Four Season Harvest, claims they create a microclimate one-and-a-half to two zones warmer.
Another reason I really like the idea of cold frames is that they can simply slide over existing garden beds. With a little forethought in design, they can be constructed to work seamlessly with existing double-reach beds or raised garden boxes, allowing us to move the protection to the plants rather than have to work within a very confined space. This, of course, makes it possible to have plenty of crops on the go long after the first snow. It also means that we can slowly expand our winter growing capacity (and habits) instead of dropping all that green into a greenhouse.
Lastly, I’m really into not using plastic, specifically plastic sheeting, which tends to tear and become both useless and a pollutant on site, let alone in the landfill. In contrast to hoop houses, I like that cold frames can be made of natural materials and old windows, as well as be used for several years. Cold frames are said to keep the crops warmer and to be easier to harvest from anyway.
Temperature control is definitely something to be aware of with cold frames, so it’s a good idea to keep a thermometer in them. This should like be somewhere near the center and bottom of the box. There are simple automatic venting arms that’ll open them via a heat-activity cylinder, or a simple notched stick can be used to prop them open when the weather starts warming up but still drops below freezing at night.
Choosing the right crops is still imperative. These aren’t meant to grow tomatoes and green beans in the winter. They are for extending the season of cold-hardy crops, like kale, chard, carrots, beets, leeks, radicchio, cabbages, lettuces, fava beans, and so on. Temperatures inside them will definitely drop below freezing during the night. For most of these vegetables, it’s still best to wait until they thaw to pick them.
Eliot Coleman, my cold frame guru, notes that keeping the cold frames relative low makes the microclimate warmer. Rather than trying to give crops a lot of head room, he suggests keeping the lids as low as possible without inhibiting growth. It makes sense. He also suggests not tilting the lids too much as that, too, creates more head space.
When the weather is really cold, small thermal masses, such as stones or cob dividers, would help with providing warmth at night. With the right crops, however, this probably isn’t necessary. Choosing things that can freeze is a much better approach, with perhaps some experimenting as to what kind of extra passively-gained warmth can be sustained.
Don’t paint or treat the frame. Rather than expose our foods to toxic chemicals from paints and/or sealers, we can just do our best to keep the frames off of the soil and live with the fact that the wood won’t last forever. Not only does painting the frame cause chemical concerns, but also it creates a maintenance nightmare, as it will have to be redone every year.
Lastly, for those who do have glasshouse but don’t want to use energy heating them, cold frames inside glasshouses are supposed to make them all the better for providing warmth, nearly doubling the effect. It might be worth a shot.
Feature Photo: Courtesty of Ofer El-Hashahar