This past week the weather has given us a preview of beautiful, mild autumn days. But with the arrival of fall, comes frost. One of the best ways to protect your less than winter hardy plants is a cold frame. There are many ways to construct cold frames, wether permanent or mobile. Now is the time to start thinking where and what type of frame to build, so you aren’t scrambling around frost time.
First lets figure out what a cold frame is. A cold frame is a protective structure using glass or plastic to protect plants from frosts or snow. Normally a cold frame sits upon the ground, rising just a few feet from above the surface. Think of it like a little greenhouse, but all your plants are in the ground instead of in pots. The glass or plastic allows the sun to reach the plants, while capturing heat, preventing your plants from freezing. They can be movable structures, placed over whatever beds need it, or permanent establishments in your landscape. Either way, the chances of you having fresh salad green on the Thanksgiving table without a cold frame are pretty slim.
Lets talk about some construction ideas for your cold frame. We’ll start at the top. Many people use old windows as the top covering for their cold frames. The only concerns with this material are lead paint and the glass breaking. Make sure if you use windows to triple check the paint on them, especially if they come from older houses. Also, if we were to have a massive snow, the weight of said snow could possibly break the glass. As far as snow goes, you’ll have to sweep it off the top of the cold frame so the plants can still get light. A lot of gardeners use plexiglass, the same stuff used for greenhouse walls. It’s more costly than other plastics but it doesn’t yellow in prolonged sun and is pretty durable. Really any hard see through material can be used as a topper. Many people use hinges for windows and heavy plastic, to help keep the top in place while propping it open for ventilation. Others are cool with just laying the plastic over the walls and being able to completely remove the top. Whatever you prefer you can’t really go wrong. It just takes some experimenting to get what works best for you. But hey, that’s kinda what gardening is about.
Now let’s take a look at the walls. Just about the easiest, cheapest walls for a cold frame are hay bales. There are many benefits to using straw bales. First is the biodegradability. I just love how you can use the hay as mulch after you’re done with the cold frame. Second is movability. Hay bales are pretty lightweight compared to other building materials. Moving them around to find the right spot for your cold frame is super easy with hay bales. Third, they provide pretty good insulation, while allowing adequate breathability. You don’t want to smother your plants. Good ventilation is crucial. Cinderblocks are the next step up in durability, permanentness, and cost. Cinderblocks, although more costly than hay bales, will last a lot longer. They are movable but pretty heavy. They don’t cost an arm and a leg, and most people have a few lying around anyways. Just make sure if you are using cinderblock to put the holes facing upwards so the cold air doesn’t get through. An even more permenant solution is a cold frame out of wood. Avoid using pressure treated or chemically treated woods. These are toxic and will not help you out in any way. Cedar and cyprus are naturally rot resistant. This can be more costly but look a lot nicer than cinderblocks.
There are some aspects of cold frames that you should keep in mind. When building your walls pay attention to the angle at which your top will sit. Angling your top about 25 to 30 percent from front to back is a good idea. This prevent water or snow from sitting on top of the frame. If possible have your angled side facing the south. If your putting it in a sunny spot, however, it should be fine regardless of what direction it’s facing. Ventilation is another factor when using a cold frame. If It gets too hot and muggy under your frame your plants could rot and die. The most basic is a sturdy stick to prop up the top. You can experiment with different ways to prop the top open for optimal ventilation. Another aspect of a cold frame is the temperature. It’s a good idea to have a maximum and minimum thermometer placed along an inside wall. A temperature above 90 degrees F can damage seedlings and cook your plants. The minimum temperature will let you know if you need more insulation or mulch.
You can always purchase a cold frame, but you might now find exactly the size or price range you desire. They are a great way to extend your growing season and a good weekend project. They can be extensively planned or thrown together from stuff you have lying around. One things for sure though, you won’t regret trying on out! – Happy Planting!