One way to extend the vegetable growing season — both on the front end in spring and on the back end in fall — is to use cold frames, hoophouses and other simple forms of protection for your plants. A cold frame is basically a mini-greenhouse for acclimating seedlings you started indoors or growing tender plants when the weather is still too cold.
Most cold frames have solid sides (wood or cement block or straw) to block wind and some kind of a clear cover (glass or plastic) to let light and heat in. The cold frame should have a system so it can be opened in the day so the plants don’t get too hot and then closed up again at night. Cold frames should be placed in the sun, but many gardeners also like to put them up next to the house so they pick up some residual heat. You can also put a cold frame right on top of a garden bed to act as a temporary greenhouse. Most gardeners use them to harden-off seedlings or hold cool weather plants until it’s time to put them in the vegetable garden.
There are lots of videos showing how to make a cold frame using an old window, some pretty complicated woodworking tools, or PVC pipe and plastic. No need to buy a premade cold frame — use your creativity. The photo here shows a cold frame used outside of the MSHS office in Roseville. It’s a simple wood box with a corrugated plastic top. The plastic is attached to wood pieces and hinges connect the top to the box, so it can be easily opened in the day. Because the plants and wood absorb heat during the day, it’s possible to leave cold-weather plants (such as lettuce) outside even on pretty chilly nights. (If it gets really cold, throw a blanket over the cold frame for extra insulation.) According to garden author Eliot Coleman, the temperature inside a cold frame can be up between 7 and 20 degrees warmer at night than the temperature outside.
If cold frames help extend the garden season and are so easy to make, why don’t more gardeners use them? One reason may be that you really do have to keep an eye on the cold frame and the weather. If it gets suddenly warm and sunny and the lid is down, your plants could be in tropical conditions very quickly! Also, you don’t want to forget to shut the cold frame on a cool night. But if you want to harvest lettuce early and be one of the first people on the block to pick a tomato, a cold frame is a great vegetable garden investment.
Do you use a cold frame? How have you set it up?
Follow Notes from Northern Gardener throughout January as we offer our series on 31 Days to a Great Northern Vegetable Garden. Tomorrow: Frost Free Dates and What That Means.