2007-05-03 / News

Extend your growing season with a cold frame

They are easy to use and simple to build
By Roger Marshall

Psst, want some lettuce for your Thanksgiving table? Or broccoli when there's snow outside. Or maybe you want to grow some herbs to spice up the offerings at your table. If you don't want to grow vegetables or herbs, maybe you want to propagate some flowers to increase your supply next season. You can do all of the above with a cold frame and it is fairly easy to make.

First, you need three or four old storm windows. Look around for a home rebuilding project in the neighborhood and ask for the old storm windows. Chances are you'll be offered more than you need. Most old storm windows are three to four feet wide so adapt the plans shown here to the number and width of your windows. When making your cold frame, build it as large as possible. A large frame heats and cools more slowly than does a small one, plus you'll have more space for plants. As soon as you have used a cold frame, you'll be looking for ways to expand it.

If you want to start with a temporary frame, simply place a halfdozen straw bales around the area where you plan on having the frame. Your windows can be laid over the top of the bales to cover the open space between them. The open space between the bales becomes your growing bed.

Once you have decided on the size of your cold frame, you'll need to mark out the area and hammer or dig four corner posts into the ground. The posts should stand higher than the cold frame base so that the sides of the plywood cladding can be nailed or screwed to them. To these posts you will first nail or screw the bottom of the cold frame. Before you nail anything together, check the diagonals to make sure that your frame is square. If it is out of alignment you may end up with windows set out of square and cracks in the frame that can let cold air in. I do not recommend pressure treated wood for vegetable beds even though most authorities say that it is safe. That means your bottom wood frame will last about ten years before it will need replacing. If you want to extend its life, you can coat it all over with epoxy to keep moisture and rot spores out. I have no idea how long it will last after it has been epoxy coated.

With the frame in place, cut the plywood to size and nail or screw it into place. Now set your windows over the top of the wooden frame you've just made. I like to have hinges on the windows at the top to help stop the windows from sliding down the frame or blowing off completely in a wind storm. If you have really heavy windows, don't make the frame so the windows can be lifted, make it so that the back can be lowered. That way you'll get access through the back of the cold frame instead of through the windows.

When hinging the windows try to install the hinges so that the windows can be lifted completely over the back of the frame. That way if rain is forecast, you can open the frame and let it rain. If the windows only lift up three or four feet, you'll have to water the cold frame all the time.

If you want to remove the windows completely, install a clasp or some form of hook at the top and bottom of each window so that it can be undone and removed, but will not blow off in a wind storm. In summer, there is little need for a cold frame unless it is to keep deer out or to grow pumpkins, and removing the windows converts it into an ordinary growing bed, but one with the advantage that it can be closed in as colder weather comes.

You may also find that there are cracks between the windows that let cold air into the cold frame. The solution is to screw a 1/4-inch by 3/4- inch batten to the edges of alternate windows. If you put the wood on alternate windows you will learn to lift those windows first, and when they are covering the frame there will be no gaps between each window.

Cold frames then, are an essential part of the master gardener's repertoire. They prolong the season and keep animals away from your precious crops. In my cold frames I have harvested basil and parsley around Thanksgiving, broccoli in January, and started seedlings in early April. You can too.

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