Build a Ventilated Cold Frame For Winter Vegetables

This spin on a homemade cold frame is ideal for winter lettuce, carrots, beets and more. Learn how to build a cold frame and more about the benefits.
By Julia Todd Forbes
January/February 1982
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A partial blueprint of Forbes' creation.

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Build a Cold Frame For Vegetables 

Well, my needs and abilities finally got together, and the result was an 18-inch-high frame — built of 1-by-4s — that fit exactly over one of my raised vegetable patches. Its sides were enclosed in 10-mil, clear plastic (polyvinyl chloride), which was secured with tacks ("washers" made from folded bits of plastic trimmings prevent the wind from enlarging the tack holes).

Next, I cut 4-inch-wide strips of the same plastic, long enough to stretch across the width of the top and come down 4 inches on each side. Again using homemade washers, I tacked the strips in place — spacing them 1 inch apart — all along the frame's top. Eureka! My frame was born . . . and it worked!

The sun warmed up the soil and the atmosphere within the structure by a full 20 degrees Fahrenheit. And, while the frame protected the plants from the wind, it also provided excellent ventilation. Rain fell readily through the spaces between the strips and I could easily slip my hands through the slots to harvest, transplant and weed.

I hate to brag, but my invention proved to be such a darned good way to grow things that I decided to use the frames throughout my garden. For four winters now I've enjoyed lettuce, freshly picked carrots, beets, parsnips, chard and parsley!


Limit Transpiration For Better Lettuce


Even my brainchild's one possible disadvantage turned out to be an asset: I knew the plastic was bound to reduce the amount of sunshine that reached the plants and less light (I had thought) would decrease photosynthesis. Well, as it happened, the diminished light was particularly good for lettuce! It seems that moderate shading tends to limit transpiration (the loss of moisture) more than it reduces photosynthesis. Therefore, the leaves became longer, broader, thinner, tenderer and smaller-veined . . . precisely the characteristics I wanted in my lettuce.

But the assets didn't stop there! I discovered that the grow-frame also works well in summer, so removing and storing it for half the year is unnecessary. The four-season capability of the mini-hothouse was revealed when, suddenly, it was midsummer and my plants — still in their winter home — were not only growing like gangbusters, but needing less water than did those in open beds.

In fact, the only time I have to water inside the frame is when I transplant seedlings. As a result, the surface soil stays dry, while the earth beneath this layer is always moist. Furthermore, the interior of the frame is never too humid, so that nasty "brown rot" can't attack my lettuce.

In addition, when the wind flutters the plastic strips, it actually sets up afield of static electricity which, some say, stimulates plant growth. True or not, I've certainly raised some happy-looking vegetables!

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Carolyn Raiford
10/22/2012 6:54:02 AM
What area does the author live in? I don't think the slotted plastic would do well in our Salt Lake snowfalls. But then I am a novice gardener and would love to find out otherwise.

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