Cold Frame Gardeningcomments (22) August 13th, 2009
Like most vegetable gardeners, I’ve always been interested in extending the harvest beyond the confines of “the growing season.” Along with the prolonged enjoyment of puttering in the garden, I treasure the reward of continuing to eat fresh, home-grown food. The easiest and most economical way to do this is with a cold frame.
If, like me, you’re not ready for the gardening year to end with the season’s first hard frost, then maybe you’re ready for a cold frame, too. This simple bottomless box with a removable glass or plastic lid protects plants inside from excessively low temperatures, wind, snow, and rain. In doing so, it creates a microclimate that is a zone and a half warmer than your garden. My garden may be in Maine, but the plants in my cold frame think they’re in New Jersey. A cold frame in New Jersey provides Georgia weather. The result is a harvest of fresh vegetables all winter long.
|Why all this talk about winter crops when you’re still waiting for the first tomato?
Because you can’t wait until winter to plant the winter garden. The rate that plants grow diminishes with the shortening days of fall until it almost stops. By then, the plants need to have reached harvestable size. After that, they’ll hibernate successfully in the shelter of the cold frame.
I start sowing seeds in my cold frame mid-July (see A cold frame timetable), but that's because I live in Maine. You'll need to adjust your planting schedule for your local climate.
Bottomless box with a skylight
The beauty of a cold frame is that it’s simple. Mine would have been familiar to a gardener a hundred years ago. Each one is a bottomless box made of 2-in. thick planks, 12 in. high at the back and 8 in. high at the front, and covered with glass frames, called lights.
|Download plans, a materials list, and instructions for building this cold frame.|
If you have access to old storm windows, you can use those as lights. If you don’t want to build your own frame, however, you can buy a sturdy ready-made polycarbonate-glazed cold frame for a few hundred dollars (companies that sell cold frames include Charley's Greenhouse & Garden and Peaceful Valley Farm & Garden Supply).
Grow cold-tolerant crops
One of the keys to success is to focus on vegetables that thrive in, or at least tolerate, the cold. No, a cold frame isn’t going to put vine-ripened tomatoes on your table in January. But it will easily provide you with the best carrots you’ve ever tasted, firm-fleshed leeks and scallions, succulent cooking greens, and a host of salad ingredients.
Most of the popular vegetables Americans grow are “chilling sensitive,” which is to say they don’t appreciate temperatures below 50°F. There are, however, many “chilling-resistant” crops, vegetables that survive winter’s freezes, providing they have some protection.
Spinach is one of the best such crops. It yields all winter in the cold frame. So does chard. Scallions are good, too—even better from the cold frame than from the outdoor garden. Carrots are outstanding. I make a big sowing on August 1 and enjoy delicious, tender baby carrots all winter. A thick layer of straw applied in late fall protects them from temperature swings. The cold turns some of the carrots’ starch to sugar—my kids liked them so much they used to call them “candy carrots.”
A cold frame is a bonanza for salad lovers. Lettuce is available until about mid-December, when it finally succumbs to repeated freeze-thaw cycles. That doesn’t bother me, though, because there’s a true winter salad green for cold-frame growing, and that’s mâche, also known as corn salad. This traditional midwinter European salad crop is far hardier than lettuce. I’ve found that mâche can be harvested while frozen and still look beautiful after it thaws (scallions also can be harvested frozen). The others need to be picked unfrozen. But that is really no problem because the interior of the cold frame gets above freezing on most winter days, even when it’s cloudy.
After learning about mâche, I began looking to Europe for more winter salad ingredients. Frisée endive proves quite hardy, as does radicchio. But my three favorites are cultivated weeds. One is claytonia, a California plant also known as “miner’s lettuce” because it was eaten in Gold Rush days. The round, succulent leaves on thin stems have a fresh, sweet taste and are second in hardiness to mâche. The Europeans imported claytonia and domesticated it to what they call “winter purslane.”
|Salad greens thrive in a cold frame. Two of the author's favorites are claytonia, which has heart-shaped leaves (lower left), and minutina, the grassy-looking row down the center.||One sowing of spinach yields from falll until spring when grown in a cold frame. Harvest it leaf by leaf, cutting 1 in. above the crown.
|Instead of beaing locked in frozen earth, cold-frame leeks are a cinch to harvest, and more of their flesh remains edible. Photo: Barbara Damrosch.||One of the best cold-frame crops is mâche, a European salad green. Harvest and eat the entire plant, which has a mildly nutty flavor. Photo: Barbara Damrosch.|
How many varieties you’ll be able to grow in your cold frame depends on where you live and the severity of the weather. For gardeners in Zone 6 and south, a cold frame will guarantee bounteous harvests. In the frigid winters of Zone 3 there are only five crops—spinach, scallions, mâche, claytonia, and carrots—that you can dependably harvest all winter, and only mâche during the coldest periods. If you want to increase the number of things you can harvest, erect a plastic tunnel over the cold frame, which will make a quantum leap in protection and crop variety.
Managing the cold frame
On a sunny day, a cold frame can quickly become a hot frame. You don’t want to be saving your crops from winter, only to have them cook before you can harvest them. A cold frame can overheat, even on a cloudy day. Keep the temperature below 60°F during the day by opening the frames a little. I can vent mine manually, because I work from my home. I recommend installing temperature-activated ventilating arms to look after this task for you. I like the Univent control from Charley’s Greenhouse & Garden because it’s the only one I know of with a quick-release feature that allows you to open the light all the way. It’s always better to err on the side of overventing. If you’re going to be gone all day, or if you’re unsure about the weather, vent.
|Vent the frame to prevent overheating. Heavy glass lights must be vented manually. If you use a lilghtweight glass substitute, you can install an automatic device to do this job for you.|
|Set up a min/max thermometer inside the cold frame and shield it from direct sun with a board.|
I’ll leave a new snowfall on the frames for a few days as insulation if it’s bitter cold. Heavy wet snow, though (6 in. or more), could break glass panes. I’ve found I’m less liable to break them myself if I remove the snow with a broom instead of a shovel. Once, when I was away, my frames spent three weeks under snow, and the plants didn’t suffer.
Occasionally people tell me that by fall they want a break from gardening. But that’s the best part: You can have your break and eat it too. After early fall, very little gardening takes place. Just some watering until November and then no more until spring. Venting can be handled automatically. I’ve found no weeds or pests in my cold frames. Harvesting is the main task, which is the aim of all this activity anyway.
And how good is the harvest? I can best describe it as a season of continuous delight for the table as well as for the soul. Even after all the years I’ve been growing in cold frames, I still marvel at the contrast of the weather outside and the bounty within. I still find myself coming into the kitchen with a basket of fresh greens for the evening salad and exclaiming, “You wouldn’t believe what’s available out there!”
“Yes, we would. We’ve heard it before,” say the eager eaters. “Please stop enthusing and make the vinaigrette.”
Read more about cold frames...
by Eliot Coleman
from issue #4
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posted in: Projects, greens, fall garden, cold frame