Cold Frame Gardening

comments (6) August 13th, 2009

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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.

Minutina, a relative of the plantain weed, is popular in Italy. Minutina has slender, lightly fringed leaves that have a salty taste and crunchy texture. And in the chicory family I found ‘Biondissima Trieste’, a non-heading type which can be planted thickly to yield abundant quantities of delicious, tender, round to finely indented leaves. Arugula, dandelion, escarole, mizuna, parsley, sorrel, and mustard and turnip greens are also chilling resistant.

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.

A cold frame timetable

Probably the most important point in using a cold frame is to start your plants early enough. I begin in mid-July, sowing seeds for slow-growing or heat-tolerant crops like scallions, chard, parsley, and for winter greens such as escarole, endive, dandelion, and radicchio. On August 1, I sow my entire winter carrot crop. By mid-August, I’m seeding mustard and turnip greens. And in September I sow all the salad greens that germinate in cooler soil—mâche, spinach, claytonia, arugula, and mizuna, as well as radishes. From time to time, spaces open up where a crop finally gives in to winter, or where I’ve harvested whole plants such as mâche. I sow seeds for new crops in these holes. Arugula, mâche, spinach, claytonia, radishes, and lettuce will germinate during the winter. If they fail, I try again. As the sun climbs higher in late winter, these seedlings are poised for rapid growth, and about the time my earlier crops are finished, the winter-sown ones are ready for harvest.

Sown the first of August, these carrots are eating size by the time the snow flies. Protected by a layer of straw, they'll stay crisp and sweet all winter.

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posted in: Projects, fall garden, greens, cold frame

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