Once you’ve gotten used to fresh, homegrown food, it’s a terrible shock (financially as well as flavor-wise) to go back to buying it—even if your ambitions, like mine, stretch no further than salad fixings, herbs, and handfuls of kale for the soup pot. In warm climates, growing crops outside for much of the year is relatively straightforward, but in colder areas, things get a bit more complicated.
If you have the room and aesthetic tolerance for a plastic-wrapped structure in the backyard, building a modular greenhouse, à la organic gardening authorities Eliot Coleman and Barbara Damrosch, is well within the capability and budget of most DIYers. But there are smaller-scale devices and strategies that work on the same principle—capturing the sun’s warmth by day and radiating it back at night—that are even easier to implement.
In The Garden Primer, Damrosch recommends that unless you live in a frost-free climate, you should have some form of protection handy in case an unexpectedly early fall cold snap threatens to do in the tomatoes and basil. A big sheet of clear plastic or even a tarp or old blanket will work—anything you can throw over a large number of plants and weight at the sides with stones if necessary. “On a still night no weighting is needed,” she adds. “In fact, it’s usually on still, clear nights that frosts occur.” Got it. She’s also had good luck with a so-called floating row cover, basically a garden blanket made of spun-bonded polyester. One popular brand is Reemay (available at numerous online sources), which allows 75 percent light transmission and provides frost protection down to 30°F.
To protect small individual plants in the ground, Damrosch uses plastic gallon jugs with the bottoms cut out—a practical if inelegant riff on traditional garden cloches. Either stick the plastic jugs into the soil around each plant, “or partially cut the bottom to make a flap, which you can anchor with a stone. If you leave the jug on during the day, be sure to unscrew the top in case it gets hot.”
For sheer versatility and dependability as a season extender, however, the cold frame wins, hands down. A cold frame is usually a bottomless wood box with an easy-open (for harvesting), adjustable (for ventilation) top made from a repurposed window, shower door, or tempered-glass patio door, but it can also be jury-rigged from straw bales, stacked bricks, or concrete blocks covered with plastic or translucent corrugated fiberglass. A cold frame should be positioned so that its transparent top is angled toward the sun; the ideal spot is next to a south-facing wall of a building, in order to maximize every bit of warmth and wind protection. For centuries, kitchen gardeners have relied on cold frames not just for raising cold-weather food crops, but also for overwintering ornamentals and herbs in pots and hardening off seedlings in the spring.
You’ll find design plans and detailed how-to info on cold frames on any number of websites and in books such as Eliot Coleman’s Four Season Harvest and The Winter Harvest Handbook, both of which make great bedtime reading. His August 2009 piece for Vegetable Gardener magazine is full of sterling advice too; in particular, he gives a shout-out to a couple of gardening supply houses that sell ready-made cold frames, Charley’s Greenhouse & Garden ($89.95) and Peaceful Valley (from $149).
Aside from planting the right crops in a cold frame (I’ll get to that in a sec), the most important thing to remember is temperature control. The soil inside a closed cold frame heats up much faster than it does out in the open, and you don’t want to cook those vegetables before their time. “Keep the temperature below 60°F during the day by opening the frames a little,” Coleman writes in Vegetable Gardener, and although you can easily vent a cold frame manually by propping open the top, a temperature-activated ventilating arm is mighty handy. He likes the solar-powered Univent control ($49.95) from Charley’s Greenhouse & Garden because it allows you to open the top all the way.
Choosing the right crops for a cold frame is key. Among the vegetables that flourish in cold weather are arugula, brussels sprouts, carrots, chard, kale, leeks, mâche, radishes, and spinach. Check out the helpful fall and winter vegetable planting guide at Ed Hume Seeds for more details.
Equally important is sowing time. Plant carrots and leeks right now for a winter harvest; they’ll be mostly grown by the time the cold weather hits and will keep in the soil all winter long, getting sweeter and more delicious with each frost. Coleman lives in Maine, and by mid-August, he’s sowing mustard and turnip greens. In September, he sows 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,” he writes, “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.” It all sounds delicious.