Growing Vegetables

in Winter   


    It’s easy to find locally grown fresh produce in summer, but come winter what’s a salad-hungry Granite-stater to do? If you’re a gardener, the answer is easy - grow your own winter hardy vegetables.  While tomatoes and corn won’t flourish through a New Hampshire winter, there are dozens of crops that will, including lettuce, broccoli, kale, chard, collards, spinach, turnips, carrots, beets and parsley. All it takes is some low-tech protection from the cold and careful planning.

Many New Englanders already use cold-frames to extend the growing season. The photo to the left was taken in the early 1950s - it was built on the south-side of our house by my in-laws out of cast-off windows. The  bottomless boxes are usually built with one side higher than the other so that the top is slanted. It is then covered with a hinged window of glass or plastic. The box is stashed in a warm spot in the garden (next to a south-facing wall is a favorite location) and the plants are grown inside the box.

    Cold frames can be great for small crops like baby lettuce, but you have to watch them like a hawk. Leave the top closed on a sunny day, and you’ll fry the seedlings. Forget to close it at night, and you’ll have a salad slushie in the morning. Luckily, garden supply companies sell temperature triggered automatic openers that can keep an eye on your veggies while you’re at work. Still, even the most tricked-out cold frame is no match for a killing frost; when nighttime temperatures regularly fall below 24 degrees, cold frame veggies either die or go dormant.

    However, if your cold-frame is inside a bigger cold frame, you should be able to harvest fresh vegetables all winter. This technique has been perfected by Maine market-gardener Eliot Coleman and is the mainstay of his profitable vegetable business. Coleman’s latest book, “The Winter Harvest Handbook: Year-Round Vegetable Production Using Deep-Organic Techniques and Unheated Greenhouses,” is a must-read for anyone interested in harvesting salad greens and other vegetables when the snow is flying. 

    Eliot Coleman spent years studying the techniques of intensive market growers in France, the so-called maraîchers, farmers who tilled tiny plots within the city limits of Paris during the nineteenth century. Using the city’s abundant horse manure as both a fertilizer and a source of warmth for tender fruits vegetables, the maraîchers not only fed their fellow-citizens year-round, but exported high-quality produce all over Europe. 

    Eliot Coleman still hews to the maraîchers’ basic tenets - keep soil fertility high using organic materials like manure; plant intensively and rotate crops; figure out what varieties grow best in your conditions; and protect crops from the cold to extend the season. Coleman, however, has the added advantage of modern materials and technology, such as special fabrics that keep plants warm while letting in the light, plastic-covered greenhouses and drip irrigation.

    He estimates that every layer of protection a farmer gives his vegetables is akin to moving the garden 500 miles south.  By growing his winter crops inside an unheated plastic-covered greenhouse, and covering them with spun-poly fabric when the temperatures drop below freezing, he is able to create a Zone 8 microclimate on his Zone 5 farm.

    Cold isn’t the only problem for northern farmers, though. Light (or the lack of it) is another limiting factor for winter growers, but it’s not a deal-breaker. You might be surprised to learn that Concord, New Hampshire sits at a latitude of 43.208 degrees - about the same as the sunny Mediterranean city of Marseilles. Even with a winter cloud cover, we get enough light in New Hampshire to grow dozens of vegetables right through the darkest part of the year.

    As long as daylight lasts ten or more hours, most cold-hardy plants (with some protection from extreme weather) will continue growing.  If such plants are well-established by the time days grow short, they will fall into a dormant state during the dark days of winter, not really growing but not dying either, ready to be picked whenever you like.

    Around here, the big drop-off in light happens between October and November, when there’s a 27% reduction in day-length. November, December and January all have around 9 hours of light a day. In February, day-length jumps back up by 25% and finally tops ten hours.

    What this means is that with proper protection, vegetables will grow (albeit slowly) right into November. Those that aren’t harvested for Thanksgiving and Christmas feasts, if they have had sufficient time to grow before November, will go dormant for the three darkest months and then spring back to life and growth in February. Even plants that have been killed to the ground by frost will often regrow in February, and seeds planted in midwinter, if watered and protected, will sprout in the growing light of late winter.

    Greenhouses, though, are relatively expensive and take up a great deal of space, which is why in recent years New England farmers have embraced “high tunnels.” High tunnels look
something like miniature hoop-houses. They’re constructed of 10 foot lengths of 1/2 inch electrical conduit bent into a hoops about 5 feet across. The ends of the hoops are inserted 5 or 6 inches into the soil; the hoops are spaced about 5 feet apart so that they form the skeleton of a tunnel.

    The garden bed beneath the hoops is planted either with seeds or seedlings and then the hoops are covered with light spun-poly fabric that’s held in place with sandbags or rocks. Some of these crops, like fast-growing mesclun, will be harvested before winter really sets in. Others, such as onions, are timed to get big enough during the fall growing season that they can survive winter dormancy, reemerging to produce super-early crops in the spring.

    High hoops can also be used to protect already established crops that are frost-tender, like peppers and tomatoes, as well as to extend the season for well-established cold-hardy crops like carrots and lettuce when an early freeze threatens. They are also invaluable in spring to protect crops from cold weather and pests like flea beetles.

    For crops intended to overwinter and resume growing in spring, an additional layer of plastic is added over the spun-poly fabric in late November or early December, when the weather gets to be too much for the delicate fabric. The plastic, when pulled taught and firmly weighted at both ends of the hoops, should stand up to snow and wind all winter. When spring arrives, the plastic is lifted to vent the tunnel and removed altogether when the weather settles.

    The trickiest parts of four-season growing are choosing the right varieties and timing their planting. Though certain kinds of vegetables are regarded as cold-hardy, broccoli and lettuce for instance, not all varieties of broccoli and lettuce are ideal for winter production. Seed catalogues geared toward northern growers, like Johnny’s and Fedco, are full of good advice about which varieties do best during cold, short days, as is Eliot Coleman’s book (click here for a list of cold-hardy vegetables and recommended varieties).

    Keep in mind that certain kinds of vegetables are sensitive to day length and you’ll need to seek out varieties that don’t mind short days. Many kinds of onions, for instance, will only form bulbs when days are long and nights are short and these varieties won’t be happy in a winter greenhouse. However, the short-day varieties of onions usually grown in the South, such as Walla Walla Sweet and Vidalia, planted in fall will overwinter in high hoop tunnels to produce a very early onion crop in spring.

    When to plant is the other difficult part of the equation. Seed catalogues and packets usually show the number of days it takes from seeding (or in some cases, transplanting) to maturity. For example, Black Seeded Simpson, a cold-hardy lettuce, is, on average, ready to harvest 42 days after seeding. In fall, however, when the weather is colder and days are growing shorter, it could take a week or two longer to get a mature head of lettuce.


     To be on the safe side, add twelve to the number of days to maturity on the seed packet. For outdoor seeding, use this number to count back from the first expected frost in your area to find a planting date. In Concord, there’s a 50% chance we’ll see a frost by October 1. So if we figure Black Seeded Simpson lettuce planted as a fall crop takes 54 days to maturity, that means we should seed it in the garden by the beginning of August - if it is not going to be protected in any way.

    Black Seeded Simpson, though, as well as many other varieties of lettuce, can take temperatures well below freezing and bounce back again, especially if it has the protection of a cold-frame, greenhouse or high tunnel. This means lettuce can be seeded as late as September for fall and winter crops that will be protected. In fact, lettuce started inside and transplanted as well-established seedlings can be transferred to a high hoop or greenhouse as late as October.

    Keep in mind that you want transplants to be big enough to be well-established when the cold and dark set in, but if they’re too big they may tend to bolt or be frost sensitive. Even Eliot Coleman with his years of experience says he’s still experimenting with varieties and planting schedules. 

    Plan on plenty of experimentation and some failures along the way, but don’t give up for the rewards are well worth the effort. Last year for the first time we harvested fresh food from our unheated greenhouse during every month of the year. There’s nothing that cures the winter blues faster than a homegrown salad in January.