Cold Frames: Gardening in Colorado throughout Winter

wintergarden 008aEliot Coleman, a market grower in Maine, has made a name for himself by extending the growing season with cold frames. Curly endive, mâche, spinach and radicchio leaf out from month to month as he harvests for gourmet East Coast restaurants. Is it possible that Colorado could support the same crops in midwinter that a gardener in chilly New England is able to sustain?

"There’s no reason why we can’t grow greens throughout the year," says Longmont farmer Marilyn Zabielski, who relies upon the Eliot Coleman style with a large hoop house cold frame on her property. Marilyn grows heirloom tomatoes, salad greens and unusual cucumbers—anything that strikes her fancy to provide quality fare from her Sunny Hill Salad Gardens. Although she doesn’t supply salad greens in the winter to anyone outside her own family, the large hoop house allows her to begin gardening months before the outside air and soil temperature is hospitable to many vegetables.

"I’ve been playing with this for years," says David Whiting, Colorado State University Extension consumer horticulturalist specialist, who also oversees the Master Gardener program. "Some years it works superbly well. Then, in other years, it doesn’t."

coldframeKen053Ken Singer, an avid gardener when it comes to roses and fruit trees, also tinkers with a simple tent cold frame in his Colorado backyard. Inside is a thermometer that measures the highs and lows within the tent. He has built a raised bed situated on a concrete slab alongside his house. "It’s zone 7 there," he explains, about the south facing area. "I can grow plants like gladiolas."

Like so many Colorado gardens, Ken’s is filled with microclimates. He grows baby arugula, spinach and radishes in his cold frame. "Lettuces have never been successful for me," he says with chagrin. But the list of salad greens now available from seed makes lettuce less the backbone of a good salad than ever. Now you can find seed packets of greens like mâche, a European salad staple, as well as curly endive.

Trying to find accurate information to grow salad greens in a cold frame is tough to come by. The concept has not spread widely enough to garner much attention. Most of the information is anecdotal. While Ken hopes that his raised bed warms up quickly, Dave Humphrey keeps his cold frame—a nearly identical tent to Ken’s—firmly on the ground. "I think that the ground will hold more heat in the winter than a raised bed," he says, "although in the spring a raised bed might help a little more."

coldframearugula055Dave is setting up a plastic cold frame on a small patch in his backyard. Although his bed is directly on the ground, he mounds earth around to prevent heat loss and anchor it securely. Like Ken, he’s been most successful with arugula. "And the prices in the stores!" he laments, makes it worthwhile to grow his favorite salad green.

Only David has tended a winter vegetable garden year after year under trial conditions so that he firmly has a grasp on what’s required. His garden is slightly raised—only to prevent him from stepping on and compacting the garden soil. He has constructed a hoop structure from sidewalk reinforcing mesh, covered with clear plastic that is clamped in place. The clamps allow him to open the plastic walls, even slightly, as the sun warms the plastic. He also keeps a thermometer inside that tells him the highs and lows of the interior.

"I’ve tried about everything," he says, "Generally, I start seeds indoors under lights and a heat mat. But most people don’t have that. Then, I’ll transfer them into the garden. Any of the cool weather crops will be easy. I stay with the most traditional: spinach, lettuce, broccoli, chard. My beds are not too high—a 2-by-4, 2-by-8, 2-by-6, because I don’t want the heat loss of a really high bed. The covering has to open in the heat of the day, at least a crack, unless I have full cloud covering all day long. The last time I was measuring heat, the interior reached 140 degrees when not opened."

David doesn’t mulch inside the cold frame because he wants the soil to warm up in the daytime. But there’s one exception. Sometimes he sows spinach and lettuce seeds inside, mulches with straw or leaf compost and opens the plastic cover just a crack. The greens will germinate, set down roots, but not truly grow until around February. Suddenly as the light lengthens, they’ll grow vigorously and by mid-May, he’s harvesting mature leaves.

By mid-October and November, light dwindles. He’s found that germination may take place with cool crops, but they simply won’t grow. Instead, they’ll wait patiently for the day to lengthen. But if he has planted in late August or early September, he can extend the growing season well into mid-winter. The plants have already germinated and grown vigorously. At that point, he’s keeping them from extended freezing in his cold frame and they will remain in a mature state for months.

coldramehoops020Some green and leafy plants enjoy cool, even cold temperatures. A good crop to begin with is spinach. If temperatures drop so steeply that the spinach won’t grow, the seeds or seedlings simply wait patiently for spring weather and will suddenly leap into action. Even without a cold frame, planting spinach seeds in the fall is one way that many Colorado gardeners get a jump on spring.

A few greens, like arugula, are sweeter and more flavorful when grown within cool temperatures. As soon as the soil heats up, arugula bolts and becomes bitter tasting. Ken's arugula may be only a couple inches high, but the flavor is notable. Kale, parsley, dill, spinach, broccoli, lettuces, radishes, mâche, curly endive, mizuna, and bok choy are candidates for winter gardens. Among lettuces, some are far more cold hardy than others. But most lettuces like temperatures between 50 and 60 degrees and may not germinate until the temperature reaches 60 degrees.

A choice worth considering is a mesclun mix of chicories. Perhaps the best known is radicchio, or red chicory. Chicory hails from the same family as lettuces. When harvested young, chicories are sturdier than lettuces in cold weather, but equally delicious. Most chicories are tender when young, but become tough and bitter as they age. You can find mesclun salad mixes that are entirely lettuces, or entirely chicories.

David has taken to tinkering with holiday lights and an aluminum space blanket when temperatures drop drastically. The lights are strung under the plastic and woven within the mesh. Both, he says, have helped. "I have tried putting an aluminum space blanket on the frame on cold nights and I have dropped to zero outside, but not had freezing inside the cold frame. The blanket has to come off every morning. I’ve tried C7 Christmas lights—the old kind we used to put on trees. I put them on a timer. It goes on at 5 in the afternoon and comes off at 8 in the morning. Using large single bulb lights, I found, creates one hot spot and the rest of the frame has cold spots. But by using the Christmas lights, I can add from six to eight degrees. It’s not so much how cold the frame gets, but how long it stays cold that matters."

coldframespinach014Along with a steep drop in temperatures, winds can wreak havoc on a cold frame. Both Ken and Dave agree that their biggest problem is anchoring a plastic tent when the Front Range winds pick up. Ken uses heavy nails hammered into the wooden timbers that make up his raised bed. Dave pins down his tent with spikes but also mounds dirt around the sides. Both their cold frames are a clear plastic stretched over a small tent frame, and not the reinforcing mesh that David prefers. It may be that winds along the Front Range make many of the commercially available cold frames less effective in our region than making a sturdier frame on your own.

David says he can make general predictions for plants, but there’s always a year when nothing seems to survive. "I’ve had lettuce and spinach go through the winter some years and not others. But I can add two to six weeks of the growing season to nearly any vegetable. Cool season vegetables are not frozen at 32 degrees and both lettuce and spinach can take a cold nip. Broccoli, too. Two things gardeners should know is that their soil can heat up too much in a cold frame and there is the danger of over watering. Most years, any one of the cool season crops is ideal."

Photos from top to bottom:

  • Ice forms on the seed heads of dill
  • Ken Singer surveys his arugula seedlings
  • Ken's arugula: tender, but promising for winter salads
  • Plastic is stretched over heavy wire mesh and placed over an existing vegetable bed (author's garden)
  • Spinach and dill will take a cold snap; duration rather than temperature drop is a deciding factor


Seeds for winter gardens:

  • Botanical Interests in Broomfield, Colorado, offers a variety of winter salad greens including mâche, endive and all the other more traditional favorites. They're at:
  • Sees West Garden Seeds in Albuquerque, New Mexico: open-pollinated, certified organic seeds, short-season vegetable seeds at:
  • Cook's Garden in Vermont, specializes in a great array of salad greens for home gardeners
  • John Scheepers Kitchen Garden Seeds in Connecticut offers a pack of winter vegetable seeds for gardeners at:
  • Le Jardin Du Gourmet in Vermont: European varieties of vegetables not readily available elsewhere, at:

Recommended reading:

  • "The New Organic Grower’s Four-Season Harvest," by Eliot Coleman, Chelsea Green, 1992. The essential book for extending the growing season with a cold frame.