Thanks to the growing interest in diverse local foods, customers who visit farmers’ markets are purchasing fresh vegetables well past the usual dates for summer favorites such as sweet corn, tomatoes and melons. With some planning, growers can offer a variety of interesting cold-hardy vegetables through on-farm markets, fall/winter CSAs and year-round farmers’ markets.
There are four ways in which a grower might fit cold-hardy vegetables into a year-round growing scheme: cool weather spring and/or fall crops that are harvested prior to severe cold weather; crops that are kept alive as far into winter as possible; crops that are hardy through winter; and crops that are planted in fall then overwintered and harvested in early spring. Each situation is dependent on location, grower’s needs, potential market, labor availability and resources such as high tunnels and row covers.
Why do some species thrive in cold temperatures? Nathan Johanning, extension educator, University of Illinois explains how some plants manage to survive the freeze/thaw cycle that occurs throughout the winter months. “Multiple freezing and thawing causes the cells to break because of the expansion of water inside the cells,” he said, describing the process that occurs in non cold-hardy plants. “The water crystals are very sharp, and once they’re frozen, the cell is disrupted and the plant succumbs.”
Cold-tolerant species can withstand frequent freeze/thaw cycles because these plants have lower water content during the coldest periods of the year. While water levels are lower, soluble sugars and salts are increased in the vascular system; which Johanning compares to the way in which salt can be used to melt ice. Plant cells that have a higher salt and sugar concentration will withstand colder temperatures, even below 32 degrees.
Heat and light also play a role in winter and early spring growing. “Heat comes from both solar radiation and also from the earth’s ambient temperature,” said Johanning. “Plants need sunlight, and they also need heat. The earth traps and stores heat. During summer, when there is excessive light and heat, management is aimed at keeping plants cool. In fall and winter, sunlight and heat are limited and must be captured and used efficiently to maintain a growing environment. The angle of the sun (which influences day length) and intensity of light in winter also affect plant growth.”
The term daily light integral (DLI) describes the total quantity of light received in a given area in a day. DLI is expressed as moles (particles of light) per square meter per day. When the DLI drops below 10 for the day, plant growth slows down considerably. “Remember that things such as row covers or high tunnel also reduce light,” said Johanning. “You’re already in a season where there is less light and lower light intensity, and the structures we use to protect plants also limit light. We have to work to maximize the efficiency of any light we have.”
When temperatures are around freezing, cool-season crops show little or no growth, even though the plants are still alive. However, plants respond to even slight increases in temperature, which is why it’s worth using whatever methods are available to maximize temperature. Johanning says that soil temperature is a good indicator of the temperature that directly affects the plant. “If the soil is dry, it is likely to be just as cold as the air temperature above the soil,” he said. “If the soil is moist, the water acts as an insulator and the system has a greater mass, so it takes more energy to change the temperature.”
Moist soil will drop to around freezing, but it will stay at that temperature and resist some of that change. “As water freezes, there is some heat released, and that keeps the temperature below 32 degrees as long as the freezing process is active,” said Johanning, adding that soil will remain above freezing for several days or nights even when there is a drastic drop in temperature.
Row covers create an igloo effect — a microclimate — and can provide up to eight degrees of temperature protection. Overall weather conditions affect this temperature range, but in most cases, it’s fairly consistent. In most situations, row covers must be managed daily. Soil should remain exposed and not covered with plastic. On sunny days, row covers should be pulled back to expose the plants and soil to sunlight. It also heats the environment. As sunlight decreases, but before it gets cold, pull the row covers back over the plants. In cold conditions, two layers may be appropriate.
Growers interested in cold-hardy crops should be aware of climate influences in their area such as large bodies of water, mountains and air currents. Winter precipitation, both type and amount, are also factors that influence crop success. Monitoring winter temperatures and weather conditions will help the grower determine patterns. Growers should also be aware of microclimates on the farm, some of which may be more or less conducive to cold-hardy vegetable crops.
What are the best cold-hardy crops to plant? The challenge is to select vegetables that are both hardy and marketable. A crop of turnips or kale won’t be of any value if customers aren’t willing to purchase them. An early-season customer survey is a good way to determine whether it’s worth establishing cold-hardy crops. Be prepared to help customers become familiar with vegetables that they may not have purchased in the past by providing cooking tips and storage recommendations.
A good seed catalog should provide information on varieties that thrive in cool temperatures. Become familiar with the minimum temperatures that various species will tolerate. Cabbage, broccoli, cauliflower, Napa cabbage, endive, mizuna, green onions and bok choy can survive temperatures as low as 25 degrees, while arugula and tatsoi can withstand just a few degrees colder at 22. Beets, celeriac, mustard greens, Italian parsley, radishes and turnips can handle 20 degrees, while Russian kale, cilantro and some lettuces can withstand a dip to 15 degrees. Chard, carrots, collards, daikon, rutabaga, leeks and large-leaf spinach tolerate 10 degrees, and chives, mache, parsnips, salsify, Jerusalem artichoke and some spinach varieties survive down to zero.
The seed catalog should also provide information on which varieties are best for fresh eating and which are best for storage. This is an important consideration for marketing, and can potentially boost sales if customers understand how to store certain cold-hardy vegetables at home.
Seeds should be selected based on cold hardiness, and for certain species, like broccoli, consider summer bearing qualities as well. Growers should experiment with varieties and keep careful notes as to which varieties perform the best.
Make notes on weather conditions and variety performance as the season progresses so that you’ll be better equipped to select the appropriate seeds for the following growing season. It’s also helpful to take photos throughout the season to enhance your notes. “If there are five varieties that are cold-tolerant, try all five in a small area,” said Johanning. “See what works best for you. There are a lot of regional differences in environment.”
Establish succession plantings and at a high enough volume that there is enough growth to maintain sales through the slow regrowth period. Keep in mind that plant growth is slowest through early February. “There’ll be a long period where you don’t get a lot of regrowth in plants like lettuces and other greens,” said Johanning. “Make sure you plant a large enough volume so you don’t completely run out of the crop.”