We are amazed at how incredibly productive our hoop house is, infinitely better than a cold frame! The rate of growth of cold-weather crops is much faster inside; the crop quality, especially leafy greens, is superb. Plants can tolerate lower temperatures than outdoors, as they have the pleasant daytime conditions in which to recover. Salad greens in a hoop house can survive nights with outdoor lows of 14 degrees Fahrenheit (–10 degrees Celsius). Winter gardening in a hoophouse is much more pleasant than dealing with frozen row covers and hoops outdoors.
We grow mainly salad crops, cooking greens, turnips, radishes, scallions and bare root transplants for setting outdoors in February and March. We aim to harvest greens from the hoophouse after the outdoor crops slow down, and the turnips after the stored outdoor fall turnips have all been eaten, or as an occasional delectable alternative.
We reckon on two hours work each day in winter in our 96- by 30-foot-high tunnel. Aim to keep the temperature in the 65 degrees Fahrenheit – 80 degrees Fahrenheit (18 degrees Celsius – 27 degrees Celsius) range during the day, opening the windows and doors as needed. If the sun is shining we usually open the windows around 9 am and close them around 2:30 pm (a few hours before dark) to store some of the warmth. Even in cold weather, plants need fresh air! High-density cropping can really use up the carbon dioxide in a closed hoophouse very quickly. When this happens, photosynthesis crashes and plant growth becomes limited. Soil high in organic matter contains high levels of organisms that produce carbon dioxide, which helps. Dense plant canopies can trap this near soil level, where it is most useful.
Our main task each day is harvesting. In the winter of 2009–2010, we had frozen soil or snow on the ground outside for a month (very unusual in central Virginia). Despite this we were able to keep a hundred people in fresh salad and cooking greens (with turnips and scallions for variety) for the whole month. Aside from harvesting, jobs include planting new crops, clearing old ones, spreading compost, hoeing, hand weeding and supplying water as needed. We have drip irrigation. In the middle of winter, not much water is needed, and we try to only water when a relatively mild night is forecast.
To harvest in the darkest days of winter you’ll need a good supply of mature crops to take you through. What has already grown before this period will provide most of your harvests. When the daylight is shorter than 10 hours a day not much growth happens. In Central Virginia, latitude 38 degrees North, this period lasts from November 21 to January 21. Soil temperature also affects growth rate. For us, December 15 to February 15 is the slowest growing time. For most of the winter, our hoophouse plants are actively growing, not merely being stored for harvest (as happens in colder climate zones and outdoors), so we can continue sowing new hoophouse crops even in December.
Growing Greens And The Hazards Of Nitrate Accumulation
During periods of short daylight length, there is a health risk associated with nitrate accumulation in leafy greens. Nitrates are converted in the body into toxic nitrites, which reduce the blood’s capacity to carry oxygen. Also, nitrites can form carcinogenic nitrosamines. Plants make nitrates during the night, and convert them into leaf material during the day. It takes about six hours of sunlight to use up a night’s worth of nitrates. In winter, a small handful of leafy vegetables can exceed the acceptable daily intake level of nitrate for an adult, unless special efforts have been made to reduce the levels. Spinach, mustard greens and collards contain about twice as much as lettuce; radishes, kale and beets often have two and a half times as much. Turnip greens are especially high, at 3 times lettuce levels.
• Grow varieties best suited for winter.
• Avoid fertilizing with animal-based fertilizers; use organic compost.
• Ensure soil has enough Phosphorous, Potassium, Magnesium and Molybdenum.
• Water enough but not excessively.
• Provide fresh air as soon as temperatures reach 68 degrees Fahrenheit (20 degrees Celsius), so that carbon dioxide levels are high enough.
• Harvest after at least four (preferably six) hours of bright sunlight in winter.
• Avoid harvesting on very overcast days.
• Avoid over-mature crops and discard the outer leaves. Harvest crops a little under-mature.
• Refrigerate immediately after harvest, store harvested greens at temperatures close to freezing; use crops soon after harvest.
• Mix your salads; don’t just eat turnip greens.
• November: spinach, lettuce leaves, mizuna, arugula, beet greens, tatsoi and brassica mix for salad, radishes and scallions.
• From December: baby lettuce mix, Tokyo Bekana, Maruba Santoh, chard, kale and turnips. Kale grows whenever it is above 40 degrees Fahrenheit (5 degrees Celsius).
• January till mid-March; the bigger greens, including Senposai, pak choy, Chinese cabbage and Yukina Savoy
How to Harvest Winter Vegetables
Don’t harvest frozen crops - wait till they thaw. With fall-sown crops the aim is often to keep the same plants alive through the winter. November-January is not a good time to sow replacements.
With leafy vegetables, highest productivity is from “Cut and Come Again” crops: cut the tops of the plants above the growing point with scissors or shears every 10 to 35 days. Leaf-by-leaf is the method we use for kale, collards, chard and spinach. Never remove more than 40 percent of the total leaf area: less than half of the leaves, with a safety margin. We say “Leave eight for later.”
Once spinach plants start to look a bit past their peak, we “crew-cut” or buzz-cut them. Initially we harvest lettuce by the leaf, leaving the center to keep growing, and switch to harvesting the heads in late January, when growth begins to pick up. Whole plant harvesting works well for small plants like tatsoi and corn salad. A direct-seeded row can be thinned over time by harvesting out the biggest plants on each visit.
Pam Dawling is the garden manager at Twin Oaks Community in central Virginia. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available at Sustainable Market Farming. Pam blogs on her website and also on Sustainable Market Farming Facebook Page.
Photo by Twin Oaks Community, Wren Vile, Ethan Hirsh, Pam Dawling.
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