Most people wouldn’t normally associate the depths of the northeast winter with an abundance of newly harvested, locally grown produce. However, a trip to your farmers market this month will reveal a surprising array of fresh leafy greens and vibrantly colored vegetables populating farmstalls. The exploding interest in and demand for locally produced food over recent decades has fueled widespread advancements in traditional winter farming methods that extend the growing season of hardier plants even when temps dip below freezing, and snow and ice cover the ground. These new technologies and materials not only facilitate year-round crop production, they can also minimize or eliminate the need for chemical inputs including pesticides, insecticides and synthetic fertilizers. Read on to learn more about the widely used “protected agriculture” techniques that lie behind those bunches of sweet spinach, kale, bok choy, radishes and other seemingly incongruous items you can spot at the farmers market all winter long.
Greenhouses
Greenhouses are structures with walls and a roof made from transparent material that allows sunlight to penetrate inside. Legend has it that greenhouse growing was invented by Roman scientists and engineers circa 30 AD to satisfy Emperor Tiberius’s penchant for munching on a fresh cucumber every day. Rudimentary Roman greenhouses consisted of raised beds mounted on wheels overlaid with frames made from transparent glazed stone or mica to protect plants from winter conditions and allow for year-round cucumber production. Centuries later, greenhouse growing gained popularity in Europe after glass became widely manufactured and was introduced to the United States by Andrew Faneuil, a prosperous Boston merchant who built the first American greenhouse in 1737.
The transparent walls and roof of a greenhouse work by allowing all visible rays of sunlight to pass through the panels and trapping it as thermal energy. This process heats up the air inside to create a warm, regulated growing environment for the plants within, while the structure itself protects them from extreme conditions such as wind, heavy rain, frost and snow. When sunlight is not available to provide natural solar energy, such as at nighttime or on overcast winter days, commercial greenhouses are typically heated via gas, propane, or fuel oil. Modern smart greenhouses utilize new technology by connecting to computer systems that control lighting, temperature, moisture, humidity and ventilation to maintain optimal climatic growing conditions.
Traditionally, greenhouses have been made from glass panels mounted on an aluminum, steel or wood frame. In recent years, advances in twin-wall polycarbonate, a virtually unbreakable, thermally insulative polymer that can transmit plenty of light and withstand severe weather, have made it equal to or even better than glass in many ways. Even without supplemental heat, the durable design and superior warmth retention of these modern greenhouses allow local farmers to grow a diversity of crops throughout harsh northeast winters from cold-hardy leafy greens such as spinach, chard, collards, kale, and arugula to root vegetables including turnips, parsnips, carrots, and beets. Many winter-grown vegetables will benefit from a phenomenon known as cold-sweetening which happens when the naturally occurring sugars intensify in plants cultivated in chilly temperatures. So, it’s not your imagination when farmers market-bought spinach tastes so much sweeter in the winter than it does in other months.
Polytunnels
A polytunnel (also known as a polyhouse, hoop greenhouse or hoophouse, grow tunnel or high tunnel) is an elongated semi-circular, or sometimes square-shaped, structure typically made from an aluminum or steel frame covered in UV-protected polyethylene plastic film. As with greenhouses, the inside of a polytunnel heats up using radiation from the sun’s rays that filter through the opaque sheeting. Equipment to control the interior microclimate and its temperature levels can be installed to supplement the passive solar heating during the winter.
Unheated polytunnels are commonly used to stretch the growing season of different fruits and vegetables for a few weeks, including tomatoes, summer squash, pumpkins, broccoli, cauliflower, cabbage, celery and kohlrabi. In addition to season extension, polytunnels are employed to allow cold-hardy crops to overwinter whose resilience is not quite strong enough for them to survive outdoors. Once summer temperatures arrive, the plastic sheeting of a polytunnel can be removed to avoid overheating the plants and allow for unprotected growing.
Cold Frames
A cold frame differs from a conventional greenhouse or polytunnel in that it has four solid walls typically constructed from treated wood on an open bottom with a clear, transparent top (usually a glass pane) to capture the sun’s rays. Cold frames are built low to the ground and are unheated so depend entirely on solar radiation and their insulating walls to create a safe, boxed-in microclimate for plants. Farmers use cold frames to extend the backend of the growing season into the beginning of winter, while others use this method to overwinter plants. Some also use it to germinate and sprout their spring seeds early within a protected setting or to harden off seedlings started indoors before transplanting them fully outside. Low-growing, cool-season crops such as lettuce, radishes, and scallions grow well inside a cold frame environment.
Row Covers
Row covers are transparent or semi-transparent flexible materials, including spun-bonded, woven plastic, polyester or polypropylene, that are used as protective covering for low growing vegetables. They create a microclimate for plants by providing a small amount of warming in a similar way to unheated cold frames, greenhouses and polytunnels. Because they are relatively inexpensive to install and easy to move, row covers are popular with farmers looking to extend growing seasons and protect plants from cold, frost, wind, insects and other pests. A blanket of snowfall can enhance their warming effect by providing an extra layer of insulation on top. Row covers can lengthen the fall harvest of summer salad, spinach, cucumbers, peppers, tomatoes, and green beans.
