The traditional growing season ends for many with the first hard frost, but the possibility of harvesting fresh produce does not have to stop there. Winter growing utilizes the natural cold tolerance of specific crops and employs protective structures to create a favorable microclimate. While plant growth slows considerably during the short, dark days of winter, the goal shifts from rapid production to successfully holding plants in a harvestable state. This approach allows gardeners and farmers to extend the harvest period well past autumn, often providing fresh food throughout the coldest months.
Understanding Plant Cold Tolerance
Plants endure cold temperatures using biological mechanisms that protect their cellular structure from freezing damage. A primary defense involves cold acclimation, where exposure to non-lethal, low temperatures triggers a cascade of internal changes. This process must occur gradually, as a sudden, severe drop in temperature can still be fatal to unacclimated plants.
A critical adaptation is the accumulation of soluble sugars (sucrose and glucose) and amino acids (proline) within the plant cells. These compounds act as natural cryoprotectants, effectively lowering the freezing point of the cell’s internal liquid, the cytosol. This is similar to adding antifreeze to a car’s radiator, preventing the formation of damaging ice crystals inside the cell walls.
The formation of ice is generally restricted to the spaces between cells, known as the apoplast, which draws water out of the cell. Some cold-hardy species also produce specialized ice-binding proteins (IBPs) that control the growth of these external ice crystals, preventing them from expanding and piercing the cell membranes. For certain biennials and perennial crops, a sustained period of cold is necessary for vernalization. This physiological requirement, typically several weeks between 32°F and 50°F (0°C and 10°C), triggers the plant’s transition from vegetative growth to flowering the following spring. Without this chilling period, plants like cabbage or winter wheat will not produce reproductive structures.
A Selection of Winter Edible Crops
Winter harvesting relies on selecting species and varieties with high cold tolerance.
Leafy Greens
Hardy leafy greens form the foundation of most cold-weather harvests due to their ability to continue slow growth and sweeten after a light frost. Spinach is highly cold-tolerant, often surviving temperatures down to 0°F (-18°C), with its flavor improving as it converts starches to sugars in response to the cold. Kale is perhaps the most resilient, with varieties like ‘Red Russian’ able to withstand temperatures as low as 15°F (-9.5°C), sometimes lower with protection. Less common but valuable are Mache (corn salad) and Claytonia (Miner’s Lettuce), which tolerate temperatures around 5°F and 0°F (-15°C and -18°C) respectively, maintaining texture better during freeze-thaw cycles.
Brassicas
The Brassica family provides other robust winter crops. Brussels sprouts are exceptionally frost-hardy and can withstand hard freezes. Mature cabbage heads can survive temperatures dropping to 15°F (-9.5°C), though young plants are more sensitive. Exposure to cold improves the eating quality of both Brussels sprouts and kale.
Root Vegetables
Root vegetables offer another path for winter harvest, relying on the insulating properties of the soil rather than active growth. Carrots and parsnips, sown in summer and left in the ground, can be harvested throughout the winter. Parsnips require a hard frost to convert their starches to sugars, dramatically improving their flavor profile. Similarly, turnips and beets gain sweetness after a light frost and can be stored in the soil under a thick layer of mulch.
Perennial Herbs
Hardy perennial herbs can also provide fresh clippings, although they typically enter a state of semi-dormancy. Thyme, chives, and oregano are among the most cold-tolerant, and can often be harvested intermittently even when growth halts. Certain rosemary cultivars, such as ‘Arp,’ are bred for better cold resistance, but generally require protection in the coldest climates to survive temperatures below 15°F (-9.5°C).
Techniques for Extending the Growing Season
While cold-hardy crops possess natural tolerance, physical structures are often necessary to maintain a harvestable environment by moderating temperature swings and blocking wind. Cold frames are simple, unheated greenhouses, typically bottomless boxes with wooden sides and a transparent top (glass or polycarbonate). They trap solar heat during the day.
A cold frame can elevate the internal air temperature by approximately 5°F to 10°F above the outside ambient temperature. This passive solar heating requires active management; the lid must be vented on sunny days to prevent internal temperatures from rising above 85°F (29°C), which can scorch the plants. Low tunnels, or hoop houses, are larger, portable versions that use flexible hoops of wire or PVC covered with a transparent material over an entire garden bed.
Covering materials are categorized by weight and their corresponding frost protection level. Lightweight spun-bonded polypropylene fabrics, often sold as Agribon, are rated for different degrees of protection:
- Lightweight covers (e.g., AG-19, 0.55 oz/sq. yd.) provide 2°F to 4°F of frost protection while transmitting 85% of light.
- Medium-weight covers (e.g., AG-30, 0.9 oz/sq. yd.) offer 4°F to 6°F of protection with approximately 70% light transmission for more severe cold.
Deep mulching is a technique focused on insulating the soil and protecting the root zone from extreme cold and freeze-thaw cycles. A layer of organic material, such as straw, shredded leaves, or wood chips, acts like a blanket to stabilize soil temperature, not to generate heat. Applying a 2- to 4-inch layer of mulch after the ground has frozen in late autumn is recommended, as this keeps plant roots dormant and protected from damaging frost heave.