A greenhouse provides the ability to extend the growing season, transforming winter from a dormant period into a productive one. By moderating the internal environment, gardeners can cultivate fresh produce even when the outside temperatures fall below freezing. The success of year-round food production hinges on selecting the appropriate crops and tailoring the greenhouse environment to meet their specific needs, ranging from simply providing shelter to supplying active heat and light.
Growing in Unheated or Cold Greenhouses
For many home gardeners, the goal is to cultivate crops that thrive in cold conditions, relying on the greenhouse for protection from wind, snow, and temperature swings. This approach is highly successful with cold-tolerant plants, as the chill triggers a biological response that increases sugar content, leading to sweeter flavor. Hardy leafy greens are the foundation of an unheated winter garden, with varieties of spinach like ‘Space’ and ‘Kolibri’ being particularly cold-resistant and capable of surviving temperatures down to 15°F (-9°C) under a protective layer of row cover.
Other robust greens include kale, such as ‘Winterbor’ and ‘Redbor,’ and various brassicas like collard greens and Swiss chard, which maintain productivity even with minimal light exposure. To prevent fungal issues, it is helpful to space these plants slightly wider than in summer and ensure adequate air circulation. Root vegetables also perform well, provided they are grown in deep beds or containers that insulate their roots from hard freezes.
Carrots, beets, radishes, and turnips can be grown for a steady winter harvest, often sweetening after a light frost. To regulate the temperature fluctuations inside a cold greenhouse, gardeners can employ passive heat storage techniques. Large water barrels painted black absorb solar energy during the day and slowly radiate that stored heat back into the structure at night, helping to raise the minimum overnight temperature by a few degrees.
Producing Warm-Weather Crops with Supplemental Heat
Cultivating warm-weather favorites like tomatoes, peppers, and cucumbers during winter requires a significant shift from passive protection to active environmental control. These fruiting vegetables originate in tropical or subtropical climates and demand a minimum temperature, often needing the greenhouse to remain above 50°F (10°C) even at night. Achieving these temperatures consistently often involves active supplemental heating systems, such as electric heaters, gas furnaces, or hydronic heating loops that circulate warm water.
Beyond temperature, the short, overcast days of winter necessitate supplemental lighting to compensate for low natural light. High-pressure sodium (HPS) or specialized LED grow lights are used to extend the photoperiod, ensuring plants receive the necessary 12 to 16 hours of light daily for optimal growth and fruit production. Without this additional light, plants become leggy and produce little or no harvest, making the effort and cost of heating largely ineffective.
Tender herbs like basil, dill, and mint also fall into this category, requiring warmth and light to maintain active growth for fresh culinary use. A heated environment allows for the cultivation of exotic crops, such as dwarf citrus trees, figs, or strawberries, which require a consistent, frost-free temperature range, typically between 65°F and 75°F (18°C and 24°C). The energy cost associated with maintaining these tropical conditions makes this approach more intensive, but it yields unique and high-value produce otherwise unavailable in cold months.
High-Yield, Quick-Harvest Winter Crops
Certain crops are ideal for maximizing space and providing an extremely rapid return on investment. Microgreens are a prime example, offering a harvestable product within a short window of 7 to 14 days from seeding. These immature seedlings, including radish, sunflower, and pea shoots, are grown densely in shallow trays and are highly nutrient-dense.
The short cultivation cycle allows for continuous rotation, making them excellent candidates for tiered shelving or vertical growing systems within the greenhouse. While many microgreens can tolerate cooler temperatures, providing a consistent environment, often around 65°F (18°C), ensures the most reliable and rapid growth. Another fast-return technique is forcing, where root vegetables or bulbs are encouraged to produce edible shoots or leaves.
Forcing chicory and endive in a dark, cool space yields tender, blanched leaves, and using kitchen scraps like scallion bottoms or leeks allows them to regenerate new green tops for quick cutting. Mushrooms also fit this high-yield, quick-harvest model, thriving in the controlled humidity and temperature environment of a greenhouse, often utilizing compost or wood block growing mediums. These quick crops offer a steady stream of fresh, specialty produce throughout the winter months.