A greenhouse provides an invaluable opportunity to extend the growing season, offering a controlled environment that protects sensitive plants from harsh winter conditions. Utilizing a greenhouse during the colder months requires a shift in focus from summer management, primarily centering on mitigating severe heat loss and compensating for low natural light. By establishing proactive structural integrity and implementing strategic environmental controls, growers can maintain a productive, frost-free space through the darkest part of the year. The success of winter greenhouse operation depends on a balanced approach, encompassing passive insulation, active heating, supplemental light, and specialized plant care.
Pre-Winter Structural Preparation and Passive Insulation
The first step in winterizing a greenhouse is to maximize the structure’s ability to retain heat without relying on external energy sources. Begin with a thorough cleaning of the exterior glazing panels, removing dirt, algae, or debris that significantly reduces light transmission. Maximizing the limited winter sunlight is paramount, as a dirty panel can decrease light levels by as much as 20%.
Structural integrity must be assessed by inspecting all vents, doors, and seams for small gaps or cracks. These openings allow warm air to leak out through infiltration, accounting for substantial heat loss, so they should be sealed with weather-stripping or silicone caulk. Once sealed, passive insulation can be added by lining the interior walls with horticultural bubble wrap, which traps a layer of air to create a thermal barrier. Bubble wrap is effective because its air pockets improve the insulation value (R-value) while still allowing light to pass through.
Consider adding thermal mass objects, such as large barrels painted black and filled with water, inside the greenhouse. These containers absorb solar energy during the day and slowly radiate that stored heat back into the space at night, helping to stabilize temperature fluctuations. Installing a temporary layer of plastic sheeting or an internal thermal curtain on the northern wall further reduces heat loss where sunlight transmission is minimal. Maintaining clear pathways and clutter-free benches promotes proper air circulation, which is necessary for uniform temperature distribution and disease prevention.
Active Temperature Control and Heating Strategies
After structural preparations are complete, active heating strategies are necessary to maintain a desired temperature differential against freezing outdoor air.
Calculating Heating Needs
The required heating capacity is measured in British Thermal Units per Hour (BTUH). This calculation considers the greenhouse’s total surface area, its glazing material’s heat transfer coefficient (U-value), and the difference between the desired indoor and expected minimum outdoor temperatures (Delta T). For example, a structure with double-glazed polycarbonate loses significantly less heat than one with single-pane glass, directly impacting the necessary BTUH output of the heater.
Common heating options include forced-air gas or propane heaters, which are efficient for larger spaces, or electric fan heaters for smaller setups. Regardless of the heat source, circulation fans are necessary to prevent heat stratification, where warm air accumulates near the roof while cold air settles around the plants. These fans should run continuously to mix the air, ensuring that the heat is distributed evenly throughout the plant canopy.
On sunny winter days, the greenhouse can rapidly overheat, even when outside temperatures are low. Brief, strategic daytime ventilation, often referred to as “airing,” is important to prevent temperatures from soaring and to expel overly humid air. Opening vents or doors for a short period during the warmest part of the day allows for the exchange of stale, moist air with fresh, drier air, reducing the risk of fungal diseases.
Supplemental Lighting and Photoperiod Management
Winter’s short days and low sun angle significantly reduce the light energy available for plant photosynthesis, necessitating supplemental lighting. Light systems serve two primary purposes: increasing light intensity for growth and managing the photoperiod to control flowering or dormancy. Supplemental lighting for growth aims to increase the Daily Light Integral (DLI), which is the total amount of photosynthetically active radiation received over 24 hours.
High-intensity lights, such as High-Pressure Sodium (HPS) or high-output LEDs, deliver the necessary light intensity for active growth. HPS lights efficiently convert electrical energy into Photosynthetically Active Radiation (PAR) and are favored for their long lifespan and the secondary benefit of generating heat. LEDs offer greater energy efficiency and spectral control, allowing growers to target specific light wavelengths for optimal plant response.
For photoperiod management, much lower light intensity (often just 10 to 20 foot-candles) is required to extend the perceived day length for light-sensitive crops. Timers are set to ensure plants receive a total of 12 to 16 hours of light, which is sufficient to maintain vegetative growth or inhibit unwanted flowering. Positioning reflective materials, such as white paint or foil, on walls or benches helps maximize the utilization of both natural and supplemental light by bouncing photons back toward the lower canopy.
Specialized Plant Care and Cultivation
The controlled environment of a winter greenhouse requires a significant adjustment to plant management routines, particularly concerning water and nutrient delivery.
Watering Adjustments
Due to lower light levels and cooler temperatures, plant metabolic rates slow down, drastically reducing transpiration and water uptake. Overwatering is a common winter mistake and can lead to cold, saturated soil, which encourages root rot and fungal issues. Watering frequency must be reduced, and the soil moisture should be monitored closely, only watering when the top layer of soil is clearly dry. When watering, do so in the morning on a sunny day, allowing excess moisture to evaporate before nightfall and helping to prevent cold damage.
Humidity and Disease Control
Humidity control becomes a major focus because the closed, heated environment can quickly become a breeding ground for diseases like Botrytis cinerea (gray mold). Circulation fans and brief ventilation cycles are important for lowering the relative humidity. Ideally, the level should be kept below 70% to inhibit fungal spore germination.
Fertilization and Crop Selection
Fertilization regimens should be adjusted or paused entirely, as plants are not actively growing and new, soft growth is highly susceptible to cold damage. Focus cultivation efforts on cold-hardy crops, such as lettuce, spinach, kale, and other winter greens. These crops thrive in the cooler, lower-light conditions of the prepared greenhouse.