A greenhouse can be used for growing during the winter months, extending the growing season beyond natural climatic limits. The structure captures solar energy and protects plants from harsh weather, but successful winter cultivation demands active management of the internal environment. Achieving continuous productivity relies on the grower’s ability to supplement the natural light and heat that become scarce during the coldest time of the year. This requires a strategic approach focused on maintaining adequate temperatures and compensating for the significant drop in available light.
Maintaining Essential Winter Temperatures
Temperature control is the most resource-intensive challenge for winter greenhouse operations because heat loss is constant through the structure’s cladding. Thermal management begins by maximizing the structure’s insulation and increasing the R-value of the walls and roof. This is commonly achieved by retrofitting the interior with materials like horticultural bubble wrap, which traps an insulating layer of air. Alternatively, growers can install double-layer polyfilm or polycarbonate panels that create an air buffer.
Even with excellent insulation, most winter climates require supplemental heating to keep temperatures above critical thresholds for plant survival. A simple frost-free environment can be maintained at a minimum of 3 degrees Celsius (37 degrees Fahrenheit) to protect dormant or hardy plants. Active growth for many cold-tolerant crops requires a minimum night temperature closer to 7 degrees Celsius (45 degrees Fahrenheit).
Heating systems vary widely. Electric fan heaters offer precise temperature control via a thermostat and circulate warm air evenly. Propane or natural gas heaters provide powerful heat but require careful ventilation management to prevent the buildup of combustion byproducts. Passive solar techniques, such as incorporating thermal mass like water barrels, help stabilize temperatures by absorbing solar heat and slowly releasing it at night.
The choice of heating system must align with the target temperature zone required by the chosen crops. Maximizing efficiency means not heating the entire volume more than necessary. Growing tender plants like citrus trees requires a higher minimum temperature than overwintering hardy herbs. A well-sealed structure, combined with proper heating and air circulation, forms the thermal barrier necessary to sustain life.
Addressing Winter Light Deficiencies
Beyond temperature, limited light availability in winter is a significant obstacle to maintaining plant growth. Shorter days and frequent cloud cover drastically reduce the Daily Light Integral (DLI), which is the total amount of photosynthetically active radiation (PAR) a plant receives over 24 hours. Plants typically require 12 to 16 hours of light daily for optimal growth, making supplemental lighting necessary to meet this photoperiod requirement.
Supplemental lighting systems aim to boost the PAR, the light spectrum between 400 and 700 nanometers used for photosynthesis. High-Pressure Sodium (HPS) lights have historically been popular due to their efficiency in converting electrical energy into usable light. Modern Light Emitting Diode (LED) fixtures offer greater energy efficiency and precise spectral control. LEDs allow growers to provide specific wavelengths, such as blue light (400-525 nm), which influences plant structure and leaf development.
The intensity of supplemental light is measured in micromoles per square meter per second. Typical supplemental light intensities for growth range from 40 to 80 micromoles per square meter per second. Smart control systems with light sensors operate the supplemental lights only when natural light levels are too low, ensuring plants receive a consistent DLI. By strategically using timers and sensors, the grower can effectively extend the “daylight” hours by 8 to 12 hours during the deepest winter months, ensuring continuous photosynthetic activity.
Selecting Appropriate Winter Crops
The most practical and energy-efficient plants for winter greenhouse cultivation are cool-season crops that naturally tolerate lower temperatures and light levels. These plants require less supplemental heating and lighting, making their production significantly more cost-effective during the dark months. Examples include many leafy greens, which thrive in the cooler environment:
- Spinach
- Kale
- Swiss chard
- Various lettuce varieties
Root vegetables like carrots, radishes, and beets are also well-suited, as they can be grown in containers or raised beds within the greenhouse structure. Certain brassicas, including some varieties of cabbage and broccoli, can be successfully brought to harvest. Many herbs, such as parsley and thyme, continue to produce foliage throughout the winter.
Growing warm-season crops, like tomatoes, peppers, or cucumbers, is possible but requires the maximum input of both heat and light. These plants have much higher demands, often needing minimum temperatures above 15 degrees Celsius (60 degrees Fahrenheit) and a high DLI to fruit successfully. A greenhouse can also be used to protect perennial plants like citrus trees or fig trees from freezing. This is usually for dormancy protection rather than active winter growth, preserving the plant until spring.