Winterizing a greenhouse involves physically and environmentally preparing the structure and its contents to survive cold weather. This annual preparation is necessary for the longevity of the structure and safeguards tender plants that cannot withstand freezing temperatures. Properly winterizing prevents heat loss, manages condensation, and maintains a stable environment, which is more efficient than applying emergency fixes during a cold snap. The goal is to create a controlled microclimate that minimizes plant stress and reduces utility costs throughout the winter months.
Structural Preparation and Sealing Drafts
A thorough structural inspection and cleaning must precede insulation efforts, as a compromised structure undermines heat retention. Begin by clearing the interior of spent plant debris, soil spillage, and algae growth, which harbor overwintering pests and disease spores. Cleaning the glazing panels inside and out maximizes the transmission of limited winter sunlight, which is important for plant health.
Meticulously check the entire frame for damage, focusing on the glazing material. Cracked glass panes or tears in polycarbonate and polyethylene sheeting must be immediately repaired with appropriate sealing tape or caulking to prevent air and moisture intrusion. Even a small hole creates a thermal leak, compromising the structure’s ability to hold heat.
Next, seal all air gaps around doors, vents, and the foundation where cold air infiltration is common. Use weather stripping, foam tape, or silicone caulk to seal the perimeter of movable parts and where the frame meets the foundation. Ensure that all automatic and manual vents, along with the main door, close tightly against drafts. A simple smoke test can reveal unseen leaks that require additional sealing. This foundational work forms a tight envelope, making subsequent heating efforts more effective and less expensive.
Enhancing Insulation and Heat Retention
Once the structure is sealed, focus on incorporating materials that passively retain solar energy and slow heat transfer. Lining the interior walls and roof with horticultural-grade bubble wrap is effective, as it is UV-resistant and allows maximum light transmission. The trapped air creates an insulating layer, similar to double-pane windows, which can reduce heat loss by up to 50%.
For greater insulation, especially on the north-facing wall, consider using reflective foil insulation or an opaque material, as this surface receives the least direct sunlight. Installing an inner layer of poly sheeting or a thermal curtain system creates a second dead-air space. This dramatically increases the structure’s R-value and reduces outward heat radiation. Thermal curtains are useful for deployment at sunset, creating an insulating blanket over the plants when the temperature drop is severe.
Strategic placement of thermal mass materials helps stabilize internal temperature fluctuations. Large, dark-colored containers, such as 55-gallon drums filled with water and positioned in sunny areas, absorb solar heat throughout the day. Water retains this absorbed energy and releases it slowly as radiant heat into the air overnight, mitigating sharp temperature drops. Incorporating these elements reduces the demand on active heating systems.
Managing Winter Heating and Ventilation
Even with excellent insulation, most greenhouses require active heating during the coldest periods, managed by accurate thermostatic control. Electric fan heaters are common because they do not introduce combustion byproducts and provide even air distribution, preventing cold pockets. Propane or kerosene heaters are often used for supplemental heat but release water vapor as a byproduct of combustion, which rapidly increases humidity levels.
If using fuel-burning heaters, regular maintenance is mandatory, including checking the heat exchanger and exhaust for cracks that could allow carbon monoxide gas into the growing space. A reliable thermometer and thermostat ensure the heater only activates when the temperature drops below the desired set point. Backup heating systems or a plan for a temporary heat source should be in place for equipment failures or power outages.
Ventilation remains necessary in winter, despite the instinct to keep the greenhouse sealed. Stagnant air and high humidity create an ideal environment for fungal diseases like botrytis and powdery mildew. On warmer, sunny winter days, briefly opening vents or doors allows for an exchange of moist, stale air for drier outdoor air, often called a “winter purge.” Horizontal air flow fans should run continuously to circulate air within the structure, preventing moist, cold microclimates near the plant canopy.
Preparing Plants for Cold Weather
The final aspect of winterizing involves adjusting plant care practices to transition specimens into a period of lower metabolic activity. Significantly reduce the frequency and volume of watering, applying water only on sunny mornings so that foliage dries completely before nightfall. Excessive moisture in the soil or on leaves, combined with cooler temperatures, increases the risk of root rot and fungal disease.
Cease the application of high-nitrogen fertilizers, which encourage tender, new growth highly susceptible to cold damage. Instead, encourage plants to “harden off” by slightly stressing them, triggering a cellular process that increases freezing resistance. Before moving plants into the greenhouse, perform a thorough inspection and treatment for pests, such as aphids or whiteflies, to prevent infestation in the enclosed environment.
Plants that naturally enter a dormant phase, such as perennials or woody ornamentals, should be placed in the coolest section, requiring minimal light and water. For plants that continue to grow, ensure they are spaced adequately to promote air circulation and maximize light exposure. Regularly remove snow accumulation from the roof panels, as a heavy load can cause structural failure and severely reduce the light reaching the plants below.