Greenhouses offer significant protection against frost, but this protection is not guaranteed and depends entirely on the design, materials, and management of the structure. Frost occurs when the air temperature drops to the freezing point of water, \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). This causes water vapor to condense and freeze as ice crystals on plant surfaces, which can rupture cell walls and lead to irreversible damage known as frostbite. A greenhouse acts as a managed environment that primarily works by slowing the rate of heat loss to the outside air, often keeping the interior air temperature a few degrees higher than the ambient surroundings. This modest thermal buffer is often enough to prevent the temperature from crossing the critical freezing threshold during light or short-duration frost events.
How Greenhouses Trap Heat
The primary mechanism that allows a greenhouse to protect against cold is a physical process often called the greenhouse effect. Shortwave solar radiation, which includes visible light, easily passes through the transparent or translucent covering material like glass or polycarbonate. Once inside, this radiation is absorbed by internal surfaces, including the soil, benches, and plants, causing them to warm up.
These warmed surfaces then re-emit the absorbed energy as longwave infrared radiation, which is perceived as heat. The key difference is that the longwave radiation does not pass as readily back through the covering as the incoming shortwave radiation did. The glass or plastic material reflects or absorbs a significant portion of this outgoing longwave heat, effectively trapping the energy inside and creating the temperature differential.
The structure acts as a thermal buffer, raising the minimum overnight temperature compared to the outside ambient air. For an unheated structure, this passive heat retention typically maintains an interior temperature approximately \(2^\circ\text{C}\) to \(5^\circ\text{C}\) (\(4^\circ\text{F}\) to \(9^\circ\text{F}\)) warmer than the outside minimum temperature. This difference is often sufficient to prevent frost formation when the outside temperature is only slightly below freezing. The structure also mitigates radiative heat loss to the cold night sky and reduces advective heat loss caused by wind.
Limitations of Unheated Structures
While a greenhouse provides a thermal advantage, a purely passive, unheated structure has significant limitations, particularly in regions with harsh or prolonged winters. The retained heat is not limitless and must be replenished daily by solar gain. During extended periods of cloud cover or multi-day deep freezes, the internal surfaces eventually cool down, creating a “heat sink” that reduces the structure’s ability to maintain a protective temperature buffer.
When the outside temperature drops substantially below freezing for many hours, the passive temperature differential becomes insufficient. For instance, if an unheated greenhouse only maintains a \(3^\circ\text{C}\) buffer and the outside temperature drops to \(-6^\circ\text{C}\) (\(21^\circ\text{F}\)), the interior will still fall to a dangerously cold \(-3^\circ\text{C}\) (\(27^\circ\text{F}\)). The amount of protection is also highly dependent on the construction material; a single layer of plastic film or glass provides much less insulation than twin-wall polycarbonate or triple-layer systems.
Wind exposure dramatically compromises a passive system, accelerating advective cooling. Poorly sealed doors, vents, and panel gaps allow warm air to escape rapidly and cold air to infiltrate, further overwhelming the passive retention capabilities. These factors mean that relying solely on the greenhouse structure is often inadequate for protecting sensitive plants during severe or sustained cold snaps.
Active Heating and Thermal Storage
When passive heat retention is insufficient, growers must turn to methods that either actively add heat or augment the structure’s natural heat storage capacity. Active heating involves external energy sources, such as electric, propane, or natural gas heaters, which are typically thermostatically controlled to activate only when the internal temperature approaches the freezing point. Propane and natural gas heaters require proper ventilation to prevent the buildup of combustion byproducts, like carbon monoxide and ethylene, which can be toxic to both plants and humans.
Electric fan heaters offer clean, forced-air circulation, which helps distribute the heat evenly and prevents pockets of cold air from forming near the ground. For any active heating system, maintaining a minimum nighttime temperature of at least \(4^\circ\text{C}\) to \(7^\circ\text{C}\) (\(40^\circ\text{F}\) to \(45^\circ\text{F}\)) is a common strategy to protect most tender plants from frost damage. The size and efficiency of the heater must be carefully matched to the size and insulation level of the greenhouse to ensure adequate performance and manageable energy costs.
Complementing active heating is the use of thermal mass, which passively stores solar energy absorbed during the day. Water is the most commonly used and effective thermal mass material due to its high specific heat capacity. Large containers, such as dark-colored plastic or metal barrels filled with water, absorb significant heat when exposed to direct sunlight. At night, these “thermal batteries” slowly radiate the stored heat back into the greenhouse environment, stabilizing the temperature and reducing the severity of overnight temperature dips.
Internal Plant Protection Methods
Even within a heated or thermally augmented greenhouse, localized, secondary defenses can be employed to protect the most sensitive plants from microclimates of cold air. Cold air naturally sinks, meaning that plants placed on the floor or near the perimeter walls may experience temperatures several degrees lower than those elevated on benches. Elevating containers off the ground helps them avoid the coldest zone of air accumulation.
One effective secondary defense is the use of specialized row covers, often called frost cloth or horticultural fleece. These lightweight, permeable fabrics can be draped directly over individual plants or entire rows, trapping the heat radiating from the plant and the soil surface underneath. Depending on the material’s thickness, these covers can provide an extra \(3^\circ\text{C}\) to \(6^\circ\text{C}\) (\(5^\circ\text{F}\) to \(10^\circ\text{F}\)) of protection.
Wrapping individual pots with insulating materials, such as bubble insulation or straw bales, protects the root systems, which are often more susceptible to cold damage than the above-ground foliage. Additionally, watering the soil the day before an anticipated frost is beneficial; moist soil retains significantly more heat than dry soil, and this stored heat slowly releases overnight, providing a thermal boost to the immediate root zone of the plants.