A cold frame is essentially a bottomless box topped with a transparent lid, acting as a mini-greenhouse to protect plants and extend the growing season. This simple structure relies on solar energy to create a warmer microclimate for hardening off seedlings or cultivating cool-season crops in early spring and late autumn. The biggest challenge to maintaining this warmth is the nighttime temperature drop caused by radiative cooling and cold air infiltration. While the transparent cover allows sunlight in, it also allows heat to radiate rapidly back out into the cold night sky, and gaps permit outside air to seep in and displace the warmer air.
Utilizing Thermal Mass and Structural Sealing
The most fundamental approach to stabilizing cold frame temperatures through the night involves implementing thermal mass and ensuring the structural integrity of the enclosure. Thermal mass refers to materials that absorb solar heat slowly during the day and then release that stored heat slowly overnight. Water is particularly effective for this purpose due to its high volumetric heat capacity.
Placing several dark-colored containers, such as plastic jugs or barrels filled with water, inside the cold frame allows them to absorb heat energy throughout the sunny hours. During the night, the water releases its stored energy, helping to moderate the air temperature inside the box and prevent freezing. Bricks, stones, or a deep layer of moist soil also function as thermal mass.
Maximizing the effectiveness of this stored heat requires minimizing air leaks and drafts. The cold frame should be positioned on a site that receives maximum sunlight, often facing south, and preferably against a solid structure like a house or fence to shield it from cold winds. Any gaps or cracks between the frame and the transparent lid, or at the corners of the box, should be sealed using weather-stripping or caulk. By addressing both thermal storage and air sealing, the cold frame can consistently achieve a moderate nighttime temperature increase of approximately five to ten degrees Fahrenheit over the ambient outdoor temperature.
External Insulating Layers for Night
While thermal mass helps to stabilize the temperature, external insulation provides a temporary, physical barrier to slow the rate of heat loss after sunset. This strategy is highly effective, especially when outdoor temperatures are expected to dip toward freezing. The key is to apply these layers immediately before dark to trap the day’s accumulated heat and remove them promptly at sunrise to allow for renewed solar gain.
Effective external insulators include thick materials with high R-values, which measure thermal resistance. Rigid foam boards, such as extruded polystyrene (XPS) or polyisocyanurate (Polyiso), are excellent for this task. These boards can be cut to fit snugly over the transparent lid, significantly reducing heat radiation.
Alternatively, gardeners can use readily available materials:
- Old blankets, quilts, or thick burlap sacks draped over the entire structure.
- Straw bales placed around the perimeter to insulate the sides and base.
When using any external cover, ensure that the entire transparent surface is completely obscured, as heat loss through the glazing is the primary concern at night.
The insulating covers must be removed every morning to prevent the cold frame from overheating once the sun rises, a condition that can damage or kill the plants. Leaving the covers on for too long can also lead to excessive humidity buildup, which promotes fungal diseases. The application and removal of these external layers is an active part of managing the cold frame.
Supplemental Heat Sources
For nights when temperatures plummet well below freezing, or when attempting to grow temperature-sensitive plants, passive measures may need to be augmented with temporary, active supplemental heat. These methods provide a direct heat source to prevent the internal temperature from reaching a destructive low.
One method involves utilizing specialized electrical heating elements, such as soil heating cables or propagation mats designed for horticultural use. These devices are typically thermostatically controlled to maintain a consistent soil temperature, often around 60 degrees Fahrenheit, which encourages root growth and protects the plant base. Any electrical components used must be rated for outdoor conditions and connected safely to prevent fire or shock hazards.
A more traditional, non-electric method for generating long-lasting heat is the use of a “hotbed,” which relies on the biological process of decomposition. Fresh, nitrogen-rich organic materials like horse manure, mixed with straw, can be packed beneath the cold frame area. Microorganisms breaking down this material generate considerable heat, with internal temperatures potentially reaching 125 to 155 degrees Fahrenheit.
This biological heat is released slowly over several weeks, providing a gentle, consistent warmth to the soil layer above it. A simpler, temporary option involves placing a single low-wattage incandescent light bulb inside the frame, provided it is safely secured and shielded from moisture, to offer a minimal but effective radiant heat source that can prevent temperatures from dropping below the freezing point.