A cold frame is essentially a bottomless box topped with a transparent lid that functions as a miniature, unheated greenhouse. This simple structure captures solar energy, creating a warmer micro-climate inside that benefits plants by extending the garden’s growing season. Gardeners primarily use cold frames to protect seedlings from late spring frosts, “harden off” young plants started indoors, or grow cold-tolerant vegetables well into the late fall and winter.
Gathering Low-Cost Materials
The concept of a cheap cold frame hinges entirely on salvaging and repurposing materials. For the frame walls, scrap lumber is an ideal choice, which can often be found at construction sites, through online classifieds, or by disassembling free wooden shipping pallets. Using wood that is six to twelve inches wide is suitable for the walls, creating a box that is deep enough to shelter maturing plants.
The transparent lid, often referred to as the sash, is the most important element for trapping solar heat. Old window sashes are a traditional choice, such as storm windows, but it is important to ensure they do not contain lead paint if they are very old. Other readily available options include clear acrylic sheeting, polycarbonate panels leftover from roofing projects, or even the tops of large, clear plastic storage totes. If a rigid lid is not available, a thick, six-millimeter UV-resistant plastic sheeting can be stretched over a light frame.
Hardware can also be sourced affordably to attach the lid and allow for easy access. Simple door hinges, perhaps salvaged from an old cabinet or door, work well for the back edge of the frame. For a handle, a piece of rope, a leather scrap, or even a simple block of wood screwed into the sash can serve the purpose of lifting the lid.
Assembling the Cold Frame Structure
Construction begins by determining the size of the box based on the dimensions of the transparent lid you have acquired. If you are using a salvaged window, the frame must be built to perfectly accommodate that window, minimizing air gaps for better heat retention. Once the dimensions are set, cut the side pieces of lumber to length for the front and back walls.
To maximize solar gain and water runoff, the back wall must be taller than the front wall. A common height difference is six inches, with the back wall often twelve to eighteen inches high and the front six to twelve inches high. This creates a south-facing slope, which helps the light penetrate more directly into the frame, particularly when the sun is lower in the sky during winter. Cut the two side pieces of lumber at an angle to connect the front and back walls, creating this necessary slope.
Join the four wooden pieces at the corners using long screws, ensuring the corners are flush. Pre-drilling the holes is a recommended step, especially when using reclaimed or pallet wood, as this prevents the wood from splitting when the screws are driven in. After the box frame is complete, attach the transparent lid to the taller back wall using salvaged hinges. The final structural step involves sealing any gaps between the wood pieces or around the sash with caulk or weather stripping to make the frame as airtight as possible.
Siting and Climate Management
The placement of the finished cold frame is important for maximizing its effectiveness. The structure should be positioned in a location that receives the most sunlight, which typically means a south-facing orientation. Placing the frame against a solid wall, like the side of a house or shed, can offer shelter from prevailing winds and allow the wall to reflect light and absorb heat.
Temperature regulation requires daily monitoring to prevent plants from overheating. On sunny days, even if the outside air is cold, the interior temperature can quickly soar due to the greenhouse effect. If the exterior temperature is expected to rise above 40° to 50° Fahrenheit, you must vent the cold frame by propping the lid open a few inches with a stick or block. This allows excess heat and built-up moisture to escape, preventing the development of weak, “soft growth” that is easily damaged by cold.
The lid must be closed again in the late afternoon before sunset to trap the residual warmth for the night. For extremely cold nights, insulation can be provided by covering the entire frame with old blankets, straw, or an insulating mat. Incorporating thermal mass objects inside, such as dark-colored water jugs or stones, can also help by absorbing heat during the day and slowly radiating it back out at night, stabilizing the temperature.