A cold frame is basically a miniature greenhouse that uses only solar heating to maintain warmth, so extending the length effectiveness of a cold frame is a matter of leveraging a cold frame’s ability to acquire, store, or retain solar energy. There are several approaches to accomplishing the desired result.
The double pain approach which basically a top and bottom of a cold frame cover of have sealed pains of glass, or plastic with ¼ to ½ inch of air trapped in the middle for insulation. This approach retains heat in the same way double pained windows do in a home.
Second is water storage method, which takes solar energy and stores it in water containers that are place in the cold frame. For example, placing several empty two or three liter bottles painted black (to soak up more heat) and filled with water provides storage for solar energy. Using water to store solar energy is an ancient technique employed by many ancient peoples.
Rig a solar heating water heating panel
Insolate the sides of your cold frame with hay bales
Place your cold frame where it receives a full day’s sun; thereby, providing more time for the cold frame to warm.