A greenhouse is a natural wood collector and it usually does not have to be warmed during the day, even in sunlight. But it is another story when the sun goes down and unless you’ve got a means to give heat, the temperature will quickly equalize using the exterior environment. You can use the heat that the sun generates within the ground during the day to keep it warm at night by keeping that heat in a heat sink. Filling old paint buckets with water works well, since water is an efficient heat-storing medium.
Seal the greenhouse roof and all of the walls using silicone caulk to prevent dropping the heat through small cracks and gaps. Keeping the greenhouse as airtight as possible will keep it warmer at night, without generating extra heat.
Paint the outsides of many plastic 5-gallon containers using flat black enamel. Make sure that the containers have lids and paint these too.
Fill the containers around three-quarters full with water, put on the lids loosely and spread them around the ground in places where they’ll be in full sunlight for as long as possible. Catch them there. The sun will heat the water during the day, and the camel will radiate the heat back to into the greenhouse through the night.
Supplement the heat created by the collectors by keeping a compost pile in the greenhouse. Heat is a pure by-product of composting. Add more organic stuff to the pile every day, as you flip it with a pitchfork.
Line the sides and roof of the greenhouse with plastic bubble wrap as a supplementary measure. Each air-filled compartment acts as a heat sink and will radiate heat to the greenhouse for hours after the sun goes down. Bubble wrap also provides extra insulation.