Greenhouses are designed to store solar heat. During the day, the glass or plastic that the greenhouse is made from draws and holds the sun's heat. The heat is slowly released throughout the evening and night, keeping plants warmer than if they were outside. While the greenhouse design automatically protect plants inside from some threat of frost, plants in an outdoor greenhouse can still be susceptible to especially cold temperatures. There are a few ways that greenhouse plants can be protected from frost.
Fill containers, such as buckets and planters, with water and place them around the interior of the greenhouse. The water will absorb heat from the sunlight during the day like the greenhouse itself and will release stored heat slowly in the evenings.
Insulate the glass or plastic walls of the greenhouse with bubble wrap. The same amount of light will be absorbed into the greenhouse, but the extra insulation will prevent the heat from escaping as quickly.
Lengthen the hours of daylight. Throughout late fall and winter there are fewer daylight hours, which provides less solar heat for the greenhouse. Adding heat or grow lights to the greenhouse and turning them on during the hours between nightfall and the time that you go to sleep will add additional hours of light and provide more heat throughout the night.
Use a portable space heater to warm the greenhouse during the coldest parts of the night. This is the easiest to do if your greenhouse has electricity. If it doesn't, you can stretch an outdoor extension cord from your home or garage if necessary.
Combine more than one method to create a constantly warm greenhouse. Insulating the walls and using a space heater, for instance, will both provide heat and prevent it from escaping. This can be especially important in cold northern climates.