Keeping a greenhouse warm at night, or on very cold days, is a challenge. Install a thermometer so you don't have to guess at the temperature. How warm the greenhouse is depends on what you're growing. Tropical plants and warm-season vegetables like tomatoes and cucumbers have to be kept warmer than cool-season vegetables like spinach and lettuce. Spring bulbs may only need a few degrees above freezing to do well. If you live in a frigid area, space heaters may be your only heating option.
Tack cup hooks on the inside of the greenhouse away from the plastic or glass.
String old-fashioned Christmas lights, not LED lights, around the interior of the greenhouse, making sure the lights don't touch the plastic walls. Since heat rises, the lower you can put the lights the better.
Keep the lights from touching plants. Turn them on at dusk. Before you go to bed check the temperature in the greenhouse. It may be necessary to add additional lights.
Add additional lights by fastening cup hooks into wooden shelving toward the middle of the greenhouse. If the shelves are metal, tape the light cords to the shelves with electrician's tape.
Fill gallon-, 2 1/2-gallon or 5-gallon containers with water. Larger containers may be used if you can find them.
Wrap the jugs with black plastic. The plastic absorbs the heat and warms the water. The warm water will give off heat to the greenhouse during the evening and night. The jugs may be spray painted black, and then you won't need the black plastic.
Stack the containers against the side of the greenhouse that gets sun most of the day. You may have to install shelving to hold the containers.