Any small structure that follows the principles of a greenhouse can be considered a miniature version of the structure. The most common form of miniature greenhouse used by gardeners is the cold frame. Gardeners use cold frames to start seedlings in flats outdoors and to keep frost from injuring the young, tender plants. Seedlings started in cold frames will harden off faster than seedlings started indoors and moved outside. You can build a quick, temporary cold frame meant to last only one season by using straw bales and an old storm window.
Collect manure for starting your cold frame. Horse manure is a good choice because it is dry and compact, but you can also use cow manure, according to University of Missouri Extension. Break up a straw bale. Mix manure and straw so that your mixture is two-thirds manure and one third straw. Pile manure into a pile 10 days before starting the cold frame. The manure will compost and heat the greenhouse. Then re-pile the manure and allow it to heat again. The manure is now ready to pack into the bottom of a cold frame.
Dig a hole in the ground that is 5 feet 8 inches long by 3 feet wide. The hole should be 30 inches deep, and should be on the south side of any structure, such as a home or fence.
Layer gravel into the pit to a depth of 6 inches to help with drainage. If the manure does not drain, it will not decompose and will not produce heat.
Pack manure into the pit on top the gravel in an 18-inch layer.
Put soil on top of the gravel in a 6-inch layer.
Place four hay bales in a square around your pit to form the walls of your cold frame. Place a second bale of hay on the north side of the hay bales to create a slightly-taller wall and give the cold frame southern exposure.
Place a window and sash over the straw bales.
Break up the final bale and stuff straw into the triangular spaces between the window sash and the hay bales to insulate the structure and prevent heat from escaping.