I built this small greenhouse out of a salvaged wooden garage door some years ago. I used the existing windows on the door for the front and the top of this box: there is no bottom. Since then, I can really extend my growing season. It warms up nicely inside but when the temperature really starts to drop outside it helps to insulate around the pots with leaves or straw.
Since the garage doors are made already in strips approximately 20 inches wide, making this box is fairly easy. Take the door apart and cut three 54 inch long sections: top, front, and back. The front and the top of the box should be made from the part of the door with windows and the back without. Also, cut two more pieces 18.5 inches wide and 22 inches tall: the sides. I can’t remember why now but I put two strips along the bottom front and along the back top (see arrows) making a combined height of 22 inches.
Screw the four sides of the box together with exterior wood screws. Next place the top on but do not attach: this design doesn’t use hinges. Use a stick to let out excess heat when needed. The box opens completely for planting and harvesting. Paint or stain the whole thing inside and out. I topped my box with a piece of Plexiglas. This gives an added layer of insulation. To attach the Plexiglas to your top, pre-drill holes in it first making sure that your exterior grade wood screws can freely fit through. I learned the hard way that the fiberglass will start to crack if the drill hole isn’t big enough.
A good place to keep your small greenhouse is up against your house. It will be slightly warmer here. Pick a spot that gets a lot of sun. I like it on my deck for quick harvesting and convenient watering. I use mine in the spring to get an early start growing seedlings. And I use it in the fall and winter to grow lettuce and other cold hardy plants. I also love to keep a parsley plant growing most of the winter: I love fresh parsley. During the deepest part of the winter I store potted plants that have died back but can’t stand excessive cold.