This step-by-step photo tutorial on the DIY site Instructables shows how to make a sturdy greenhouse with reused windows and fairly inexpensive materials that can be found at a home improvement store — lumber, screws, caulk, etc. The windows and lumber can be purchased for cheap at a local Habitat for Humanity ReSale outlet or you can probably score some for free from a junk yard or from people or businesses getting rid of old ones.
The flooring of this particular design utilizes rocks for drainage and to help keep heat inside. These can be purchased from a home improvement store, but you might even be able to get them for free from someone looking to replace ones in their driveway.
For the roof, these plans used a piece of siding from an old shed. Any waterproof, lightweight hard material would work for the greenhouse roof, though. Again, get it from looking to get rid of it or from a Habitat for Humanity store.
The author of this project also lists tips on properly sealing cracks and gaps, ventilation, how to keep it heated inside during the cold winter months, and has even written updates on upgrading the design.
If you've got a free weekend, a major green thumb, a big back yard and plenty of gumption, then give this upcycling project a go. It will protect your plants during harsh weather and keep them flourishing and healthy year-round.
Image and instructions by cheft via Instructables.