Obtain Free Materials
Construction sites will often give away scrap lumber, according to Mel Bartholomew, the author of "Square Foot Gardening." Take note of sites in your city where building is taking place and ask whether scrap lumber is available. Bricks are an alternative option to lumber and can also be obtained for free from a building site. Get permission before entering a work site, and make sure it is okay with the person in charge for you to take the materials. Another option for free or very inexpensive materials is to search classified ads in your area for people selling leftover building supplies.
Use Smaller Lumber
Many raised garden beds have walls that are 2 inches thick and 12 inches tall. However, most plants can grow in just 6 inches of soil, according to Mel Bartholomew. If you are growing root vegetables, build a few raised beds that are 12 inches deep just for these plants; the remainder of your raised beds can be 6 inches deep. Also, if you purchase the lumber to construct the beds, consider using boards that are 1 inch thick, which is less expensive than 2-inch lumber and will likely last as long.
Build Large Beds
Consider building just a few large raised garden beds instead of many smaller ones. Two small raised beds, each 2 feet wide and 4 feet long, might look pretty, but they use a lot of materials, considering the limited planting space they create: 16 square feet of space requires 24 feet of materials to enclose. If you used the same 24 feet of material to make one large raised bed that's 4 feet wide and 8 feet long, you'd create 32 square feet of planting space. Try to avoid building raised beds wider than 4 feet, which makes it difficult to reach the plants in the center of the space.
Consider Alternative Materials
You can make raised garden beds out of almost anything, including many items people throw away. For example, a used tire makes a great small garden bed if you lay it on the ground and fill it with dirt. In fact, its dark color and insulation help warm the soil early in the summer for better plant growth. Another idea is to take the springs out of an old box spring and use the wooden frame as a raised garden bed.