Many materials are suitable for raised bed construction including rocks,concrete blocks, landscape timbers, or similar rot-resistant material. Untreated lumber, such as fir, will rot and breaks-down within a few years. Insect-and decay-resistant wood such as planks of black locust, cedar, Osage orange, redwood, or oak. cypress, redwood, or cedar. These rot-resistant woods are more expensive but are long-lasting and safe. Note: Do not use pressure-treated wood or railroad ties for your raised bed garden. Avoid wood containing copper sulfate, creosote, cuprinol, or compounds containing penta chlorophenol,which can be toxic to plants.  Plan raised beds that are 3 feet wide for children and 4 feet for adult gardeners. Raised beds are typically 8 to 12 inches high to allow for adequate root development. For most wheelchair users, 27" is a comfortable working height.Turn the boards "heartwood in" so that if they warp, they will curve slightly outward at the middle. When constructing a wooden frame, install support stakes halfway down the length of the bed to prevent the wood from bowing after it is filled with soil. Use 3-inch galvanized screws, rather than nails, in construction.  https://extension.psu.edu/how-to-construct-a-raised-bed-in-the-garden  https://extension.usu.edu/yardandgarden/research/raised-bed-gardening https://extension.umn.edu/planting-and-growing-guides/raised-bed-gardens