Raised bed gardening gives you beds that are easier to work in, require less bending or stooping, provide better drainage in heavy clay soils and produce hardier plants with higher yields. In the past, raised beds may have been constructed using railroad ties or chemically treated timbers. But because of concerns over the dispersal of preservative chemicals in soil, raised beds today may be made of stone or pressure-treated timber. You can also make raised bed walls with paving brick or stone.
Drive stakes into the ground along the corners of the bed. Tie string between the corners to mark off the walls of the bed. Use a carpenter’s level to ensure that the string stays level when you are building a raised bed on a hillside. If you are making a curved bed, lay out a garden hose to mark off the walls instead.
Cut down existing woody perennials with a pair of branch loppers. Dig out the roots and stump of the woody perennials with a pair of branch loppers. Spray perennial weeds with a systemic herbicide that contains glyphosate. Many systemic herbicides are sold in spray bottles for ease of application.
Dig a trench beneath the base of your raised bed wall that is between 16 and 18 inches wide. The trench must be deep enough to extend below the frost line of your soil--generally 6 to 12 inches. The frost line varies depending on how far north you are within the United States. Check with local builders to determine how deep the frost line extends in your area. Check that the trench floor is level with a carpenter’s level.
Mix concrete and water until the mixture is as thick as cake batter. Pour the concrete into the trench so it covers the trench up to the soil line. Press 3/8-inch rebar rods end-to-end into the concrete to reinforce the trench. Smooth the top over with a trowel. Allow the concrete to dry and cure for three to four days.
Mix concrete and water into a mortar until the mixture is the consistency of cookie dough. Apply a 1-inch layer in a 2-foot-long strip along the top of the poured concrete trench. Press the first paving stone into the mortar so that there is ½ inch of mortar between the concrete trench and the paving stone. Apply mortar to the side of a second paving stone and press it into the concrete trench next to the first paving stone. The bottom of the second paving stone should also be ½ inch from the surface of the trench, and the side of the stone should be 3/8 inch from the side of the first stone.
Tap the top of each stone with the handle of your trowel to set them in place and squeeze out any excess concrete. Wipe away the excess concrete. Continue to lay paving stones around the trench until you have completely edged the trench in a layer of stones.
Lay a second layer of stones in the same way that you put down the first layer of stones. Stagger the stones so the seams from where the first set of stones met aligns with the center of each paving stone on the second layer. Continue to assemble layers of stone until your raised bed is the height that you desire. Allow the mortar in the stones to dry and cure for up to four days.
Place a weed barrier cloth onto the ground inside the surface of the raised bed.
Mix soil for your raised bed that contains 1 part peat moss, 1 part compost, 1 part sand and ½ part well-rotted manure. Do not use ordinary topsoil or garden dirt in a raised bed. The microbes in topsoil or garden dirt will multiply rapidly in a raised bed and may injure plants. Fill the raised bed with the soil mixture.