How to Prepare Plant Beds Over Concrete

Check the concrete for slope by laying a builders level across the surface. In most instances, the concrete will already have a built-in slope for drainage.

Create a retaining wall around the plant bed. To build a retaining wall on concrete, you must use heavy building materials that do not need to be bolted down to keep them in place, so build walls from railroad sleepers or stone blocks. Make sure the retaining wall is narrow enough that you can easily reach the middle of the bed for plant maintenance.

If the concrete had no natural slope when you checked it with the level, cut a piece of 1/2- to 3/4-inch-thick plywood to fit the interior of the retaining wall you built. Place a wood two-by-four on the ground inside one wall of the retaining wall, and place the board so that it lies on the two-by-four on one side and slopes to the ground on the opposite side.

Use a drill to make holes for drainage all along the bottom of the retaining wall and in the board you laid across the bottom, if you had to create your own slope. If you build with stone blocks, use a masonry bit to drill the holes, to prevent damage to the drill.

Fill the plant bed with soil mixture. Pete Lane of Ohio State University's Horticulture and Crop Science department recommends mixing an even amount of soil from your yard, compost, and sand. Once you add the soil to the plant bed, you can begin planting just as you would in the ground.