Raised garden beds offer several advantages over traditional, in-ground growing. The benefits of raised beds include the ability to use customized soil mixtures, excellent water drainage and less weeding. Using a raised bed also allows you to grow plants you might not otherwise be able to due to factors such as a black walnut (Juglans nigra, U.S. Department of Agriculture plant hardiness zones 4 through 9) in your yard or heavy clay soil. A raised garden bed allows for easy access and, if tall enough, eliminates bending and squatting. While you can make a raised bed any size, keeping it within a specific range makes access for planting, watering and maintaining the bed much easier.
A width of less than 4 feet works best for most raised beds to allow access to the center without stepping in the bed. If you don’t have access from all sides of the bed, make the sides 18 inches to 2 feet wide so that you can easily reach the entire planting area.
While any length can work, keeping the beds under 6 feet in length means you won’t have to add stakes or supports to the sides. If the bed measures longer than 6 feet, you will probably need supports so the sides don’t bow or collapse. Raised beds generally range in length from 3 feet to 12 feet.
The height of the bed varies depending on several factors, including your needs, construction materials and your soil conditions. A raised bed might reach 6, 12 or 18 inches high, depending on the materials used and whether you have poor soil. Taller beds require more support through staking the sides or reinforcing block and brick with rebar or mortar. Elevated beds sit on legs or a support frame. The bottom of elevated beds should reach no higher than 30 inches to eliminate arm fatigue when reaching to maintain them. Wheelchair-accessible beds work best at a height between 24 and 30 inches.
Standard depths for raised garden beds range from 6 inches to 18 inches. If you have poor soil or toxic plants in your yard, a bed that's 12 to 18 inches deep gives roots more space. Elevated raised beds require at least an 8-inch soil depth because the plant roots will not have ground underneath in which to spread.