Aside from the technicality that raised garden beds measure at least 6 inches tall, no other written or unwritten gardening rules dictate the maximum height for a raised bed garden. Raised-bed gardens may range from heights of 6 inches to 6 or more feet above grade. Without height limitations, gardeners may choose to build raised-bed vegetable gardens to fit their personal needs.
To provide the benefits that make raised-bed gardening different from ground-level gardening and planting in hills or mounds, raised beds must measure a uniform height of at least 6 inches. Soil elevated at least 6 inches above grade provides better water drainage than ground-level soil, warms more quickly and allows earlier plantings.
Raised-bed gardening is no different from ground-level gardening when it comes to planting and root depths. Regardless of the total height of your raised-bed garden, you will continue to plant transplants and seeds at the depths described on packaging for ground-level gardening. Heights of raised beds will not prevent plants from rooting to their maximum depths, as long as the topsoil below the raised bed has been tilled and prepared. If the soil below a raised bed is not adequate for sustaining roots, raised beds at minimum heights of 24 inches will provide safe rooting for deep-rooting garden vegetables such as tomatoes.
Raised beds make vegetable gardening easier for handicapped and elderly gardeners. Beds measuring at least 2 feet tall allow gardeners of all ages and physical abilities to garden without the back, knee and neck pain associated with tending ground-level gardens. In addition, containerised or supported raised beds with heights between 10 and 12 inches offer better protection to plants in high-traffic gardens and reduce maintenance issues.
A shallow raised garden bed measuring between 6 and 8 inches tall, or deep, is easy to build without supports. Simply till the garden, pile soil and level piles to measure between 6 and 8 inches thick. Taller raised beds will need supportive walls to prevent piled soil from eroding to expose plant roots. To create walls for raised beds form frames from boards or stack brick, block, stone, hay bales or old tires, building on until you reach the desired heights for the raised beds.