Build for at least 12 inches (30 cm) of soil and you have room for almost every crop you would put in a backyard bed. That number comes from University of Delaware Cooperative Extension, and it is the strictest of the published figures. Below that line, raised garden bed depth turns on what sits under the frame. Roots that can slip out the bottom into open ground need far less soil in the box than roots that hit concrete.
Push a trowel into my own two oldest beds in midsummer and it comes up cool and damp. Pale roots thread out of the 12 inches of soil and down into the clay underneath. Neither bed has boards across the bottom. The cedar frames have gone gray at the corners, and the beans root deeper than those boards are tall. Utah State University Extension says beds shallower than 12 inches should have no bottom, so roots can reach the soil below them.
The published numbers spread wide because of that split. A bed on open ground is a mound with sides, and the dirt under it counts toward raised bed soil depth even though you never hauled it in. Set the same frame on a patio or in a lined trough and the box holds every root you will ever grow there.
Check the ground under your site before you argue about inches. Peel the sod back in one corner and push a spade in. If it slides down easy, roots will follow it, and a shallow frame over that ground gives them far more room than the boards suggest. If the spade stops on gravel, builder fill, or a slab, treat the box as the entire root zone and build it deep.
Depth also decides how often you stand there with a hose. Deep soil holds more water between waterings than a shallow layer does, and roots in the bottom half stay cool while the surface bakes dry in a heat wave. On open ground the plants chase that moisture down past the frame. Over a slab they cannot, so the box has to hold all of it.
Line the recommendations up and the range makes sense. University of Maryland Extension gives bed heights of 2 to 12 inches (5 to 30 cm), in beds 2 to 4 feet (60 to 120 cm) wide and any length you want. Utah State asks for 6 to 12 inches (15 to 30 cm) on framed beds, and 8 to 12 inches (20 to 30 cm) above grade on unframed ones. Delaware sits at the top of the stack with its 12 inches of soil.
Measure those minimums against your own ground before you pick one. The lowest minimum bed height on the list, 2 inches (5 cm), only makes sense over ground roots can enter, not on a box dropped on a hard surface. Raised beds give greater yields per unit area because they add rooting area, and that gain shows up when roots can spread. Over pavement I would build to 12 inches and skip the argument.
Depth is an ergonomics call as much as a growing one. Delaware notes that beds can be built much taller for gardeners who have trouble bending or who need wheelchair access, and the plants do not care how high the frame goes. Soil above 12 inches mostly costs you fill, which is real money on a big bed but nothing a tomato will complain about. If bending is what keeps you out of the garden, build up.
Read the full article: Raised Garden Bed Specs Backed by Research
