Most plants will grow within the space you allow them. They’ll survive with a minimum of soil depth, but they’ll thrive if you give them as much room as possible for their roots to branch out and breathe.
For example, shallow-rooted plants like lettuce may do better in soil that’s high in clay and doesn’t drain well.
Raised beds built over grass or dirt typically don’t need to be more than the standard 8 to 12 inches in height because the roots can sink into the subsoil.
Hydrozoning is the practice of grouping plants with similar water needs together in order to conserve moisture and irrigate more efficiently.
For example, a cucumber plant sends down a single tap root 3 to 4 feet deep. The majority of its roots, however, extend outward about 2 feet and are concentrated just below the soil surface.