The finite space in a raised garden means it’s important to consider the mature nature of the plants selected for it. A two-foot azalea that’s charming in a nursery may want to be 12 feet; well-established lilies can spread to the point of excluding other plants.
Simply because raised beds are raised, they can be harder on the back to mulch. Buckwheat hulls, while more expensive than wood mulch or leaf mold, are a great option: They’re very light and easy to spread, they’re decorative, and add nutrients to the soil as they rot down. As with any mulch, keep the hulls from touching the base of the plants themselves, and don’t mulch too deeply. Two inches is adequate, four inches is too much.
Raised beds, especially small ones, are rarely appropriate for trees and large shrubs. Use tall annuals or perennials to add focus and height: Ornamental grasses, ferns, foxglove, coreopsis tripteris, bee balm, some black-eyed Susans, balloon flowers and delphiniums are among the plants that can reach four feet or more.
When planning the dimensions of a raised bed, it’s important to remember that all gardens need maintenance. The stunning effect of a very high or very wide bed is lessened if it’s choked with weeds or spent blooms; consider access points in planning. Wide terraces can include paths or steps for easy access, or decorative flat rocks can give the gardener a place to step.