The height is a matter of choice. If you want them quite high, that's fine, but even if lower, they have a number of claimed advantages.
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Raised bed gardening is a form of gardening in which the soil is formed in 3 ?
4 foot (1.0?1.2 m) wide beds, which can be of any length or shape. The soil is raised above the surrounding soil[1] (approximately 6 inches to waist-high), is sometimes enclosed by a frame generally made of wood, rock, or concrete blocks, and may be enriched with compost.[2] The vegetable plants are spaced in geometric patterns, much closer together than conventional row gardening.[2] The spacing is such that when the vegetables are fully grown, their leaves just barely touch each other, creating a microclimate in which weed growth is suppressed[2] and moisture is conserved.[3] Raised beds produce a variety of benefits: they extend the planting season,[2] they can reduce weeds if designed and planted properly[2] and reduce the need to use poor native soil. Since the gardener does not walk on the raised beds, the soil is not compacted and the roots have an easier time growing.[4] The close plant spacing and the use of compost generally result in higher yields with raised beds in comparison to conventional row gardening. Waist-high raised beds enable the elderly and handicapped to grow vegetables without having to bend-over to tend them.[4]
Chris