Raised bed gardening is a form of gardening in which the soil is formed in 3-4 foot (1.0-1.2m) wide beds, which can be of any length. The soil is about 0.5-1 foot (15-30cm) above the surrounding soil, sometimes enclosed by a frame generally made of wood or concrete blocks, and enriched with compost made from leaves and grass clippings.

 The vegetable plants are spaced in geometric patterns, much closer together than conventional row gardening. The spacing is such that when the vegetables are fully grown, their leaves just barely touch each other, creating a microclimate in which moisture is conserved and weed growth suppressed. Since the gardener does not walk on the raised beds, the soil is not compacted and the roots have an easier time growing. The close plant spacing and the use of compost generally result in higher yields with raised beds in comparison to conventional row gardening.

Raised beds lend themselves to the development of complex agriculture systems that utilize many of the principles and methods of Permaculture. They can be used effectively to control erosion and recycle and conserve water and nutrients by building them along contour lines on slopes. This also makes more space available for intensive crop production. They can be created over large areas with the use of several commonly available tractor-drawn implements and efficiently maintained, planted and harvested using hand tools.

Cite: Wikipedia