The advantages of raised-bed growing include spacing plants closer together than in conventional row gardening. This spacing allows for growing more plants, useful for vegetable and herb growing. Raised-bed practices also apply to smaller fruiting plants and for cutting gardens.
Raised Bed Principals
Raised bed gardening allows plants to develop more robust root systems because they don't have to fight through boggy, rocky or clay soils. Temporary or permanent walls hold soil which gardeners lay on top of the ground. This soil, usually good-quality topsoil amended with fertilizer, provides the best possible nutrients and texture for crops. In addition, the design of most raised beds maximizes garden space while minimizing pathways.
Plant spacing depends partially on the shape of the raised bed. Although no hard-and-fast rules apply, gardeners often use rectangular beds no longer than 4 feet in width---to help them reach crops from either side of the bed. Theoretically, the rectangles are quite long, but remember that you'll have to walk around them to get to the other side. So, 8 feet is a good length to aim for.
When it comes to putting plants in raised beds, think "blocks," not "rows." In row planting, there are two types of spacing to consider: the distance between the plants and the distance between the rows of plants. In block gardening, the same amount of space exists between all plants. Garden author Barbara Damrosch notes that more vegetables are grown in a raised bed if the plants grow diagonally to one another, rather than in straight up-and-down lines.
Advantages of Close Spacing
Planting vegetables and other crops close enough for their leaves to touch blocks the sunlight underneath those leaves, helping suppress weeds and conserve water. In addition, yields increase significantly. The University of Ohio reports one experiment in which yield increased by .6 pounds of food per square foot with row gardening to 1.24 pounds of food with raised-bed gardening.
Understanding Spacing Directions
Often, seed packets will give a range of suggested distance between plants. If the packet or seedling information dictates 2 ½ to 4 feet between eggplants, for instance, choose 2 ½ feet for raised-bed spacing. Many experienced wide-row gardeners space plants even closer than the smallest distance suggested by nurseries.
Remember that plants on the perimeters also need room to grow, so always start the plants a few inches or feet from the edge, depending on the crop.
Extension services like Colorado Sate University are good spring boards for spacing suggestions. For example, the Colorado site suggests spacing peppers 15 inches apart, trellised tomatoes 2 feet apart, trellised cucumbers 1 foot apart, radishes 2 inches apart, lettuce 7 inches apart, corn 12 to 30 inches apart and carrots 2 to 3 inches apart. Damrosch recommends setting strawberries 12 inches from one another to allow room for the "daughter" plants which emerge from the original plants.
Flower distances vary greatly; check the seed packet or bedding plant information and use the smaller of the recommended distances.
Small berry bushes and dwarf fruit trees also work well with raised-bed planting. In general, use one large plant per bed, or perhaps two in a 4-by-8-foot bed. Underplant them with shade-tolerant vegetables and herbs such as lettuce, mint or chives.