One reason raised beds are so effective for increasing efficiency and yields is that crops produce better because the soil in the beds is deep, loose, and fertile. Plants benefit from the improved soil drainage and aeration, and plant roots penetrate readily. Weeds are easy to pull up, too. Since gardeners stay in the pathways, the soil is never walked upon or compacted. Soil amendments and improvement efforts are concentrated in the beds and not wasted on the pathways, which are simply covered with mulch or planted with grass or a low-growing cover crop. Also, the raised bed’s rounded contour provides more actual growing area than does the same amount of flat ground.
Raised beds also save time and money because you need only dig, fertilize, and water the beds, not the paths. You don’t need to weed as much when crops grow close together, because weeds can’t compete as well. Gardeners with limited mobility find raised beds the perfect solution—a wide sill on a framed raised bed makes a good spot to sit while working. A high frame puts plants in reach of a gardener using a wheelchair. For best access, make beds 28 to 30 inches high, and also keep the beds narrow—no more than 4 feet wide—so it’s easy to reach to the center of the bed.
Building Raised Beds
The quickest and easiest way to make a raised bed is simply to add lots of organic matter, such as well-rotted manure, compost, or shredded leaves to your garden soil. In the process, mound up the planting beds as the organic content of the soil increases. Shape the soil in an unframed bed so that it is flat-topped, with sloping sides (this shape helps conserve water), or forms a long, rounded mound. The soil in an unframed bed will gradually spread out, and you’ll need to periodically hill it up with a hoe. A frame around the outside edge of the bed prevents soil from washing away and allows you to add a greater depth of improved soil. Wood, brick, rocks, or cement blocks are popular materials for framing. Choose naturally rot-resistant woods such as cedar, cypress, or locust. If you choose some other type of wood (don’t use chemically treated wood), keep in mind that you’ll need to replace it when the wood eventually wears and rots away.
The traditional way to make a raised bed is to double-dig. This process involves removing the topsoil layer from a bed, loosening the subsoil, and replacing the topsoil, mixing in plenty of organic matter in the process. Double-digging has many benefits, but it can be time-consuming and laborious. See the Soil entry for details.
If your garden soil is difficult—heavy clay, very alkaline, or full of rocks—you may want to mix your own soil from trucked-in topsoil, organic matter, and mineral amendments. Then you can build beds up from ground level, without disturbing or incorporating the native soil. You may also need to add extra materials to raised beds if you want them to be tall enough for a gardener in a wheelchair to reach easily.
This is a no-till option for building raised beds and great soil. It is similar to sheet composting and allows you to build raised beds without stripping grass or weeds off the site. You can also build a lasagna garden on top of an existing vegetable garden site.
If you are starting on a new site, first cut the grass as short as possible and/or scalp the weeds at ground level. Next cover the bed with a thick layer of newspaper (6 to 10 sheets) to smother existing vegetation. Use sheets of cardboard or flattened cardboard boxes if there are vigorous perennial weeds on the site. Either wet down the newspapers as you spread them or have a supply of soil or mulch at hand and weigh them down with handfuls as you spread. Be sure to overlap the edges of the newspaper or cardboard as you work.
Gardening in layers. To make a lasagna garden, spread newspapers or cardboard to smother existing vegetation, then pile on layers of grass clippings, chopped leaves, kitchen scraps, finished compost, and topsoil.
After that, begin layering organic matter on top of the site. Combine materials as you would in a compost pile, by mixing “browns” and “greens.” Add layers of organic materials such as grass clippings, finished compost, chopped leaves, kitchen scraps, coffee grounds, seaweed, shredded mail or newspaper, garden trimmings, used potting soil, sawdust, and weeds (don’t add ones that have gone to seed or perennials with vigorous rhizomes, which will spread and grow in the bed). You can also add topsoil, which will help speed things along. Make a pile that is 1 feet or more deep, and top it off with a layer of mulch to keep weeds from getting a foothold. Then wait several months for materials to decompose.
You can build a lasagna garden any time of year. Building one in fall to plant in spring is a good idea, and there are plenty of leaves available for chopping and adding to the mix. If you’re building in spring or summer, you can speed up the time when it will be ready to plant by adding extra compost and topsoil in the mix. Top the bed with 2 to 3 inches of topsoil and/or compost for annual crops (more for perennial plants) and then plant seedlings directly into the topsoil/compost mix.
Rich soil coupled with intensive gardening practices are what make raised bed gardening so successful. Intensive horticulture has been practiced for centuries in many parts of the world. In America, one of the best-known methods is French intensive gardening. Intensive gardening methods all have their own disciplines, but all use raised growing beds, close spacing between plants, careful attention to building and maintaining soil fertility, and succession planting to make the best use of available growing space.
Applied skillfully, intensive growing methods can (and consistently do) produce harvests 4 to 10 times greater than might be expected from a conventional garden. But intensive gardening also demands more initial work, planning, and scheduling than row gardens. If you wish to convert to intensive methods, it’s best to start gradually. For example, you could try building one or two raised beds each gardening season for a few years.
Plantings managed using intensive planting systems require fertile, well-balanced soil rich in organic content. Without plentiful additions of compost along with soil amendments, intensively gardened soil soon loses its vitality. Cover crops or green manures also help keep the soil fertile.
Close Plant Spacing
One reason that raised beds are so productive is that they are planted intensively, putting as much as 80 percent of a garden’s surface area into crop production. Pathways and spaces between crop rows make up the remainder. Plants are placed close together over the entire bed, usually in a triangular or staggered pattern, so that their leaves overlap slightly at maturity. This allows for more plants per square foot and produces a continuous leafy canopy that shades the bed, moderates soil temperature, conserves moisture, and discourages weeds. Close spacing also means plantings must be carefully planned according to each crop’s growing habits, including root spread, mature size, and water and nutrient needs.
Another technique used to maximize harvests in raised bed gardens is succession planting, which is the practice of rapidly filling the space vacated by a harvested crop by planting a new crop. This can be as simple as following harvested cool-season spring vegetables, such as peas or spinach, with a planting of warm-season summer crops, such as beans or squash. Once harvested, those crops could be followed by a cold-tolerant fall crop such as spinach. Another technique is to stagger plantings at 1- or 2-week intervals to prolong the harvest. Advanced intensive gardeners also interplant compatible short-, mid-, and full-season vegetables in the same bed at the same time. They then harvest and replant the faster-growing plants two, three, or even four times during the season.
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