Building Raised Beds
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.
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.
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.