If your soil does not drain well and gardening is in your blood, you should build raised beds. If your land is sloping severely, terraces will help prevent erosion. Terraces are essentially raised beds using existing soil, and are quite common in many Asian countries and in South America.
For beds used exclusively for growing flowers or fruit, the walls can be built of almost any type of material. However, if the raised beds are to be used for growing root crops and greens, avoid using lumber treated with copper chromium arsenate. The arsenate in the wood moves into the soil, where it can be absorbed by roots and translocated into leaves of plants. There is no evidence that it will translocate into fruit or seeds.
Wood treated only with micronized copper can safely be used for building raised beds. Copper is an essential plant nutrient. Other species of wood that can be used without chemical treatment are redwood and cedar. The fibers in these species are composed primarily of lignins, making them rot-resistant. However, they will rot in time. Lining the inside walls with four- to six-millimeter polyethylene sheeting, to minimize soil contact, will increase their useful life.
A Bay Weekly reader recently told me he built his raised beds with four- and six-inch-diameter black locust logs. Dry black locust is used by cattle farmers for fence posts because it resists rot for at least 40 years. This reader built his walls three logs deep and secured them by drilling three-quarter-inch-diameter holes and pounding rebar through the logs into the ground.
Cement board and cement blocks can also be used for building raised beds.
If the raised beds are to be used for growing flowering plants, greens and small fruit, they need only be eight to 10 inches deep. Root crops need a depth of 14 to 16 inches.
A common mistake in filling raised beds is using potting mixes. Most potting mixes are extremely high in organic matter, and their volume shrinks rapidly. The cellulose and hemicellulose in the organic matter oxidize and are digested by microorganisms, causing shrinkage. Another drawback of organic matter is its inability to retain water and nutrients, thus making it necessary to water and fertilize frequently.
Raised beds should be filled only with sandy loam soil. To drain well, the soil should contain a minimum of 60 percent sand and not exceed 15 percent clay. The remaining components will be silt and organic matter. Organic matter can always be added by incorporating compost into the top four to six inches prior to planting.
If you have a choice, purchase a manufactured soil containing 65 percent sand, 15 percent clay, 12 percent silt and eight percent organic matter. For soil for growing vegetables, flowers and most small fruit such as strawberries, raspberries and blackberries specify a pH between 6.0 and 6.5. For growing blueberries, azaleas, rhododendrons, andromeda and the like, specify a pH not to exceed 5.0.
Raised beds can be planted earlier due to warmer soils and offer higher yields per square foot. It’s also easy to use plastic mulch to control weeds and to conserve moisture in raised beds. Design them three to four feet wide, and the center will accommodate most mulching-grade plastics and can easily be reached from either side. Raised beds will also make you do less bending.
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