Introduction
In places where soils are difficult to work, or are generally infertile, gardening in raised beds can solve a number of problems. Growing plants in a raised bed allows a gardener to control the type and fertility of the soil or mix, which improves the likelihood that plants will thrive and produce higher yields.
Because a raised bed has a smaller area to be kept moist, gardening in one can limit water waste. In addition, the bed can be raised to a level that is most comfortable for the gardener. No matter what environment, raised beds can be useful.
What is a raised bed?
Many kinds of planting beds can be considered “raised.” It can be as simple as an area where enough amendments have been added so that the level is higher than the surrounding soil. In this case, there may or may not be walls built to confine the improved soil. More often, however, the term is used to describe a discrete area that is walled and considerably higher than the surrounding ground. The height depends on what is being grown and what the gardener needs. It is filled with material that could be completely different from the original soil – usually a mix with a high level of rich compost added to other components. In some ways, a raised bed could even be a particularly large plant pot.
Why build one?
When bending and reaching are difficult, a raised bed can be at a height that is more comfortable for the gardener. It can be helpful in areas where garden soil is difficult to work because it is a heavy clay or very rocky. Plants often die when they are growing in soil that drains poorly, so raised beds should contain well draining soil or mix. Vining vegetables, or plants such as mint that might have a tendency to become invasive, can be better managed when they are grown in a limited area.
What plants benefit?
Vegetables are often grown in raised beds, but they can provide a good setting for many plants, including flowers and small shrubs. A raised bed is really a mini-environment that is designed for the plants growing in it. Virtually any plant can benefit when local conditions do not meet the needs of the desired plants. Many people, for instance, want to grow fresh vegetables. These plants evolved in places where soils were mildly acidic or neutral, and relatively fertile. Neither of these conditions is dominant in the desert Southwest where soils tend to be alkaline and infertile. The raised bed can provide a setting that replicates those plants’ native environment more closely.
Where to place it?
A sunny location is usually best. If possible, choose a site that is brightest in the morning and noon time. Because soil drainage is possibly the single most important factor in plant success after light, a raised bed should be placed atop an area where water can drain readily without damaging the surface beneath it. Since it will contain growing plants, it should be located where there is easy access to water. If high winds pose a problem, it should be in a sheltered location.
Sizes
The size of the raised bed will depend on the gardener. How much space is available? If a bed is built so one could walk around it, then it should be about 4 feet wide, providing an easy reach of 2 feet from either side. If it is built next to a wall, the width should be less, to reduce the possibility that someone will need to walk on the bed itself. Many people opt for 8- or 10-foot-long beds, although that is by no means critical. The length is less of a concern, and can be as long as is practical. Remember, getting to the other side means having to walk around it.
Lettuce and green leafy vegetables can grow well in a bed as shallow as 1 foot or less. To maintain good form for root crops such as carrots, which might be 8 inches long, the bed must be at least a couple of inches longer than the longest carrot.
In every case, drainage is critical. Standing water at the bottom of the planter results in a muddy, airless area where roots cannot work and will ultimately die. Good drainage is a function of the depth of the bed, the growing medium and where it is built.
Building materials
The choice of materials for a raised bed is really up to the gardener. The walls must withstand the pressure of both the growing medium and the weight of the water that soaks it, not to mention the plants growing within them. There are many choices for the raised bed walls, and the selection depends on the gardener’s aesthetics and budget. Boards made of wood or composite materials, bricks, blocks, or plastic can be used. Any of them can hold soil or planter mix, and can be attractive. Each kind of material has benefits and drawbacks.
Wood is attractive, but certain lumber varieties or wider boards can be costly. If using wood, take care not to use wood that has been treated with anything that might be taken up by the plants (Fig. 5). At one time, wood was “pressure treated” with arsenic compounds, but that practice has been largely discontinued. In some parts of the world, when wood gets wet it has a tendency to mold and rot. In dry areas (less than ~ 12 inches of annual rainfall), this is less of a problem.
Plastic and aggregate materials vary greatly in their strength. With either wood or plastic, longer boards need extra support. If they are used for high (more than 2 feet) walls, they may bend or buckle under the weight of the planting material. In addition, plastics tend to be less durable under hot, dry desert conditions.
Blocks come in a wide variety of styles and can be decorative or simply be construction blocks. Decorative blocks are usually expensive; construction blocks are often high in salts that must be leached if plants are to survive. Because their weight stabilizes them, block raised beds may not need mortar.
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