Raised beds can be attractive and great for growing vegetables and flowers. Wood that can be used for raised bed gardens can range from expensive rough-cut cedar or oak lumber to pressure treated 2-by-12s.
Landscape contracting students designed and constructed some very substantial raised beds behind the greenhouses right behind the Tyson Building, home of the plant science department. The beds are taller than traditional raised beds, and would be great for older gardeners. To ensure you can work from both sides comfortably, construct the beds about 4 feet in width.
The main objective of raised-bed gardening is to provide optimum water drainage. Even if good garden soil is used in a raised bed, it should be amended with organic matter to ensure good drainage. If you use good garden soil as part of the growing medium, prepare a mixture that includes two parts good soil, one part peat moss or compost and one part either perlite, vermiculite or very coarse sand when the beds are filled.
If poor quality soil or no soil is available, you can prepare a suitable growing medium from equal parts of peat moss or compost and vermiculite or perlite. If you are going to fill large planter boxes or raised beds, mix the largest amount of growing medium that you can easily handle or move in a single operation. Generally a cubic yard, or 27 cubic feet, can be handled at one time.
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This amount of medium can be prepared from two 6-cubic-foot bales of peat moss and three 4-cubic-foot bags of perlite or vermiculite. The wet expansion of the peat will eventually equal the required 27 cubic feet for the cubic yard of material. Make certain to use horticultural-grade vermiculite as construction grades of this same material may contain toxic levels of potassium.
Dampen the peat and begin to mix it with the vermiculite or perlite to form a cone-shaped pile. Dump each shovel of material over the pile and allow it to run down the side to promote mixing. When the pile is roughly mixed and formed into a cone, spread 6 pounds of 5-10-10 fertilizer, 2 pounds of superphosphate and 5 pounds of ground limestone over its surface. Continue to mix the pile and form a cone until the amendments are distributed uniformly through the mixture. A mechanical mixer such as a cement mixer also produces a uniform blend of materials.
To fill small raised beds or containers, consider using a commercial planting medium. Follow the fertilizer directions on the package. If no directions are found with the commercial growing medium, incorporate a slow-release fertilizer according to the directions offered by the fertilizer manufacturer.
As the material is placed in the container or raised bed, it should be firmed to reduce the settling that occurs during the growing season. Take care not to compress the growing medium, which destroys the porosity necessary for good drainage and water movement. Fill the beds to within an inch or two of the top for some water retention during rainfall or irrigation.
Planting is much the same in raised beds as it is in ground beds, except that plants are often set in wide rows with plants spaced equal distance apart to use the full growing area. Another point to note is that in many raised beds, there is little or no natural upward movement of water to replace what is lost or used by the crop. Even in seasons with normal rainfall, it may be necessary to water raised beds to maintain a uniform level of moisture in the growing medium. The texture and quality of the growing medium will govern the amount of watering.
Bill Lamont is a professor and extension vegetable specialist in the department of plant science at Penn State and can be reached by email at firstname.lastname@example.org.