Raised Bed Construction - February 9, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
The rockiness and steep slopes of the Verde Valley’s upland areas can make it very challenging to grow annual flowers and vegetables. Where soils do exist, rocks can be removed and the soil amended, but often, the amount of rock or the presence of shallow bedrock make this very challenging. Here, gardeners have little other choice than to use containers and/or raised beds. Containers vary greatly in size, materials, cost, and provide a limited amount of growing space. For dedicated gardeners without a natural garden plot, constructed or walled raised beds are often the best option.
Although there will be initial expense and labor in constructing walls for raised beds, the finished product should last for many years and allow for deep soil amendment. The choice of framework for walls depends on the availability and expense of the construction material, as well as the desired appearance of the final product in the landscape. Naturally rot-resistant lumber, such as redwood or cedar, may also be used. If you have an ample supply of native rock onsite, it can be dry-stacked or mortared together. Other possibilities include concrete blocks, bricks, or synthetic lumber made of recycled plastic.
National gardening publications have raised concerns about the safety of using treated lumber for raised beds in food gardens. The US Environmental Protection Agency phased out the consumer/residential use of lumber treated with arsenic containing compounds in 2003 and those treatments have been modified to use copper containing compounds. After reviewing the available information, I would recommend that copper treated lumber be avoided for use in raised beds used for food production. Similar concerns exist for creosote and the use of railroad ties for use in raised beds for food production. Here, it is thought that older railroad ties can be used. However, injury may occur if ties are still oozing black, sticky creosote or smell intensely. Here, you may attach a heavy plastic liner between the creosote and soil.
Raised beds can vary depending on the gardener's goals and the topography. Hillside terraces may require different bed dimensions than those used for flat-land vegetable gardens. On hillsides, follow the contour of the land and adjust the depth of beds according to the slope of the hill. On flat ground, you can be more creative. Make sure pathways are wide enough to allow equipment and/or wheelchair access depending on your goals and needs. My garden is on a gentle slope and uses a 25’ x 18’ perimeter wall of dry-stacked granite that was gathered onsite to create one large terrace.
A convenient width to use for raised beds is 4 feet. Here, the center of the bed is accessible from either side. Lumber for constructing beds is readily available in 4-foot length multiples, minimizing the amount of sawing necessary and the amount of waste produced during construction. If the bed is accessible only from one side, limit the width to 3 feet. Most gardeners find it uncomfortable to reach farther than 3 feet to tend the bed. Bed height can vary, but 18 to 24 inches is fairly common. The length of a raised bed is not critical. However, you should divide long distances into shorter beds (24 feet is probably a good maximum length). Three foot wide walkways are usually adequate. These can be mulched with wood chips to minimize weeds.
After you have built your raised beds, you are faced with finding suitable soil to fill it. This can be a difficult decision and quality topsoil may be challenging to locate and expensive. Bagged products are expensive and often contain lots of organic materials and lack actual mineral soil. This is an important decision. Rather than explaining any further here, you may read my column titled Importing Topsoil from March 21, 2001.
Remember to construct enough bed space to allow for crop rotation. Crops of certain plant families need to be rotated with crops in other families to avoid increasing the incidence of disease and other pests. You cannot have one small raised bed filled with tomatoes every year without eventually running into disease issues. For additional information about raised beds, I have included links to several publications on the web version of this column.
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(Christopher J. Starbuck, University of Missouri Extension)
Building a Raised Bed Garden
(Priscilla Files, Michael Arnold, Douglas Welsh, and Frank Dainello, Texas A & M Cooperative Extension)
Garden Use of Treated Lumber
(Richard Stehouwer, Penn State Cooperative Extensione)
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Last Updated: January 31, 2011
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