On Sept. 27, 2018, we conducted a backyard farming field day at our main campus in Ardmore, Oklahoma. This educational event was designed to acquaint participants with tools and techniques they can use in a backyard setting to produce their own food.
One of the key components on display was the raised garden bed and container exhibit, which features beds and containers of all shapes, sizes, composition and pricing for both kit and home-built (DIY) structures. Some structures are commonplace, having been used for generations. Others are novel designs. Participants were able to examine these growing structures and learn about construction and maintenance requirements.
We have constructed roughly 30 beds or containers and have plans to construct many more as resources permit. It is my opinion that all of the beds and containers we are currently evaluating have the potential to produce bountiful vegetable crops, if properly managed. However, some are more difficult to manage because of design. This is also true when it comes to durability. Some growing structures are more prone to structural failure based on composition and design.
Based on many years of raised bed and container garden construction and growing experience, I offer the following comments and recommendations.
One of the topics that often confuses new gardeners is the difference between a container and a raised bed. Many of the display structures look like beds but are technically containers because they have bottoms. A bed may contain a different soil type than the soil it rests on, but because it is bottomless there is no barrier to prevent the movement of water out of the bed. This is an oversimplification of the process as soil depth, soil type, pore space and capillary action all contribute to drainage. Garden soil may be used successfully in a raised bed but should never be used in a container due to drainage issues. A soilless mix (potting soil) is recommended for container gardening because of superior drainage characteristics.
For more information on growing mixes for container production, refer to my Noble News and Views article, “Container Gardening: Here’s What You Need to Know” at www.noble.org/container-gardening.
As a rule, kit structures lack durability. The kit structures we demonstrate range in price from $200 to $500 depending on size and composition. Most of the structures are wood-framed with a few made of vinyl or recycled plastic panels. Many components are undersized to keep construction and shipping costs to a minimum. Also, much of the hardware used to connect the structure is undersized or there is not enough to ensure a good connection. Consequently, side panels bow out, come lose or break apart, and screws pull out. This is especially true with the wood-framed structures that come equipped with boards often less than one-inch thick.
Kit structures might look great while viewing online but fall short of delivering in the garden. This is not to say there are not good kit structures on the market, but be ready to pay far more than $500 for a durable one with decent size.
Plan on spending a little extra money to coat raised garden bed liners prior to using. Wood bed liners are susceptible to rot and metal liners to corrosion. Using pressure-treated wood or a rot-resistant wood such as cedar is a good first step, but additional protection is recommended to extend the life of your structure.
Preventing water absorption is the key to extending the service life of wood. To seal out moisture, consider applying several coats of a rubber-based sealant to the interior surface of all wood liners. Commercial oil-based wood sealants and preservatives may be used on exterior surfaces but should not be used on interior surfaces due to potential health risks.
Galvanized, corrugated sheet metal roof panels are commonly used as a liner. If exposed only to rain water, these panels can resist corrosion for decades. However, the service life is greatly reduced when in contact with soil. Without additional protection, these sheet metal liners can rust out in as little as five years. Moist soil is much more corrosive than water because fertilizer salts (electrolytes) in the soil speed up the rate of corrosion. A coating is recommended to prevent contact with the soil solution. Consider applying several coats of a rubber-based sealant to the interior surface of all metal liners.
Concrete blocks and landscape wall blocks are excellent building materials for use in raised garden bed construction. Both may be loosely stacked using an overlapping pattern for maximum stability.
To minimize shifting, the first run of blocks should be set on a packed gravel base. It is tempting to use mortar on block walls to prevent block slippage, but this should not be done unless a concrete footing is used. Without a concrete foundation, the soil is prone to shift, causing the wall to crack. When using hollow core blocks, it is best to dry stack the blocks and use sections of rebar or steel pipe “pins” inserted through the core openings and into the soil to limit block movement. Once pinned, the cores can be filled with sand or gravel.
When selecting a style of growing structure and composition, you should consider the initial construction cost and the maintenance cost.
One material we have found that offers both a low initial cost and a low maintenance cost is a discarded tire. As part of our raised garden bed and container exhibit, we display multiple growing structures made from used tires.