Both small- and medium-scale growers can benefit from gardening in a raised bed. Raised bed gardens not only provide a clean, organised look to a garden, they also reduce the back strain caused by frequent bending to weed. In addition, raised beds extend the growing season because the soil in a raised bed warms more quickly than the ground around it in the spring. Raised beds can be made of many materials including pressure treated woods.
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In the middle of the 20th century, manufacturers introduced pressure treated wood to the construction industry. These woods are infused with CCA, chromated copper arsenate, which helps the wood not rot when used outdoors. The copper in the solution injected into the wood acts as a fungicide and prevents fungus from breaking down the wood fibres. The arsenic kills any wood-eating insects such as termites. Unfortunately, copper is toxic to aquatic organisms and arsenic is toxic to humans and other mammals.
Removal of CCA Wood
Pressure treated wood helps reduce the strain on forests and reduces the labour and costs of having a home garden because its lifetime is longer than untreated woods such as pine. But despite some universities claims that CCA wood was safe, other scientists disagreed and the United States Environmental Protection Agency advised against using CCA wood near foods or animal feeds. Eventually, the EPA pressured CCA manufacturers to withdraw CCA woods from the market. However, because of its longevity, such woods are still available as salvage or repurposing from decks and other outdoor structures. If you aren't sure if what you have is CCA wood or not, it's not advised to reuse it near your vegetable garden because arsenic and copper leaching will increase as the wood breaks down.
Pressure treated alkaline copper quaternary lumber has replaced CCA lumber. ACQ lumber has no arsenic but instead has very high levels of copper. While this is likely safe for vegetable gardening, ACQ lumber comes with its own problems. The high levels of copper in ACQ lumber prevents the wood from rotting but it greatly accelerates the rotting of most devices, such as nails and screws, used to secure the boards together. Copper is an electrical conductor, and that electrical charge disintegrates the screws and nails over time. Using hot-dipped galvanised fasteners will reduce the problem of fastener degradation.
An additional consideration when choosing pressure treated lumber for a raised bed garden is that organic growers cannot use ACQ lumber and be accredited. Also, according to the MSDS for ACQ lumber, aside from the high levels of copper, these timbers are infused with formaldehyde. While manufacturers expect that these toxic elements won't leach into the soil in sufficient amounts to affect the food, they do add a burden to the ecosystem. If you prefer to reduce your impact on the world around you, consider using pine lumber and treating it with linseed oil, a product made from flax seeds, or a mixture of warmed linseed oil and beeswax to extend the life of your raised bed gardens.
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