Answer: Redwood and cedar are the longest-lasting, but you're right they are more expensive. There are two sides to the story concerning the safety of pressure treated wood in gardens. One says they are safe, citing studies showing little or no
leaching of arsenate for years. The other side has its own studies and says it does leach at dangerous levels. So, I err on the cautious side and recommend gardeners not use pressure treated wood, especially for food plants. Try cedar, redwood, hemlock, plastic wood or cement blocks as alternatives. You could also line the inside with plastic to reduce the risk of the chemicals leaching into the soil, but that interferes with drainage.
There is a article covering pressure-treated wood in detail at the following site. It states that recent research shows that more arsenic is leaching out of the wood treated with CCA (chromated copper arsenate) than previously thought.
Concrete blocks work well for raised beds. There are also some recycled plastic ready-made raised beds, but I don't have any experience with these. My redwood beds last the best.
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