Answer: While they would certainly last a lifetime, the safety of using wood saturated with creosote is questionable. There are a number of treated landscaping materials on the market, and you're wise to be selective when building a raised bed garden. Stay far away from any timbers treated with creosote. They last forever, but can leach petroleum products into the soil.
There are two sides to the story concerning the safety of pressure-treated wood in food gardens. One says they are safe, citing studies showing little or no leaching of arsenate for years. The other side has it's 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.
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