Can you use pressure treated lumber on a garden
Thank-you for contacting Toronto Master Gardeners with your question.
Please take a look at the following article published by the Toronto Master Gardeners:
Another article, published in Fine Gardening, has a broader discussion of the topic:
Essentially, the experts referenced here suggest that the original concern of arsenic leaching from the wood are no longer an issue because pressure treated wood no longer contains arsenic. Current treatments do contain some copper compounds, which could be problematic though much less toxic than arsenic. One expert consulted for the article says that
if you already have the older, arsenic-treated wood in your garden, don’t panic. Plants will not take up arsenic unless the soils are deficient in phosphorus. That is not a problem for gardeners who use compost generously. As for the new copper-based wood treatments, Brown believes the actual risk is minimal. First of all, if plants take up too much copper, they will die before a gardener can eat them. In addition, if homegrown vegetables make up a small percentage of the diet, exposure to any metal taken up is insignificant. Do not use copper near ponds and streams because it is toxic to aquatic life.
Even with this somewhat reduced risk, however, you may wish to consider using alternate materials if the structure you are building is associated with a vegetable bed.
Here is the link to the whole article:
Best of luck!