Is it safe to use pressure-treated lumber for raised garden beds?


Is it safe to use pressure-treated lumber for raised garden beds? If not, what are the alternatives to help prevent the wood from rotting? For example, a plastic lining does not seem to me to be a good alternative.


Jonathan Foster, Home Horticulture Outreach Professional

It’s a complicated question because the fact of the matter is that wood is eventually going to rot, and there are several factors at play here, generating a spectrum between “no treatment, but shorter-lived” and “treated with potential toxins, but longer-lived.”

Pressure treatment has definitely come a long way in the past few decades, and products are significantly safer now than they were. Studies have definitely shown some leaching of ingredients from this wood into garden soil and onto foliage (most likely from dust kicked up from the soil), relatively little in plants themselves (many of the most concerning elements aren’t easily taken up by plants), and very low concentrations in any event. The primary concern seems to be direct contact (dermal, inhaled, and/or ingested accidentally) between a working gardener and the soil, as opposed to contaminated edible plant material. But the risk in both cases is certainly not zero. So answering whether they are safe is something of a judgement call by the informed homeowner who has looked into the topic and evaluated her own tolerance for risk.

You can read up on the history of pressure treatments, and how this impacts use in raised beds for the gardens, in this fantastic piece from the Iowa State Univ Extension. They do discuss mitigating strategies such as protective gear when installing and sheet plastic barrier as a lining.

However, my recommendation is that you use naturally water- and rot-resistant woods such as untreated hemlock or cedar, which last significantly longer than other woods and avoid the contamination risk (my own beds are made of hemlock). They do eventually succumb to breakdown from the elements, but not for several years, and the process of replacing them is typically manageable by the home gardener–they don’t all go at once, so you should only have to replace the occasional piece or two every few seasons. Mine are at least 6 or 7 years old now and I’ve not bothered to replace even the couple of timbers that are looking a little worse for wear. They’re all still completely structurally sound–once a raised bed full of soil gets established into place, even losing a board entirely rarely has much impact on the whole thing.

Happy gardening.