A couple of decades ago, lumber impregnated with chromated copper arsenate (known as CCA) was considered the answer to a gardener’s prayer. It boasted longer life than rot-resistant species like redwood, you could buy it almost anywhere, and manufacturers said the treatment chemicals, though toxic, safely stayed put in the wood. The main plus for gardeners was that the chemicals didn’t harm plants, unlike creosote and pentachloro­phenol, two previously popular wood preservatives.

But then word started seeping out that those CCA chemicals weren’t so perfectly bound up after all, that some of them, in fact, migrated from the wood into the surrounding soil. And that’s when pressure-treated wood moved into the hot seat.