An untreated wooden box is an excellent container for edibles except for the fact that, given time, it will rot. Slap on some stain or preservative paint, and things start to get complicated. Many wooden containers sold to gardeners are pressure treated to increase their lifespan and ability to stand up to moisture. The chemicals forced into wood during the pressure treating process (copper, chromium, and arsenic—also known as CCA) arrest decay and generally don’t leach into soils much after the first initial rain.
Arsenic isn’t something you want to build up in your garden soil, however. To minimize any chemical movement, scrub or power wash wood prior to usage. Older, salvaged wood containers like whiskey barrels or window boxes have also usually been treated with CCA.
If you’re working with a more fragile, antique container, add a plastic liner before planting to keep any residual leaching in check.
If you choose to decorate your own wooden container, select paints, glazes, or oils that are marked with a nontoxic label. Prior to the 1980s, use of lead paint was widespread—particularly on outdoor surfaces exposed to wind, sun, and rain. Lead paint will chip and flake as it ages, letting it dissolve into soil, so a good way to assess whether that antique bucket has been coated in it is to look for lead paint’s characteristic cracking pattern.
To go completely au naturale, use wood that is rot resistant and requires no treatment at all: Cedar is the perfect pick.
Related: The Most Toxic Stuff At The Garden Center