I recently bought some pressure treated yellow pine for building some window boxes. After building them, I sealed them with a product that would protect the wood for 3-4 years, according to the can. After building them, they sat outside under a covered porch for about a week until I installed and filled them. When I did, I noticed that the boards are pulling the screws out of the wood. The boards were straight when I bought them but have now begun to curve. Should I have purchased a different type of wood? What could be causing this? The damage is so bad, the boxes may not last the season.
Treated wood doesn't always twist/warp/shrink, but in my experience more often than not it will to some degree. For something small like window planter boxes etc. I would use non-treated lumber and stain or paint with a quality exterior grade product. Obviously if the project is a deck or similar structure subjected to the inevitable wrath of Mother Nature then you should always use PT. Or as Kris suggested you could elevate and stack PT lumber to dry under controlled circumstances to mitigate warping.