do you have info on sources and materials used in non-toxic pressure
treated lumber?
Yes, it contains copper sulphate and borax as preservatives. Some brands
are much better than others. Do not buy yours from any of the box stores
as it will not last, especially in ground contact and may warp. I am
confident that this would be safe in a greenhouse but paint it just to
me sure the grain is sealed.
Your best sources will be custom building materials suppliers providing
lumber to upscale contractors doing high dollar work. These will be
smaller companies. If you look carefully you might find exceptional
quality NTPT lumber from box stores but don;t count on it to last long
in ground contact application. Pay attention to the weight of the wood
which indicates ratio of dark and light wood seen in the grain; you want
more dark (heartwood) and less light and the grain should be straight.