"Cipher" wrote

I've settled on a pair of 2ftx8ftx2ft high boxes to start the spring. As
for the wood, I'm considering PT for the rot resistance but something
nagging in the back of my mind remembers outgassing of formaldehyde from
PT?

PT not good. Your memory is right. It has various chemicals in it that are
carcinogenic and leach to the soil in direct contact.

What a neighbor did as he wanted the looks of wood, was outer frame in PT
(for it's last) and inner frame with cement blocks. He ran black plastic
under the cinder blocks then staple gunned to the wood on the inner side.
Seems ok to me!