The timber yard I work at offers three treatments of which only one would
probably be suitable. The CCA (Chrome, Copper, Arsenic) is most
definately
not something I would want anywhere near my garden. There is a VAC-VAC
treatment which is spirit based which may be safe but offers little long
term protection - it just kills anything in the wood and then evaporates.
The one that may be suitable is AC500 which is a copper-chrome treatment.
We had it installed because CCA is outlawed next year. We have a
playground
company that buys from us because the treatment is supposedly safe for
wood
licking/chewing kids so it is likely to be safe for plants. Check with
whatever supplier you intend to buy from as to what treatment has been
used. If they don't know find another supplier. They may say pressure
treated ask what type, all treatmens are done under vacuum/pressure

Just remembered, we make plant pots to sell in the shop from rejected
decking and this is treated with AC500. They sell pretty well so this is a
good indication that it's suitable.