Timothy Murphy wrote:
Do all wood preservatives damage plants?
And what about (oil) paint?

When properly dry and hardened they shouldn't do any harm, especially
modern ones, which have to be formulated so that teething babies aren't
harmed by noshing them.

The "expert" at my local Builders' Provider
asserts that all preservatives are bad for plants ...

I'd guess that your 'expert' is mostly right, so if you are using (say)
tanalised wood, or anything that's creosoted or tarred, put a layer of
black polythene (builders' membrane, not dustbin bags!) between the wood
and the timber.

--
Rusty