In article , Chris Hogg
writes
What is the best way to stop wooden fence posts rotting below ground
level? My understanding is that keeping them out of contact with the
soil helps considerably (i.e. by setting them in concrete), but would
wrapping the bases in polythene make things better or worse (allowing
for some drainage at the base to let water out)?


You will be a millionaire if you can come up with the answer ;-)

Use biocide impregnated posts.
--
martin

I believe the problem isn't actually underground but at the point where
soil, post, air and the elements meet. If you bed the posts in cement (or
maybe concrete .. I'm not really an expert in this. I've just observed
others who are!) and build up a collar round the post sloping out towards
the soil down the post the rainwater runs off the wood = it doesn't rot.
Well it does ultimately, but it lasts a good deal longer than if you don't.
Here in Edinburgh our posts have been in situ for over 10 years and there is
no sign of rot - or even deterioration - at all.
Anne