Chris Hogg wrote:

Hello Chris

CH What is the best way to stop wooden fence posts rotting
CH below ground level?

There isn't one, sorry. Wood rots in contact with water.

Untreated softwood lasts 5-10 years. (Dependant on soil conditions)

Treated, 10-20 years. (Larch can do this untreated)

Some hardwoods (split chestnut, oak, willow) will last 50+ years
untreated.

CH My understanding is that keeping them
CH out of contact with the soil helps considerably (i.e. by
CH setting them in concrete), but would wrapping the bases in
CH polythene make things better or worse (allowing for some
CH drainage at the base to let water out)?

No, it merely traps the water against the wood. The best idea is to
either accept that the wood will rot, or understand how rotting works
and work to avoid it. Wood rots when in constant or regular exposure
to moisture. Get that moisture away from the wood ASAP and it'll last
longer.

Metposts can help, but themselves trap water where they touch the wood
and I don't think they're really a good solution since their contact
area is so small the wood tends to snap off when rotting starts before
it would if it was planted. Possibly sealing the joint with mastic
might help, but some is bound to get in through the wood itself and
you've just stopped any chance it had of evaporating, so possibly
not...

If you use concrete, flaunch the top to shed excess water and ensure
the base of the post is on stones to aid free drainage. It'll still
rot though. Ramming stones in dry has to be the best way to avoid
rotting.

Split chestnut lasts for ages, but looks very rustic and hard to come
by unless you know a forestor who still knows the art. (All done by
hand, I don't know of any retailers) Willow also lasts for a long
time, but has the habit of starting to grow again if you use it green
in wet ground. Oak is superb but expensive.

If you use tanalised softwood (as I do), never cut it below ground
level as you then compromise its protection. (ie, if you cut it off
for height, do it from the top, and angle the cut so it sheds water -
retreat (not as good as tanalising, but better than nowt))

Plastic - don't even think about it. Expensive and not UV stablised so
instead of rotting they just go brittle and shatter.

--
Simon Avery, Dartmoor, UK Ý http://www.digdilem.org/