Is putting wood ash anywhere on the garden beneficial?
Yes, I put all my wood ash all round the garden.
my understanding is that the only ash "high in potash" is the ash from garden bonfires where it's mostly small twiggy wood. Someone might well have evidence to the contrary.
I stick the ash from our woodburner in the compost bins.
This might be useful:
Treated wood ( and that includes fencing timber and posts etc ) of any kind should never be burned and used either.