I keep a separate pile of pressure treated wood, old wood from construction and other sundry "I don't know what's in it" stuff, along with old vines, branches, weeds that may have been sprayed with insecti-herbi-or-fungicides.
Otherwise all my burn barrel and wood stove ash goes on the gardens, bushes and fruit trees, with a bit saved over for the chickens in the summer. It makes a great dust bath for them when it isn't going to rain right away.
