Woodash only, don't use it if there is coal ash mixed in.
Woodash contains potash and trace minerals, so is good for building strong roots in plants. I mainly use it as an additional fertiliser to my potting composts, especially for bulbs. I also use it in the vegetable garden, eg for the brassicas. It can raise the pH of the soil so I don't use it near ericaceous plants.

I use it mixed with garden compost to mulch around tomatoes, building up the level around the feeding roots that form on the lower stem. It was a tip from Harry Dodson on Wartime Kitchen and Garden!

Thanks, I have now somewhere to put my heap of wood ash.