Yes, ash is actually a very good low grade fertilizer. It will not show results overnight but in the long run ash is actually very good for all plants. A friend of mine owned a lot before he built his house on it and we would go there to party, have bonfires, etc. After the house was built and sod installed, the next spring and all through summer there was a distinct circle in his lawn that was greener, healthier, and had less weed growth than the rest of his yard. This of course was where the bonfire pit was. Ash is good, but again, don't expect to see any instant, noticeable results.