Gosh, I'm surprised somebody didn't respond before now. We don't have any ornamental grasses, so I have no expertise to help you. However, I don't spread wood ashes around growing plants. We scatter them over the snow in the winter and let them gradually weather in.

Wood ashes do contain potassium, but they are also highly alkaline, they contain some lye, and could conceivably damage a plant before they are diluted and neutralized by precipitation.

MM