I wouldn't worry too much about tossing ash away and losing worms, lady bugs, and other beneficial garden bugs. If the soil is not too toxic, they will reappear in no time at all. I dealt with 4 years of volcanic ash and Daisy is right. It doesn't wash away, it turns to concrete and causes all sorts of problems, like poor drainage if it is worked into the soil and plugged up pipes.
During the active volcanic years we were told by Civil Protection not to try to wash away the ash.
