Answer: Wave petunias do not need to be deadheaded the way regular petunias do. The faded flowers will drop from the plant on their own and the plant will continue to branch and set new flowers throughout the season. They are practically maintenance free.
A pH of 6.8 is slightly acidic (7.0 is neutral; anything below that is considered acidic). Feeding with Muracid would be fine. As the roots of your astilbes grow, they will support larger flower plumes. Just be sure to keep the soil moist during the growing season. I think by next year your plants should produce larger plumes.
Best wishes with your garden!
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