You will send more energy back into that plant sparking more blooms. Keep up on those good cultural practices too. Any leaves that have powdery mildew, a big August problem, need to be discarded into the trash, not the compost pile. Keep them away from plants they could reinfect. If the problem is showing on your plant's leaves, it is too late to treat it this season.
Good circulation is a key.
Thin out plants that are indeed too dense. Go in with a pair of hand pruners and selectively take out stalks in your clumps that will facilitate good circulation. Remember to fertilize container plants. The extra food will keep them lush through the Fall. Fertilizing trees and shrubs could be done early in the month, but don't force too much new growth with fertilizers high in nitrogen, especially late in the month.