Woody and brushy leafy perennials will be starting to drop sap now to store energy for the dormant season of winter. Hit 'em now while they still have green leaves.
I know all the hrbicide specimen labels say to spray when the plants are actively growing, and many times, that is good advice, but it is not if all you are doing is killing off the leaves and not hurting the root system of a perennial.
And, I always keep in mind what an honest herbicide rep from one of the major chemical makers once told me.
"--- isn't in the business of killing plants--we're in the business of selling herbicide. The more you have to use and the more often you have to use it, the more we sell and the more money we make."