How and when do you control nimblewill?
Nimblewill is a warm season grass. It's very difficult to remove it from a cool season grass because it comes out of dormancy later than a cool season turf. Fall spot applications with a non-selective herbicide, such as glyphosate (Round Up) could eliminate some of the weed. Spray 12 inches beyond the edge of the weed to make sure you get it all. This is an aggressively spreading weed.
There is a product, that right now is labeled only for golf course use, that will selectively eliminate nimblewill in a cool season turgrass. Hopefully, it will be available to homeowners in the future.