This extension article discusses post-emergent treatment of crabgrass.
http://www.turffiles.ncsu.edu/PDFFil...urfgrasses.pdf
Most crabgrass is annual so pre-emergent herbicides are used---especially in residential situations. The disadvantage to this approach is you have to treat all of the turf and not just areas known to be a problem. This is costly environmentally (potential herbicide overuse and runoff) and in dollars, especially for large turfgrass installations. There is something to be said for a more targeted post-emergent approach.
If you use a spray product like you selected, make sure to where a mask or aspirator.
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