From Roch Gaussoin, Extension Turfgrass Specialist, for the University of Nebraska-Lincoln’s blog, TurfiNfo:
We are nearing the first “official” day of fall. While daytime temperatures are currently in the 80’s, nighttime temperatures are starting to decline and day length is decreasing. Summer annual grasses such as crabgrass, foxtails, and goosegrass, are still quite visible, herbicide application is not recommended in the fall – these grasses will die with the first frost.
Keep mowing sites with high annual grass populations to limit seedhead maturation and increase turf density. Winter annual broadleaf weeds such as common chickweed and henbit will be germinating soon and the window for both annual and perennial broadleaf weed control with herbicides is opening.
Fall-applied herbicides are preferred for broadleaf weed control because 1) winter annual weeds are smaller and more easy to control than when they mature in spring, 2) perennial broadleaf weeds are translocating stored energy (and properly applied herbicide) below ground, and 3) cooler temperatures reduce the likelihood of injuring turf or ornamental plants. For best control that will be noticeable this fall, herbicide should be applied by late September to mid-October. Three to 4 weeks post application a second treatment can be applied if the targeted weeds have not been effectively controlled by the initial application.
Single applications applied later in fall can still be effective if soil moisture isn’t limited at the time of application, but control may not be evident until the following spring. Herbicides are most effective when applied to actively growing weeds not stressed by extreme temperatures, drought, etc. It is also recommended that turf is not mowed within 3 days before or after broadleaf herbicide treatments. Premixed herbicides containing 2,4-D, dicamba, and/or MCPP, are effective on most winter annual and perennial broadleaf weeds.
For difficult to control weeds such as wild violets or ground ivy, herbicides containing triclopyr or fluroxypyr are most effective. Quinclorac is effective on field bindweed. If you are doing any fall renovation, including overseeding, check herbicide labels for recommended rates and intervals for applications before or after germination and establishment.