Is this the best herbicide to control barnyardgrass in my Switchgrass, Big Bluestem and Indiangrass? Will it hurt Kentucky Bluegrass "Troy" forage variety in my lawn or other turf grass in my lawn? What will kill barnyardgrass in my Switchgrass, Big Bluestem and Indiangrass now? (post emergence August 22 in Northern Illinois)
Surflan is a pre emergent herbicide that can be used in the listed turf grass species on the product label to prevent weeds such as barnyardgrass from coming up. Since none of the turf types you listed you have are on the Surflan label we cannot say for certain if it will harm your Switchgrass, Big Bluestem and Indiangrass. For your convenience I have copied the turf label instructions for you below.
"Warm Season Turfgrasses: Surflan Pro may be applied as a preemergence treatmetn for control of annual grasses and certain broadleaf weeds in established warm season turf including bahiagrass, bermudagrass, buffalograss, centipedegrass, st. augustinegrass, zoysiagrass, adn established tall fescue growing in warm season areas. Established turf is defined as a dense turf having a well-anchored root system and healthy, vigorous top growth. "
Surflan is not recommended to use on cool season turfs such as Kentucky Bluegrass which you also mentioned you had. As far as a post emergent product to control the barnyardgrass goes, there are not any selective herbicides I can find that will control barnyardgrass without possibly also harming the other desired grass species you mentioned. You would have to spot treat the areas where the barnyardgrass is a problem with something like roundup or another herbicide labeled for barnyard grass control and then overseed with any other desireable turf in that area.. If you prefer to spot treat with something other than roundup a suitable product would be Quinclorac 75 which I have copied a link to below.
Answer last updated on: 08/22/2018