Creeping bentgrass (Agrostis palustris) is a thin-bladed perennial grass that is usually quite invasive. It spreads into areas of more desirable bluegrass, choking them out. Bentgrass is occasionally used as a lawn grass in states with milder, moister climates, but rarely in cool season areas. To thrive and look good it needs a very high level of maintenance, including frequent, short mowing and frequent fertilizing. A more desirable variety of this bentgrass is widely used on golf greens here and throughout the world.
Unfortunately there is no herbicide you can use to kill these grassy weeds that will not also kill the desirable lawn grasses.
Bentgrass is shallow-rooted. Patches appear as puffy, fine-textured grasses in Kentucky bluegrass lawns. You can remove patches of Bentgrass by cutting the patch out with a hand sod cutter or shovel. Cut down to at least one-inch deep. You will need to reseed the area.
One method of controlling Bentgrass is to apply a herbicide that contains the active ingredient glyphosate. These herbicides are sold as Round Up or Kleen Up. Be aware that glyphosate kills desirable grasses as well as weeds. Apply the herbicide only on the Bentgrass patches and plan on re-seeding the area.
For Bentgrass, apply the herbicide to an area about six inches or so outside the patch of Bentgrass to kill the individual stems which are creeping outwards from the patch, otherwise, these patches will reemerge. Apply the glyphosate in spring or fall when the grasses are actively growing. Wait approximately seven days, then reseed or sod the area. If you decide to till the soil prior to establishment, and see bits of Quackgrass rhizomes coming to the surface, remove these. Or wait two weeks or so until enough new Quackgrass leaves emerge and kill the new plants with a second application of glyphosate.