Tall fescue is a wide-bladed clump grass commonly used in pastures. In a lawn, the textural difference of fescue from bluegrass is obvious because of its clumps. This interruption in uniformity makes us think of fescue as a weed.
Tall fescues have been the subject of much improvement and breeding. The resulting turf types make a good lawn if they comprise 100 percent of the lawn. However, when the pasture-type tall fescues invade a bluegrass lawn, you may note that the clumps are tougher and the lawn mower has difficulty cutting them. The tall fescue clumps often green up earlier in spring than the surrounding bluegrass. Clumps may become two feet in diameter, and may coalesce with other nearby clumps. When this occurs, there are several control options you can try.
You can dig out the clumps, but ensure that all tall fescue roots are removed. Then re-seed or re-sod after removal.
You can spray clumps with glyphosate, sold as Roundup, as they first green up in spring. The bluegrass may be relatively unharmed if it has not yet started to turn green. When using any pesticide, read and follow label directions carefully. Re-seed or re-sod after removing dead fescue clumps.
You can spray fescue clumps with glyphosate anytime they are green and growing, but this also will kill the intermingling bluegrass. If you spray the chemical in a rectangular pattern, you can easily cut out the resulting dead grass, including fescue, and replace it with a new roll of sod.
A restricted use herbicide called TFC, or Tall Fescue Control, can be applied by a lawn service. It can selectively kill fescue without harming the bluegrass. It may be slow-acting and seems to work best when applied in fall.