I recently filled in some low spots in my lawn with soil and seeded. The grass I planted has now sprouted a lot of (I think) crab grass. Do I need to rip up and re-seed? Or, would a simple chemical treatment work? It is pretty widespread in the areas I had new soil, so pulling it would be laborious but not impossible.
I would wait until next spring an put down pre-emergent crabgrass herbicide. It will not affect perennial grasses.
Very often the best tool to remove weed from your lawn is not to try to fight the weed, but to help the lawn grass to outperformance the weed.
I can personally confirm moving the lawn frequently and close to the ground effectively reduces the crap grass, because it is then left without any green parts.
