What’s the difference between pre-emergent and post emergent herbicides?
The difference is pre-emergent prevents weeds from germinating and emerging from the soil, while post emergent herbicide kill weeds after they’ve germinated and have emerged from the soil. Example, prodiamine is a pre-emergent and applied in early spring to cool season lawns to prevent crabgrass from germinating. For crabgrass that has emerged from the soil, a product like quinclorac is an effective post-emergent herbicide to eradicate crabgrass.
What’s the difference between selective and non-selective herbicides?
Selective herbicides selectively kill specific weeds and grasses, while non-selective herbicides kill virtually everything it comes in contact with. Example, glysophate which is the active ingredient in Round-Up, is a non-selective herbicide and will kill both nuisance weeds (i.e. broadleaf weeds) as well as the good grass (i.e. turf type tall fescue) in your lawn. A product like Speedzone is a selective herbicide, which is designed to eradicate a list of grassy and broadleaf weeds, while being safe for most cool season grass types.