Nobody's mentioned the other way to get rid of crabgrass. That's to
encourage the good grass to grow and choke out the crabgrass. So
rather than spending money on killing the crabgrass, spend the money
on getting the good grass to grow.
I believe the original poster said it was a new lawn. Just fertilize
and water and do everything to encourage the new grass to get good and
thick this year. Next year, instead of putting down a pre-emergent
herbicide, which incidently will prohibit you from putting down any
good grass seed, just look at all the thin spots, lay down some grass
seed and repeat the process.
It will take awhile, a few years maybe, but in the end you'll have a
good looking lawn.
BTW, the time to put down a post emergent crabgrass killer is when the
seedlings are real little. By the time you can see the crabgrass
shoots in your lawn its too late and the crabgrass is too established
to get rid of it. That of course means that most homeowners apply it
way too late in the season to do much good. It should be applied
before you even know you have a crabgrass problem.
Also, almost all of the post emergent stuff contains arsenic which I'm
not too keen on spreading all over my yard. Look at the ruckous raised
over CCA treated lumber and arsenic leeching from it and now you want
to purposely spread it over your yard?
If you do decide to go with a pre-emergent next spring, look at any of
the products containing corn gluten. its a natural product and
actually decomposes and fertilizes the lawn at the same time. But be
aware you wont be able to put down any new seed, so if your trying to
establish a new lawn, I'd wait on applying the pre-emergent killer and
through seeding and fertilizing, thicken up the lawn. That should
naturally choke out the crabgrass.
I'm not adovcating a total chemical free weed control program. I'm
probalby the first to reach for a broadleaf weed killer when the need
arises. But the post emergent crabgrass killers werent very effective
when I tried them and I got much better results by just encouraging
the good grass to grow and choke everything else out.
When I bought my house the lawn had been totally neglected for years
and crabgrass was everywhere. Just by fertilizing and reseeding, the
next year the crabgrass was markedly reduced and by the third year was
totally gone. What little did appear was an easy matter of just
pulling it up while mowing the lawn.