This year, my lawn has become overrun with crabgrass and other nasty
weeds. There are areas that are mostly weed, and areas that are mostly
grass(bluegrass/rye). This has not occurred in the 7 years I have been
here. Anyway, I would like to take some action now if possible. I have
a 100x100ft lot with my home on it.
The beginning of the ideal time to seed is still 2+ weeks away. I'd
treat the weed problems now and then over-seed if necessary in 3-4
weeks. If the lawn has been doing well until now, you may not even
need to do the seeding. I'd use a tank sprayer and treat the broadlead
weeds with a product like Weed B Gone and I'd use Acclaim on the
crabgrass. Don't apply the broadlead weedkiller on the crabgrass with
the Acclaim, it will reduce the effectiveness. Also, these work best
if the weeds and grass are actively growing, ie, you don't want it
extremely dry. If you do this now, you can do any overseeding in 3-4
weeks. If you want the option of overseeding, then I'd apply a starter
fertilizer in early Sept. That will get the existing grass going too.
As Doug suggested, cutting the grass high in the future is a good
practice. But when you're over-seeding, you want to cut it relatively
short before seeding.