Originally Posted By: doitmyself
Rolla07 has it figured out.
Are you trying to completely kill your lawn, then start over (Glyphosate)? Or, are you wanting to get rid of the weeds and over-seed the remaining grass to bring it back to a full, lush lawn?
Up in our neck of the woods, fall is a better time to sow our cool season grasses due to its physiology. In the fall it is wanting to put down strong roots and build energy for next year. In the spring its wanting to put its energy into leaf growth (vs. the roots). Then, just as it is getting established, the heat and drought tend to make it want to shut down and go into semi dormancy. The spring weeds tend to be more ferocious too. But, spring establishment is doable, it just takes more effort.
Your land grant university, Cornell, has some good local information for you. Timing can vary quite a bit comparing northern New York vs. southern, so keep that in mind.
http://warren.cce.cornell.edu/gardening-landscape/turf
Quinclorac is both a broadleaf and crabgrass herbicide that is friendly to new grass seed sowing. You are pushing the time limit, but if you get some post emergent weed control on this week, you should be able to safely sow seed by September 15. Plenty of time for it to get started, unless you are extreme north. If going this route (over seeding), you can rent a machine that over seeds the grass seed into the existing thin turf. Follow Cornell's fertilizer recommendations to avoid having it be too lush as it goes into early winter dormancy.
If you kill the entire lawn with glyphosate, it should be dead in 2 weeks and you can resow by September 1st.
Consider core aeration before re-sowing if you have extreme compaction.
Best wishes on your fall endeavor.
Quinclorac does a good job on crabgrass but did not do much to any broadleaf.