Ciscoe Morris, Seattle's Channel 5 NBC gardening guru suggests spot
spraying with straight vinegar. Especially on a hot sunny day. Within in
days you can pull those dead grassse up, stir up the soil, add a little
more lawn soil and reseed. Early spring and fall best time to do this.
It works on weeds and grasses coming up in cracks on the sidewalk. I
have a 20 ft or longer area covered with lava rock that abuts next to
the street. I spray the length of it with straight vinegar every now n
then to keep weeds down. This won't kill any seeds weeds have left
behind so have to keep at it.