I live in a townhouse with a lawn that is connected to the neighbors on
either side of me. I know for a fact that these neighbors perform no
lawn care other than occasional mowing. I, on the other hand, put down
weed & feed twice a year, overseed, and apply lime when necessary.

So guess whose lawn has been taken over by crabgrass?

The infestation literally stops at the property line. My (former) lawn
is 90% crabgrass while my negiligent neighbors have attractive grass.

The obvious conclusion is that my efforts are somehow causing/allowing
the crabgrass to flourish. I wait until the lawn is pretty high to
mow--no scalping is taking place. I have not put pre-emergents
down...but neither did the neighbors. Could the weed & feed be
weakening the grass enough that the (apparently immune) crabgrass can
take hold? If so does this mean the whole weed & feed concept is a
farce?

Thanks for thoughts,
-Kurt