Question
I must have the absolute worst lawn in our neighborhood (central Ohio). We bought the house three years ago and it had sat empty for almost two years prior to that.  The first year, the grass had obviously done by a lawn care company because there were no weeds (even though everything turned brown and dry by mid-July.  This year I waged war on the whole mess: I thatched it, watered profusely, over-seeded, and kept it neat as a pin while it was growing lush and beautiful in early spring.  Then BOING!  Within two weeks, it grew a great crop of goosegrass, nutsedge, clover, and other eyesore weeds that not even I can identify.  I am an avid gardener and I grow mainly my own perennial and annual flowers, and even though I am not a lawn expert, I am not a simpleton with it, either.  But for the life of me, I can't understand what is going on.  This is the third year I have wanted to just set a lighted match onto it, but alas, that's not going to please the neighbors.  With all your expertise out there in the cyber world, someone give me some sage advice (no pun intended).  And please tell me if I am wrong, but cound this lawn soil just be terribly compacted?  Any cheap way to remedy it before I call in the big guns?  I don't have a whole lot of money to sink into it with the economy the way it is, but I also don't want to lose my mind over it looking so badly.  HELP!  And thanks!

Answer
I am not sure but it sounds like this problem has occured before; only this time you have noticed it and want to remedy it. Take a calendar and try to plot the things you did to the lawn and when you noticed the great upsurge of weeds. I am going to make an assumption which might not be entirely fair to you and I apologise beforehand.  You noticed the weed upsurge about two weeks after they emerged. Locate that date on the calendar and work back to ten days before that. That was the time to put down a pre-emergent weedkiller. I am guessing that would have been about the beginning of April. But that is for next year. The best you can do now is to spot spray the weeds with a systemic weedkiller like glyphosate. Add a dye to the spray mixture so that if you over-spray the desirable grass you will be able to prune away the sprayed part. You can use a contact chemical which is safer in that it will kill only what has been sprayed; it will not translocate throughout the whole plant.
Please do not lose your mind over anything because you will need it to find it.