My 2 cents worth for this problem,
If you have a lawn, which is predominantly common couch [[i]Cynodon dactylon[/i]] then the best way I have found to keep it pure, is to spray the entire lawn with Roundup, which you must use at half rates [e.g. the recommended rate is 40ml/litre] from memory, so you would use 20ml in a two litre container, along with a teaspoon of dish-washing liquid, which acts as a surfactant. Of course the rate and quantity of water you use will depend upon the size of the area you are spraying.
This you must do in early Spring, and one spray will not be enough, you will need two, or possibly more, depending on the infestation, and don't spray if you are expecting rain within a 24 hr period.
In my case, when the couch is growing vigorously, the process kills every flat weed, Winter grass, Bin-Die, Creeping Oxalis etc., and to a degree, makes the couch look a 'tad' crook, but after a couple of weeks, it virtually "jumps out of the ground" you will have a pure sward.
I too, keep notes on when and where and what was sprayed, for future reference...
