[Crowbob]

If we're talking about weeds in turf grass, the best defense is a good offense. Do whatever it takes to rid the lawn of weeds (i.e., 2,4-D, weed n'feed or whatever home recipies you choose). THEN propogate a strong lawn by letting it grow to 3-4 inches (for perrenial rye, Kentucky bluegrass and fescues). I dont know about the other wimpy grasses that grow in the tropics such as anywhere south of the Michigan/Indiana border.

Turf grasses require enormous amounts of dihydrous oxide, depending on the soil. Once the shock and awe is done, turf grasses do very well. I spend 10 times as much time and energy on maintaining the lawn than anywhere else.