I don't pre-em grow in eiither. When pulging St Augstine grass we just let the weeds go until they need post-em. Pre-em will slow grow in. St Augstine grows from runners and if the runner can not put down roots they don't grow as fast. I only use Pre em on Crab grass or real problem areas. Like Stone says a health turf is its own defense against weeds. If I have a nice where weeds can get in I try and plug that area. Of course warm season turf is different than cool season turf but I think the theory is the same. I do pre-em my planting beds if they are real wet and I have a problem with weeds.
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