In portions of low lying areas of my st Augustine grass this brown patchy area and its barren n drought conditions.

dead aster?

yeah thats roadside aster. Just pull it.

So how does that get into my yard?

Wind.

CAR96 said:

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So how does that get into my yard?

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Thick grass is going to be your best defense against weeds. "Barren in drought conditions" is going to lead to weeds regardless if you're putting out any kind of a preventative. Work on thickening everything up, put out some pre-emergents if you want and you'll have less weeds. Aster is pretty easy to control with pre-emergents.

I have been trying to get it to thicken up. I had it aerated, seeded with a bermuda seed and fertilized around October 2023 but there was not much growth to last. That area holds water and is part of a natural drainage flow path.