I'm a little confused as to your exact problem. Are trying to control the turfgrass or the weeds? I have no direct experience with SA, but I do know that you are very limited in the types of selective herbicides you can use on it to control weeds. If you are trying to control it's infiltration into your garden, glyphosate should work fine but you will continue to see it creeping back in from above ground stolons unless you create some sort of physical barrier or border.

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John - Zone 6

killing St Augustine grass requires you to get out the old weed wacker, and/or a very sharp spade.
this grass spreads in long runners just under the surface of ANYTHING, and will find a way to pop up their heads anywhere you dont want them, and when you try to rip them out, you wind up doing something like pulling a seam...long anchor roots that are part of the lawn itself.
Edging the beds is the key here.
Since this is about the only grass that will grow here, and really is considered a noxious weed in other areas (and a house plant some places, honest! ) all you can do is wack away, or chop down and edge.
(zone 10 b)

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the first duty of love is to listen

Hey dragon yoga! The best way that I have found to keep this grass out of the beds is to use that black edging that is plastic, the one with the rolled top. I use a flat edged shovel and chop straight down around the bed. Then I inserted the tape edge as far as it would go leaving about an inch above the ground.
Then I used eucalyptus mulch in the bed.
(I have used those edging bricks from Lowes and Home Depot to edge the bed to make it pretty, right along with the black edging material.) This seems to add som extra protection as it smothers the grass as it attempts to "jump the fence".
on that grass!
I still have to yank out a few runners though, and an occasional stalky thing will hide among the hibiscus, but this method seems to have worked the best of everything I've tried.
Hope it works for you!


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the first duty of love is to listen