It really comes down to how much fertilizer you put on at one time. Three or four applications is fine but you can put the same amount on in more applications with less applied each time. As you build a strong lawn most weeds will be choked out. In the meantime most lawn companies can apply something either to kill existing weeds (except existing grassy weeds) or to prevent weed seeds from germinating. Some weed killers can stress St. Augustine, especially in hot weather. I suggest for now you focus on good lawn care and get the turf in dense, healthy condition and your weed problems will be dramatically minimized.
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