The single best "weed" growth preventer you can use is a good, heavy mulch. I have not found a reason yet to spend my money on "stuff" that will kill "weeds". Any preemergent "weed" killer you use will stop all emerging plant growth, even those you want to grow.

Basically any preemegent should be fine. However, it won't have any effect on your nutsedge. That is a perennial weed and will come back from the "nut" and multiply quickly. You will need to either dig out those weeds or use a post emergent specifically designed for nutsedge.

IF you are going to use a chemical "weed and feed", better carefully read the label as the product will kill a lot more than weeds. Also, contrary to popular belief, St. Augustine needs 5 or 6 hours a day of sun shine. Now, before Lou starts in about shade tolerant varieties of St. Augustine, yes, there are some new strains, they are expensive and it involves resodding your lawn. To reiterate about CGM, an application of less than 20 lbs. per 1,000 sq.ft. has limited effectiveness, and a heavy rain immediately after application will render the weed killing effect nill. If your desire is to use a natural approach, you need to understand how products work and what to expect of them. Most of the grassy weeds will die out in the heat. Others will be crowded out as the grass regains its vigor as it comes out of dormancy. The best way to control a lot of weeds is with a healthy lawn. Further, I have been praticing organics for 50 years and my present property for 35 years. Don't expect a property to turn around in 1 or 2 years as it will take that long at least to get the toxics out of your lawn and garden.

Herbicide Safety and Weed Control Comparison in Spring Seeded Kentucky Bluegrass "Early spring preemergence herbicides are often necessary in Indiana to prevent troublesome annual grassy weeds such as crabgrass and goosegrass. Additionally, these applications help to prevent the emergence of some broadleaf weeds. Most preemergence herbicides work to kill weeds by preventing cell division causing death to weed seedlings shortly after they germinate. All preemergence herbicides (except Tupersan) work to prevent the emergence of turfgrass seeds as well as weed seeds, so a turf manager cannot use a preemergence herbicide if they plan on seeding in the spring. As mentioned, Tupersan (siduron) may be used for preemergence control of annual grassy weeds in newly seeded cool-season turf. Another strategy is to use a postemergence herbicide instead of a preemergence herbicide to control crabgrass and other weeds in late May and June that is safe to use on seedling turf. Options include Drive XLR8 (quinclorac), Quicksilver (carfentrazone), Tenacity (mesotrione), and SquareOne (quinclorac + carfentrazone). These products can be most safely used very soon after seeding to control crabgrass (see label for exact details on each turf species). If the seedlings are more mature (have been mown 2-3 times following their emergence) then other products such as Q4 Plus (quinclorac + sulfentrazone + 2,4-D + dicamba), Onetime (quinclorac + MCPP + dicamba), or Solitare (quinclorac + sulfentrazone) can also be used. The objective of this experiment was to evaluate herbicides at three different application timings for use in establishing Kentucky bluegrass from seed in the spring."