Answer: If you used weed and feed, it contains both herbicide and fertilizer. Don't put anything else on your lawn or you'll burn it. A pre-emergent will stop weed seeds from germinating but won't do a thing to the existing weeds. A weed and feed will kill existing weeds (and feed your lawn). You won't need to apply a pre-emergent for another 6-8 weeks. Read the weed and feed label to see how frequently it is recommended that you apply it. When you reach that date, use a pre-emergent instead of the weed and feed.
Best wishes with your lawn!
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