How long after pre/early post emergent should I fertilize?

Well with application of Weed & Feed products one is applying herbicide and fertilizer simultaneously, so it stands to reason one can apply fertilizer immediately before or after lawn herbicide application. There is no waiting period - they do not affect each other - unless specifically stated on the products’ label.

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The 15-5-20 I bought said it’s needs water right after, figured that would effect the weed killer I put down.

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Fertilizer can be watered in with a day, two or 3 or more post application. Most post -emergent herbicides are “rain fast” within 24 hours, many within an hour or 2.