Answer: Crabgrass prevention must be applied before the crabgrass seeds germinate, in early spring, as you noted. You could use a fertilizer/pre-emergent combination product. Or, you could use a single purpose crabgrass preventer such as corn gluten along with using your regular fertilizer. If you opt to use a combination product such as Scotts, you will need to read the label for application instructions. Certainly you can switch back and forth between brands, but depending on the fertilizer content of the combination product you may need to reduce (or increase) the amount of 13-13-13 you are using to avoid over or underfertilizing. You might also want to run some basic soil tests and see how much fertilizer you really need to use based on the test results. Your local county extension should be able to help you with running the tests and interpreting the results based on what you want to achieve with your lawn care program. Good luck with your lawn this summer!
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