Answer: Crabgrass is a warm-season, annual grass, which grows best in the heat of mid-summer. It overwinters as seed and it comes up about mid-April. The turf builder you used contains a pre-emergent which will stop crabgrass seeds from germinating. However, it won't kill existing crabgrass. So, for complete control, you'll need a thick, lush lawn, mowed at a height of less than 3-inches. I'd pull or dig out the existing crabgrass and then reseed the bare areas. By digging the soil you'll remove the pre-emergent herbicide (which could keep the grass seeds from germinating).
Best wishes with your lawn!
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