Answer: Crab grass is an annual weed so it regrows from seed each year. One way to limit it is to use a pre-emergent product such as corn gluten. This prevents the seed from germinating. Since the seed germinates very early in the spring, it has to be applied about when the forsythia bloom in your area. If the forsythia have already bloomed, it is too late for a preemergent this year.
In my experience the best way to control crabgrass is to maintain the lawn in a way that keeps the lawn grasses healthy. That would include checking the pH to make sure it is suitable, fertilizing if soil tests indicate that you need to, and most of all mowing correctly.
Most lawn grasses should be mowed to about two and a half inches tall and at that height they will shade out weed seedlings. Proper mowing also helps to keep your lawn grass thick -- you need to mow often enough that you do not remove more than about one third of the height of the grass when you cut it. In spring that can mean mowing twice a week.
Your local county extension and garden center can help you with soil testing and interpreting the results. They should also be able to help you develop a lawn care maintenance plan. You could also look at a book or two about basic lawn care, one I really like is Lawn Care for Dummies. It includes detailed practical advice on how to check your soil, fertilize, mow, patch bare spots, and reseed. Good luck with your lawn!
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