When temperatures begin to warm and plants show signs of life, it's time to take action in order to maintain a thick, healthy lawn. Early spring is the best time to stop crabgrass before it germinates.
By preventing crabgrass seeds from germinating, the lawn will have a much better chance at "choking out" the weeds before they have a chance to grow.
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Early spring is also a good time to apply grass seed to bare spots and patches. However, it's important to note that using a product to prevent crabgrass germination will also inhibit grass seedlings from germinating as well. So, most recommend to either prevent crabgrass or seed the lawn, but not both if possible.
If you need to accomplish both, look for a herbicide called siduron, also known as Tupersan. This selective herbicide will prevent crabgrass germination while not disturbing the grass seedlings.