Herbicides may kill the weeds, but they don't get to the root of the problem. A healthy lawn does. Read the label carefully to identify the amount of slow-release nitrogen. Feed your lawn with grass clippings. Short clips quickly decompose, adding valuable nutrients to the soil.
Tall grass encourages deep roots and also shades out crabgrass and low-growing weeds.
Let the short grass blades fall back onto the lawn. Use a sharp mower blade. A dull blade will tear grass and provide an entry for diseases. Mow the grass when it is dry. Keep the mower deck clean.