Pre-emergent

• Pre-emergent weed killers prevent seeds from germinating, and, if they do, from sending out strong roots. According to the Purdue University Turfgrass Science Program, lawns in southern Indiana can be spayed for crabgrass as early as the first week of March and up to April 1 for effective season-long coverage.
  
  

Post-emergent

• Post-emergent chemical control is best done when the temperature is below 85 degrees Fahrenheit. The day should be clear and with low humidity, with the area well-watered.
  
  

End of Summer

• After mid-July it is best to just leave any large amounts of remaining crabgrass alone to eventually die with the frost. Small patches can be dug out by hand.