Late winter to spring is the time to spray for Guilford Grass or Onion Grass as it is sometimes called. This is a persistent weed and a problem to mow. It will need to be treated over a couple of years to eradicate. Use a slasher type mower on a low setting will help to remove the flowers, but each surviving flower will bear many seeds that can last a couple of seasons in the ground before geminating. Quotes to spray will be per square meter of spray plus time.