Here in Georgia we reccomend Simazine as a pre-emergent to be completed by December 15th to finish the year. that way it does its job, prevents po annua and onions. If you are doing split apps., round 1 starts up around January 20th with Pre-m or Barricade. Also, if you are doing split apps. remember round 1 does not have to get you more than 60 days. Round 2 we reccomend a higher rate of Barricade or Dimension be completed by mid-April since they will last longer and pull you through the summer.
If you are trying to do a single app. then you should go with the highest rate of dimension completed by end of March.