Question
I am in Dallas TX and accidentally used Preen Weed Preventer for southern gardens on my St. Augustine grass.
Active ingredient: dithiopyr. Should I be worried?  What action should I take?

Answer
Dithiopyr acts by preventing root growth and so it will prevent SEEDS from germinating or growing. We have to conclude that it will inhibit root growth on the St. Augustine turf also. These chemicals do not usually move in the soil so their effect can persist for a long time.
On the other hand, healthy St. Augustine will have many, many roots which will help it through these tough times. You can help by keeping it nourished by foliar applications of fertilizer and by adequately watering - without over doing either one.
Please be careful that you do not violate state or local regulations (watering and fertilizer restrictions!!)and always follow label directions; it is the law.