Celsius does OK in full sun, high heat, just really slow. Have to be careful about concentrations too, max can cause the St. Agustine to shock and go into decline with the unintended consquence of the broadleaf taking over again. I know this because I was agressive with an application on my own lawn I've always had excellent results with Blade & Quicksilver, but the stuff is so expensive. I'm afraid of 2-4-D in St. Augustine personally, although Ric is usually right on the money. Sure wish I could get some 40% Atrazine.