It is not too late to spray weeds and you are correct in that Weedmaster will knock down current weeds and Preen is a weed preventer but before you both products in one area consider if you would like to grow something in that area in the future or if there are plants you like nearby. Using both Weedmaster and Preen will damage some of the micro-organisms in the soil. A better solution may be to spray the weeds, let them die, then lay down newspaper with a few inches of mulch on top to keep any weeds from coming back. The newspaper and mulch will decay and add some organic matter to the area. Not sure if you are talking about an area with trees and shrubs or an area like a gravel driveway that you never want plants to grow. Marianne Binetti