Yes, RoundUp is a tank mix suggestion with Pendulum AquaCap Herbicide to control everything already growing in an area.
Yes, Pendulum AquaCap Herbicide is a pre-emergent that requires waterin in after application to get down into the soil to create the barrier. According to the Label, The efficacy of Pendulum AquaCap will be improved if the application is followed by 1/2 inch of rainfall or its equivalent in sprinkler irrigation. It is best to do this within 7-10 days of application if not right after application.
