While a pre emergent and post emergent herbicide can typically be tank mixed and applied at the same time without issue, there are a few things to consider. Princep is not allowed to be applied to any grazing or pasture areas, so if this was an intended use site you would not be able to use this product. Instead you may consider a pre emergent like Prowl H20 which is labeled for grazing and pasture areas. A pre emergent is typically applied in the spring or fall in a single or split treatment application (6-8 weeks after the first) to prevent weeds from germinating. A pre emergent usually requires a higher water volume or irrigation/watering in after treatment to move it into the root zone and off the leaf surface of the existing vegetation to be effective in the soil. A post emergent is often applied with less water and should have no irrigation or rain for around 24 hours at least after treatment. These use instructions for each product can mean its often better to apply a pre and post emergent separately so as to not render one ineffective or short changed in the application process. If both Prowl H20 and Grazon are both labeled for your grass type and target weeds, you can tank mix them to apply at the same time. 

Answer last updated on: 12/08/2022