there was a similar post about this about a year ago, i' m not sure on which forum or website. someone that was knowledgeable said that sethoxydim (which is in Poast) will decompose after a few hours upon exposure to sunlight and oxygen in the atmosphere. so with that in mind, you would want to make sure that when it' s applied to the grass, the grass is actively growing. in other words, if you apply it when it' s cool or not sunny, the product will stay on the surface of the grass and start to decompose before it' s fully absorbed by the plant. this is a waste of time and money. you want to apply it when it' s warm and sunny and perhaps a few days after a rain, when the grass is actively growing and will absorb the product and take it all the way to the roots.