The products do not persist in the soil and normally the concern is from any product drift at the time of application or as vapor drift in the hours following application. Mode of action is one of disrupting cellular structure. Products could be applied anytime the weed is actively growing for both contact or systemic products. Late Summer is often a good time as the products are more successfully moved into the root system for better control. Read the entire label before you purchase, mix, apply and dispose of any product.