Herbicide activity is often affected by two things. First, temperatures will determine how fast the plant takes in the herbicide. The cooler the temperatures, the slower the weeds will respond to the application. In cool months, it is not uncommon to take 3-4 weeks to visually see the results of the application. In the summertime in may be seen in just a few hours. Secondly, conditions play a very important role as well. In dry conditions, the plant will go more into a dormant state and its pores in the leaf tissue will close. Therefore, herbicides will not be absorbed and no control is achieved. Once there is rainfall or irrigation, the plant opens back up its pores, and herbicides, if still available, will be taken in and do their job.