Postemergence, nonselective herbicides are classified as either contact or systemic. A Non Selective Herbicide cannot differentiate between weeds you would like to kill and the plants you want unharmed. Contact herbicides kill only the green tissues that are contacted by the spray. Systemic herbicides move within the plant from the point of application to other plant parts. There are some systemic herbicides are can move easier and further in plants than others. The Amount of Movement Systemically also depends on the season you are applying the herbicide. Therefore, to obtain optimum pest control, herbicides must be applied at the time of year when weeds are most susceptible and actively growing.