Some very selective herbicides may kill only certain plants in a group, such as crabgrass killers on lawns. One reason is the safety of beneficial insects. Another reason is to avoid unnecessary non-point source pollution. This is the long term, slow accumulation of residual pesticides that accumulates in soil or ground water. Finally, the right product used at the right time, and in the correct amount, is effective, dollar-wise and more environmentally friendly.
Diagnosis is the first step in eliminating the cause of the plant problem.
Then select the appropriate method or methods of control. If the chemical control is selected, make sure to select the right pesticide for the pest problem.