Before we say anything else, it is important to note that it is difficult if not impossible for weeds to invade dense, healthy turf and so they are seldom a serious problem on well-constructed and properly maintained lawns.  Weeds can be troublesome on neglected areas or lawns on poor soil. Although hand pulling is practical when weeds are sparse, high weed populations require chemical treatment. If weeds continue to invade after proper chemical treatment, some other facet of maintenance is at fault and steps should be taken to remedy the failing.

Chemicals used to control weeds in established lawns are called herbicides and are applied as preemergence or postemergence treatments.  Preeemergence herbicides are applied before weed seeds germinate and as the the seeds or seedlings begin growth they absorb enough chemical to kill them. Permanent lawn grasses are relatively unaffected. Postemergence herbicides are applied after weed seed germination when the weeds are in active growth, and such formulations do not greatly affect affect lawn grasses. Some chemicals are selective in that they kill only broad-leaved plants but not grass.  Others are nonselective and kill both weeds and lawn grasses.

• Correctly identifying the weed and then selecting a proper chemical for its control are crucial.
• Timing is also important. For example, dandelions may be controlled in midspring and late summer in Ontario because they are growing actively and are very susceptible to selective chemicals.
• Choose a clear day with little or no air movement and temperatures of 15-25 C degrees. Such weather permits rapid absorption and uptake of the chemical with little chance of drift or spread beyond the treated area.
• Do not apply herbicides during the excessive heat.
• Follow exactly the directions on the package regarding dosages.
• Keep all containers out of the reach of children and pets.
• Wash your hands thoroughly after using the chemicals.
• Do not use the same sprayer for different pesticides because it is virtually impossible to rinse out all traces of toxic herbicides; use separate sprayers and label them clearly.