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Herbicide classification


Herbicides are classified by which types of plants they affect, how they injure the plants, or when they are applied.

The choice of herbicide depends upon

  • which weeds are being controlled
  • whether weeds are being controlled preventively (preemergent) or after they have appeared (postemergent)
  • the turf species

For common herbicides labeled for use on turfgrass refer to:

Types of plants affected

Selective herbicides

  • Kill target plants (weeds) without damaging desirable turfgrass species
  • Are toxic only to certain plants or weeds; for example, 2,4-D selectively kills only broadleaf plants and not grasses

Nonselective herbicides

  • Kill all or most vegetation including turfgrass
  • Should be used only prior to planting a lawn, during renovation, or as spot treatments

How herbicides injure a plant

Contact herbicides

  • Cause localized injury where the chemical comes in contact with the plant
  • Are most effective when applied to small young weeds

Systemic herbicides

  • Move within the plant, causing injury throughout the plant
  • Can control older weeds

When weeds are being controlled

Preemergent herbicides

  • Are applied before weeds emerge from the soil
  • Kill weed seedlings as they germinate and try to emerge
  • Are primarily used against annual grass weeds such as crabgrass and annual bluegrass
  • Are also effective against many broadleaf weeds

Postemergent herbicides

  • Are applied after weeds have emerged from the soil
  • Control actively growing weeds
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