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How to Manage Pests

The UC Guide to Healthy Lawns

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

Statewide IPM Program, Agriculture and Natural Resources, University of California
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