Pests in Gardens and Landscapes: Quick Tips

Weed Control Using Herbicides

Published   5/20

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Most weed problems can be managed by hand-weeding, mulching, good garden or landscape design, keeping lawns vigorous and competitive, and using other nonchemical methods. Herbicides (weed killers) are pesticides designed to control undesirable plants. When using herbicides, follow label directions precisely. Otherwise, products will fail to control the weeds, may damage desirable plants, or can limit your ability to replant in that area. Applying too much herbicide in an area also wastes money and can lead to it running off site and contaminating creeks and streams. Follow herbicide treatments with nonchemical methods such as applying mulches, modifying irrigation, pulling weeds, or filling cracks.

Identify the weed and select the proper herbicide for the application site.

  • Be sure the herbicide label lists the weed species you want to control.
  • An herbicide will kill all susceptible plants, not just weeds. Make sure the label says it’s safe to apply on or around the other plants where you intend to use it.
  • Be sure the weeds are in a stage that is susceptible to the herbicide. (See preemergence and postemergence below.)

Check the label for the herbicide type.

  • Herbicides that kill most plants they contact are called nonselective.
  • Weed killers that control some kinds of plants but not others are called selective herbicides.
  • Preemergence herbicides control the germinating seeds before plants emerge from the soil. They won’t control weeds that have already emerged.
  • Postemergence herbicides are used to control weeds that have already emerged. The younger the weed, the better a postemergence herbicide will work.
Some Common Herbicide Active Ingredients and Their Use
Active Ingredient1 Weeds Controlled Where Used Notes2
Most broadleaves and grasses Many situations; for large area control and as spot treatments Will injure nontarget plants if spray contacts them
Plant oils
(such as clove, lemongrass, eugenol, and other oils)
Young broadleaves In cracks and crevices or as spot treatments Organically acceptable; works on contact with plant material; best applied when temperatures are above 70°F
Most annual weeds Gardens and lawns Water or cultivate soil after applying, use after garden plants are established
Most annual weeds Lawns Often used for crabgrass control
Crabgrass, annual bluegrass, oxalis, spurge, others Lawns and landscapes Safe for use on most turfgrass species
Broadleaves Lawns Controls clover, may harm roots of certain ornamentals
Grasses including bermudagrass On broadleaf groundcovers or landscape beds Apply when weedy grasses are actively growing
2,4-D or 2,4-Dichloro-acetic acid
Broadleaves Lawns Travels through the plant’s system to kill the entire plant
1Some of these active ingredients are sold in combination with other ingredients.
2 Always consult the herbicide label for precautions and specific instructions.

Minimize the use of pesticides that pollute our waterways. Use nonchemical alternatives or less toxic pesticide products whenever possible. Read product labels carefully and follow instructions on proper use, storage, and disposal.

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