Pests in Gardens and Landscapes: Quick Tips

Weed Control Using Herbicides

Published   3/12

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Most weed problems in gardens can be managed by hand weeding, mulching, good garden design, keeping lawns vigorous and competitive, or using other nonchemical methods. However, gardeners sometimes choose to use an herbicide (a chemical weed killer) to control weeds. When using herbicides, follow label directions precisely. Otherwise products will fail to control the weeds, and they might damage desirable plants, limit your ability to replant within a preferred time frame, or waste herbicide if it gets carried away in runoff water. Follow up herbicide treatments with longer-term, nonchemical methods such as installing mulches, modifying irrigation methods, pulling weeds, or filling cracks.

Select the proper herbicide by identifying the weed and application site.

  • Be sure the label lists the weed you want to control.
  • An herbicide will kill all susceptible plants, not just weeds. Make sure the label says it’s safe to use on or around the plants in your lawn, garden, or landscape.
  • Be sure the weeds are in a stage that is susceptible to the herbicide. (See preemergent and postemergent 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.
  • Herbicides that control the germinating seeds before plants emerge from the soil are called preemergent herbicides. They won’t control weeds that already have emerged. Use postemergent herbicides to control plants that already have emerged.
  • The younger the weed, the better a postemergent herbicide will work.
Some Common Active Ingredients in Herbicides and Their Use
Active Ingredient1 Weeds Controlled Where Used Notes
Glyphosate: Nonselective, postemergent Most As spot treatments on weeds or clumps of weeds Will injure desired plants if spray gets on them.
Plant oils including clove, lemongrass. and eugenol: Nonselective, postemergent Young broadleaves In cracks and crevices or as spot treatments Organically acceptable. Won’t control older weeds or perennials. Best when temperatures are higher than 70°F.
Trifluralin: Selective, preemergent Most annual weeds Gardens and lawns Water or cultivate soil after applying. Use after garden plants are established.
Benefin: Selective, preemergent Most annual weeds Lawns Often used for crabgrass control.
Dithiopyr: Selective, preemergent Crabgrass, annual bluegrass, oxalis, spurge, and others Lawns Will injure fine fescue and bentgrass.
Dicamba: Selective, postemergent Broadleaves Lawns Controls clover and other broadleaf weeds in lawns.
Fluazifop: Selective, postemergent Grasses including bermudagrass In broadleaf groundcovers or landscape beds Apply when grass weeds are actively growing.
2,4-D or 2,4-Dichloro-acetic acid: Selective, postemergent Broadleaves Lawns Controls dandelion and other broadleaf weeds in lawns.

1 Some of these active ingredients are sold in combination with other ingredients.

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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