How can I use preemergent herbicides to control weeds?


How can I use preemergent herbicides to control weeds?


Preemergent herbicides are used to prevent weeds from growing. They interrupt the growing process of a newly germinating plant, preventing that plant from becoming established.  Preemergent herbicides will not impact mature or established plants, in particular perennials and weeds that grow from runners, crowns, and bulbs. 

Timing of Application is Important

Timing is critical when using preemergent herbicides to control weeds.  If applied too early, the herbicide will be leached or washed away before it can impact the germinating seed.  Applied too late, and the growing process the herbicide interrupts will have passed, allowing the plant to take root and become established.  The precise timing of application will vary by species.  Preemergent herbicides are particularly effective for the control of annual weeds because the time of year they germinate can be easily predicted.  Perennial weeds are less likely to germinate all at once or at a specific time during the growing season so these herbicides are less effective at preventing these weeds from getting established. 

Herbicides to Use

Consult the label to determine the appropriate application time for the weed species that is being controlled.  Common preemergence herbicides available to home gardeners contain one or more of the following active ingredients: pendimethalin, balan, dithiopyr, prodiamine, corn gluten meal, siduron, trifluralin, and oryzalin. Not all of these herbicides are appropriate to use in all lawn or garden situations. Herbicides must be used according to the label instructions on the package. Failure to follow directions may kill desirable plants or prevent other plants from being grown in the area.

Answered by
  • Specialist
  • Consumer Horticulture Extension
Last updated on
March 19, 2024