When to Use Pre-emergent Herbicides

Written by April Reinhardt (last updated March 1, 2023)

If you have a problem with weeds in your lawn, and realize that you need to use a weed killer, but don't know when to use pre-emergent herbicides, then read on. This will help you to understand how and when to use pre-emergent herbicides:

  • Instead of choosing one weed killer that will kill everything it touches, including your grass, choose a selective weed killer. Pre-emergent herbicides are selective weed killers. Look for generic and brand names of Oryzalin, Surflan, Preen, Trifluralin, and Treflan.
  • If you have full-grown or mature weeds, then you need to use a post-emergent herbicide.
  • Timing is vital when applying a pre-emergent herbicide. Always apply a pre-emergent approximately two weeks before the weeds start sprouting. A good rule of thumb is to apply a pre-emergent when the Forsythia blooms start to wither, or just before the Forsythia start to bloom.
  • Do not aerate or dethatch your lawn after applying a pre-emergent herbicide since doing so will render the protective barrier ineffective. If you disturb the soil in any manner after applying a pre-emergent herbicide, then you must reapply immediately.
  • Pre-emergent herbicides must be applied before weed seeds begin to germinate in the spring. You want the pre-emergent to already be in place when the soil is warm enough to cause germination.
  • Since a pre-emergent herbicide will not penetrate deeply into the soil, make sure that you apply the agent before weeds have a chance to grow their roots into the deep soil.

Always water after you apply a pre-emergent to make the herbicide penetrate the surface of the soil. If you don't water it in, then it will simply lay there on top of the ground and the weeds will continue to germinate underneath the herbicide and grow as if you never applied it. Always follow the directions on the product label, making the proper calculations as to how much chemical you should use as it pertains to the size of your lawn.

Author Bio

April Reinhardt

An admin­istrator for a mutual fund man­age­ment firm, April deals with the writ­ten word daily. She loves to write and plans to author a memoir in the near future. April attend­ed More­head State Uni­ver­sity to pursue a BA degree in Ele­men­tary Edu­ca­tion. ...


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