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Pre Emergent vs Post Emergent Weed Killer

Posted by Nate Bahler on Sep 14, 2022 10:45:08 AM
Nate Bahler

Weeds can be a year-round problem.  Certain weeds are more prevalent in certain seasons and, therefore, require different approaches.  For this reason,  there is a place for both pre-emergent and post emergent strategies in the lawn care industry.

What’s the difference between pre-emergents and post emergents?

Pre-emergent weed control is a proactive approach and post-emergent is a reactive approach.  

Why do we want to manage weeds? 

Not only are weeds an eye sore, they rob your healthy lawn of essential nutrients and water.  They compete with your healthy grass for air, nutrients and water. 



Pre emergent Weed Control:

  • Kill weeds BEFORE you see them
  • Inhibits germination & sprouting
  • Used during spring
  • Applied before weeds grow
  • Provides a barrier around seeds, preventing them from germinating. 
  • Proactive approach

Pre-emergents don’t work on all weeds but they do work very well on Crabgrass.  For that reason, pre-emergents play a large role in lawn care programs in the Northeast.  Pre-emergents work by creating a barrier near the surface of the lawn that prohibits seeds from germinating.  They do this by inhibiting a particular enzyme. Pre-emergents are engineered to not affect already established grass plants. 

When Should Pre Emergents be applied?

Generally speaking, pre emergents should be applied in Spring.  The optimal timing, however, is based on soil temperature. 


Post Emergents Weed Control:

  • Kill existing weeds by traveling down plant stalk into the root system
  • Weeds must be actively growing for post emergents to work
  • Used once spring has gotten under way

There are 4 kinds of post emergents:

  1. Systemic post-emergent: absorbed into plant and kills from root up
  2. Contact post-emergent: leaves coating on plant to prevent photosynthesis
  3. Selective post-emergent: targets certain weeds while leaving grass alive
  4. Non selective post-emergent: destroys everything it touches. 


Why is crabgrass so hard to manage?

  • Crabgrass will grow into thin or bare areas in your lawn and it chokes out "good" grasses.
  • It’s an annual plant - If given the space, it can create up to 15,000 thousand seeds per plant each year.
  • Its high stalk create a perfect space for lawn pests to thrive


Pre-emergents should be applied at specific times of year but post emergents can be applied whenever you see weeds.  Think of the way they work together as a one-two punch.  If you have any further questions about how either of these two methods work, feel free to ask you local lawn care company or lawn service techs!

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