Pre Emergent vs Post Emergent Weed Killer

Written by Nate Bahler | Sep 14, 2022 2:45:08 PM

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:

Systemic post-emergent: absorbed into plant and kills from root up
Contact post-emergent: leaves coating on plant to prevent photosynthesis
Selective post-emergent: targets certain weeds while leaving grass alive
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!