fighting grass lawn weeds with post emergent herbicides

Post-Emergent Herbicide

Do you feel like you’re constantly losing the battle against weeds in your lawn? You’re not alone. Many people struggle with keeping their lawns free of weeds. One solution that may work for you is using a post-emergent herbicide. 

Weeds can quickly turn a beautiful yard into an eyesore and signal an unhealthy lawn. While pre-emergent herbicides are used to prevent weeds from sprouting, it is a step people often forget or miss spots when applying. Post-emergent herbicides can help if weeds do show their ugly heads in your lawn.   

What are Post-Emergent Herbicides?

Post-emergent herbicides, also known as weed killers, are chemicals used to control already existing weeds. In contrast, pre-emergent herbicides kill weeds before they emerge or sprout above the soil line. The best defense against weeds in your lawn is to apply both pre and post-emergent herbicides.  

There are many different types of post-emergent weed killers, making it important to identify the variety of weeds you need controlled and pay close attention to the product description.

Types of Post-Emergent Herbicides

Post-emergent herbicide can be classified in a few different ways. Knowing the key differences will help you determine the most effective treatment for your lawn.

Systemic vs. Contact
(Mode of Action)

Post-emergent herbicides work in one of two ways. They can either attack the foliage or can be absorbed by the roots. Post-emergent herbicide formulas come as either systemic or contact applications.

  • Systemic 
    Systemic post-emergents are absorbed directly into the plant, causing the weed to be destroyed from the inside. Systemic post-emergent herbicides can kill both the leaves and the roots of the weed. It can take several days for systemic herbicides to work, so results will not be immediately noticeable.

    Systemic post-emergents are especially useful on perennial weeds, such as dandelion, poison ivy, and ragweed. Perennial weeds are often difficult to control, as they spread by seed and hardy roots. If the entire root is not killed, the plant can continue to reproduce and grow in your lawn.  

  • Contact 
    Contact post-emergent herbicides target the exposed part of the plant, causing damage to the leaves and making it impossible for the plant to receive the necessary nutrients it needs to continue to thrive. Contact post-emergents typically work quicker than systemic post-emergents but may not fully destroy the weed’s roots, making it possible for the weed to reappear.  

    Contact post-emergents are commonly used on annual weeds (weeds that live for a single season), such as crabgrass and chickweed, where the death of the foliage is enough to kill the entire plant. 

Selective vs Non-Selective

Post-emergent herbicides are also classified as selective or non-selective.   

  • Selective 
    Selective post-emergent herbicides are used to target specific weeds. This is especially helpful in areas such as turf where contact with the grass is unavoidable.
Pre-emergent being sprayed on weeds
  • Non-Selective 
    Non-selective post-emergent herbicides eliminate weeds along with any other plant in the application area.  While this type of post-emergent can control weeds quickly, it will also damage or kill anything else it comes in contact with, such as surrounding turf and nearby flowers or shrubs.

    Non-selective weed killers are mostly commonly used to treat hardscaped areas like sidewalks and driveways. They can also be useful when clearing off large plots of unwanted vegetation. 

Form: Liquid Spray vs. Spreadable Granules

Herbicides are also classified by the method of application.

sprayable option of pre-emergent herbicides
  • Liquid Spray
    Liquid post-emergent weed killers can come in ready-to-use bottles with spray nozzles or concentrates that must be mixed before use. Liquid weed killers cling to the weed’s leaves, making them very effective.
spreadable granules of pre-emergent herbicide
  • Spreadable Granules
    Post-emergent herbicides can also come in spreadable granular form. Granules can be distributed with a spreader, making it easy to apply to a large area, such as an entire lawn. 

Why Use Post-Emergent Herbicide

If a post-emergent herbicide is used properly it can control existing weeds and help prevent future weeds from germinating.

Pro Tip
Before choosing a post-emergent herbicide, it is important to know what weeds you are trying to control and what variety(ies) of grass you have. 

When to Apply Post-Emergent Herbicides

Early to late spring, when weeds are small but actively growing, is the best time to apply post-emergent herbicides. It is easier to kill small weeds and when weeds are actively growing because they more easily absorb the weed killer.  

Post-emergent herbicides can also be used during the summer and fall months, but it may take several applications to fully control the targeted weeds.  
Regardless of the time of year, post-emergent herbicides should be applied when temperatures range between 55-85 degrees Fahrenheit.  

  • If applied when temperatures are too hot, products may burn or damage the lawn.  
  • If applied when temperatures are too low, the herbicide will not be effective. Weeds must be actively growing for the weed killer to work. 

Pro Tip
If your lawn is suffering from drought or disease, wait until your grass has recovered before applying a post-emergent. Weeds that are stressed can struggle to take in weed killer. 

How to Apply Post-Emergent Herbicides

Once you have determined what type of post-emergent herbicide you should use, you must consider how you will apply it.  

Spot Treatment

Spot treating your yard means focusing on the weeds themselves and not the surrounding grass. Spot treatments are great for lawns with a few weeds and work well along sidewalks, driveways, and pools.  

Blanket Treatment

If a large portion of the lawn contains weeds, a blanket treatment may be warranted. Blanket treatments work best with selective post-emergent herbicides, or else you can easily kill your entire lawn. Blanket treatments may be applied using a liquid post-emergent spray or by spreading granules evenly to the lawn.

Regardless of the type of application you use, be sure to keep pets and children off the lawn until the application is dry.  

Tips When Applying Post-Emergent Herbicide

  • Monitor the weather forecast. 
    • Some post-emergent herbicides need to be watered in, while others should be allowed to dry after application. Be sure to read and follow the product label for watering instructions. 
    • Never spray on windy days. 
  • Apply when temperatures range between 55-80 degrees Fahrenheit.  
    • If applied when temperatures are too hot, products may burn or damage the lawn.  
    • If applied when temperatures are too low, the herbicide will not be effective. Weeds must be actively growing for the weed killer to work.  
  • Do not mow the lawn 3 days before or after post-emergent application so the weeds herbicide can be absorbed well into the plant. 

  • Use caution when applying to prevent drift of spray or contact with non-target plants.

  • Avoid skin contact and inhalation. 

  • Wear personal protective equipment (PPE) when applying herbicides, including long sleeves, pants, closed-toe shoes, gloves, and protective eye wear. 

  • Carefully read and follow all package directions and warnings. 

Post-emergent herbicides are a great way to control unexpected, pesky weeds that pop up in your yard. If you have any questions about selecting the best herbicide for your grass type, the Certified Turfgrass Professionals at NG Turf are happy to answer any questions about selecting the best herbicide for your grass type. Give us a call at 770-832-8608 or email 


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