How Does a Pre-Emergent Herbicide Work?

Brendon Willis
Grass Blades Close-up of Herbicide Drops

Each year in the spring and fall, you begin hearing the phrase “pre-emergent” being tossed around pertaining to lawn care. (Did you know you can also use a pre-emergent in your flowerbeds?!)  

If you haven’t already read our post about what a pre-emergent is, you should do so. But now we’ll take a quick look at the anatomy of a seed and how pre-emergent herbicides work. 

What is a pre-emergent? 

A pre-emergent herbicide is a type of pesticide used for preventing weeds from showing up in your lawn. It’s reasonable to expect that the seeds from weeds are planted by nature in your lawn every day. Whether by bird poop, wind gusts, or mowers blowing the seeds into your lawn – the spread of weeds happens all the time.  

Rather than waiting for weeds to become an eyesore, a pre-emergent stops the weed at the seed germination stage. 

When Does Pre-Emergent Start Working? 

Pre-emergents begin their work as soon as a seed “wakes up.” Weeds, like any other plant, start from a seed. So, it is important to understand how a seed works. When a seed encounters ideal temperatures and moisture, it cracks open and sends its first root (radicle) and first leaf (cotyledon).  

The seed itself has just enough nutrients to grow a first root and leaf. From there, the plant (whether grass or weed) is fully dependent on its ability to grow stronger roots to retrieve nutrients and moisture from the ground.  

This stage of a seed – post-germination, yet still just a seed – is a plant’s most vulnerable state. This is where a pre-emergent comes in and does its work. 

How does a pre-emergent herbicide work? 

A pre-emergent herbicide is applied to your lawn by either a liquid or a granular treatment. Once the lawn is properly watered, the pre-emergent chemically binds itself to the top layer of soil. 

Then, when the seed of a weed begins to germinate and sends down its first root, that root makes contact with the treated soil. When it makes contact, the pre-emergent interrupts the biological process within the seed and destroys the seed. Because the plant has no other way to get additional nutrients, the seed of a weed dies. 

Considerations When Purchasing Pre-Emergent 

There are many types of pre-emergent herbicides on the market. Different products can target specific weeds and miss others. Some products are manufactured for flowerbeds, some are manufactured for turf. Some are expensive, some are budget products. Some last longer, while others have a shorter life.  

The best way to know what pre-emergent you need is to join the free lawn help Facebook group and inquire there. There are professionals giving free lawn care advice and consulting every day in this group. 

FAQs, Misconceptions and Myths about Pre-Emergent

Does pre-emergent kill weeds? 

No, pre-emergent prevents new weeds from germinating by seed. Existing weeds that are visible and growing will not be killed by a pre-emergent application.  

Is pre-emergent the same as a weed killer? 

A pre-emergent should not be used as a weed killer. Technically it does kill a weed, but only in the first few days of its life. To kill actively growing weeds, you would need to use a post-emergent weed killer. 

Does a pre-emergent kill grass? 

A pre-emergent that is applied improperly can kill your lawn or damage it at the very least. However, even a properly applied pre-emergent can have a major adverse effect on a newly seeded or soon-to-be seeded lawn.  

Newly seeded lawns are made up of a bunch of tiny, underdeveloped roots that a pre-emergent can destructively prune. Lawns that are soon-to-be seeded will in most cases not be able to germinate as the herbicide barrier on the soil will stop grass seeds the same way they stop weeds from growing. 

Will pre-emergent prevent all weeds? 

Pre-emergent herbicides are not always labeled for the weeds you are targeting. Be sure to read the labels of the products you are researching prior to buying or applying them to your lawn or landscape. There’s not much more frustrating than applying a pre-emergent, waiting several months, then dealing with the same issues! 

How long does pre-emergent last? 

Different pre-emergents have different residual effects in the soil. Soil type also plays a role in the longevity of a pre-emergent. Typically, sandy soils lose pre-emergents quicker than clay soils. The lifespan of many products is correlated to the rate at which you apply it to the area.  

Pre-emergents are formulated to last between 1-8 months, with the most widely-used products lasting 3-4 months. Some of the better (and much more expensive) products, like the professional products we use at Willis last between 6-8 months with the proper application rate. 

Is fertilizer and pre-emergent the same thing? 

Fertilizer and pre-emergent are not the same thing and have very different purposes. However, fertilizer and pre-emergent can be applied simultaneously by a blended liquid solution or a blended granular product that is spread on the lawn.

Fertilizers provide helpful nutrients to plants, pre-emergents stop undesired plants from coming into a flowerbed or lawn area. 

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