When Should I Apply Pre-emergent on My Lawn?

Weed seeds germinate when the soil temperatures rise. This is especially so during the transition between cold and warm reasons. You should capitalize on this knowledge to capture germinating weed seeds by surprise. 

Pre-emergents don’t kill weeds, but they are effective at preventing weed seed germination. Importantly, research about the soil structure of your turf. Information like organic matter content, pH and texture are invaluable. Ensure your soil characteristics match the label to maximize on the herbicide. 

In this article, we give you answers to the most burning questions. We hope when you read it to the end you would find the information herein useful in making buying decisions. 

Should I mow my lawn before applying pre-emergent?

The growth pattern of grass is predictable in lawns that have had weed problems. It is certain that weeds will grow again. Applying pre-emergent herbicides is a reasonable course of action because they hamper weed seed germination. But, first things first. 

You need to mow your lawn from time to time to keep it healthy, and a healthy lawn is a hostile germinating ground for weed seeds. However, if it’s the first time you are breeding grass in your turf, allow grass to mature before you apply herbicides. 

Grass seedlings are sensitive and need time to entrench themselves in the soil structure, creating plenty of space in between for herbicide distribution. Mow three times or so to improve air and moisture circulation in the soil as well as strengthen glass blades. 

With mature roots and blades, herbicides can distribute uniformly across the soil structure and reach out to roots and seeds of weeds that have penetrated deep into the soil. On the other hand, if it’s not the first time your breeding grass, then apply the pre-emergent herbicide after you have mowed your lawn every other day of the week.

What temperature is required to apply pre-emergent?

Seed germination is dependent on soil temperature. But temperature decreases with depth of the seed in the soil. Temperatures below 55 degrees Fahrenheit aren’t ideal for seed germination. Seeds buried deeper into the soil take longer to germinate than seeds near the surface. 

Thus, it’s important to check soil temperature to determine the most appropriate time to apply the pre-emergent herbicide. Measuring soil temperature helps you to predict weed pattern growth; thus, beating the stubborn plant ahead of time. 

You can create a heat map and rely on it to determine whether weed growth has begun during a particular season. For example, if temperatures fall under 55 degrees Fahrenheit during spring and 64 degrees Fahrenheit during fall, the weeds haven’t germinated and you are still within the grace period to apply the pre-emergent. Else, you are late and you need to apply post-emergent controllers.

When should I apply pre-emergent (before or after rain)?

Timing is key. You can rely on weather forecasts along with heat maps so that you can apply a weed preventer just before it rains. Soil temperatures of 55 degrees and thereabouts are telltale signs. That is, you should be alert during a transition from a cold season to a hot season.

It’s the best time to apply the pre-emergent. Rain water breaks down active compounds in the herbicide. The solution of the preventer will seep deep into the structure of the soil, catching germinating weed seeds off-guard. 

And because controller effectiveness reduces under oxygen and sunlight conditions, we recommend you save half as much herbicide. You can apply the remaining half amount in a month or two. 

However, this is not a fixed amount. It could be any amount depending on the recommended dilution level in the label. And so, check the label first as it can give you clue as to how much herbicide you need to apply that would be effective when diluted.

Will heavy rain wash away pre-emergent?

The first place to check a reliable answer to this question is to check the label. You can obtain information about recommended dilution level, so the active compounds remain effective, despite dilution. Most active ingredients will remain effective with rainfall amounts ranging between 0.5-inches and 1-inch. Leaching occurs when dilution level exceeds 1-inch. 

However, beyond the dilution level, texture, organic matter, and pH affect the effectiveness of the weed preventer. This information isn’t available on the label, although you can find information about the content of soil organic matter. 

You can glean this information by studying the soil structure of your turf or lawn. Fine-textured soils such as clay or loam don’t leach easily because they are rich in organic matter. 

On the other hand, rough-textured soils such as sandy soils leach easily. Importantly, measure soil pH level. It should range between 6.0 and 7.0. If it goes above 7.0, add a suitable acid to lower it. Else if it falls below 6.0, add a base to raise pH to within the acceptable range. 

When you shop for herbicide, select the one with a high Koc value. Herbicides with high Koc value adsorb (bind) tightly to soil surfaces. Be sure to select herbicides with low solubility. Also, ensure that the soil microbes don’t easily break the active compounds down. 

How long does pre-emergent take to work?

Generally, pre-emergent granules don’t dissolve in water with ease. And so, they can take between a week and two to show their effectiveness. Thus, apply the herbicide a week or two before the weed seeds begin to germinate. The active compounds can remain effective for three months.

What you should not do after applying pre-emergent?

Don’t apply pre-emergent after the weed has sprouted. After all, some germinating weed seeds will escape the guillotine and germinate anyway. Keep in mind that herbicides remain effective for three months. During this period, don’t seed. 

Do it after. If you plan to lay a sod, wait for twelve months. Importantly, you should always water and mow your yard after you apply the weed preventer. Fertilizers can also reduce herbicide effectiveness. Avoid them when you can.


Next time you go out to shop for herbicides, you should have studied the soil structure of your turf and weather patterns. This information is important because knowledge of soil organic matter content helps you to match the label information with your yard. 

Importantly, select the herbicides with high Koc values because they are highly adsorbent; they don’t get swept away with floods. While dilution level shouldn’t be something to worry about, it gives you a clue as to whether the herbicide would remain effective after dilution.


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