When we bought our home, the only thing I knew about lawn care was that taming weeds can be a real pain in the grass. Then I discovered the miracle of pre-emergent herbicides, which knock out weeds before they take root.
Pre-emergent herbicides are specialized weed control products that create a chemical barrier in the soil’s top layer to prevent weed seeds from developing into mature plants. When combined with strategic applications of post-emergent herbicides, pre-emergents work wonders at keeping your hard-earned lawn almost weed-free.
We’ll discover what lawn care experts have to say about pre-emergent herbicides, what weeds they control, and the different varieties available.
How pre-emergent herbicides work
Pre-emergent herbicides create a chemical barrier in the top layer of soil that disrupts cell division in germinating seeds, preventing proper root and shoot development. Unlike post-emergent herbicides that kill existing weeds, pre-emergents target weeds before they emerge from the soil.
“Preemergence herbicides prevent germinated weed seedlings from becoming established; either by inhibiting the growth of the root, the shoot, or both,” says Joe Neal, PhD at North Carolina State University.
When the plant cells can’t divide, they can’t multiply, and therefore, they can’t grow or establish themselves in the ground. Without roots or shoots, the seedlings have no access to nutrients or water, so they die, Neal explains.
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Types of pre-emergent herbicides
Preemergent herbicides are available in both granular and liquid forms. Let’s explore their differences:
- Liquid weed killer spray is applied using a hose or included sprayer. It offers more precise application, better coverage, flexibility in mixing with other products, and quicker absorption into the soil.
- Granular lawn weed killer is applied using a spreader or by hand. It is easy to apply and is well-suited for smaller yards, although application isn’t as precise as with liquid.
Most pre-emergent herbicides are synthetic, but for an organic option, corn gluten is an effective pre-emergent.
In my house, we typically use synthetic granular pre-emergent because it can be more forgiving and I find it easier to apply. While I recognize that liquid pre-emergents also have lots of benefits, granular is best for us at this time. However, we are actively weighing the pros and cons of corn gluten.
Read more: Types of Pre-Emergent Herbicides
What weeds do pre-emergents control?
Pre-emergents control many common grassy weeds and most broadleaf annual weeds. They do not control established perennial weeds.
“Preemergence herbicides are generally the most effective chemical option for controlling challenging annual weeds like crabgrass and annual bluegrass,” says Dr. Becky Grubbs, a turfgrass specialist at Texas A&M University AgriLife Extension.
Annual weeds complete their lifecycle in one season. Broadleaf weeds have wide leaves with netted veins, while grassy weeds have narrow leaves with parallel veins. Both types spread primarily through seeds and compete with desirable plants for resources. I’ve outlined some common weeds of each type below.
| Broadleaf weeds | Common grassy weeds |
| Lambsquarters | Crabgrass |
| Pigweed | Foxtail |
| Common Purslane | Barnyard Grass |
| Chickweed | Annual Bluegrass |
| Field Bindweed | Goosegrass |
| Dandelion | Wild Oats |
| Henbit | Italian Ryegrass |
| Shepherd’s Purse | Yellow Nutsedge |
Read more:
- Why Do Weeds Grow?
- How to Get Rid of Weeds in Flower Beds
- How to Control Annual Weeds vs. Perennial Weeds
When to apply pre-emergents
The best time to apply pre-emergents is 1 to 2 weeks before they appear in the spring and again in the fall. However, it’s important to pay attention to soil temperatures.
“Make two preemergence herbicide applications: one in the spring to target summer annual weeds and one in the fall to target winter annual weeds,” says Dr. Grubbs of Texas A&M. Apply from February to April, when soil temperatures reach 55 degrees, and from August to October, when soil temperatures drop to 70 degrees, she explains.
Read our complete guide: When to Apply Pre-Emergent Herbicides
How to choose the right pre-emergent for your lawn
Choosing the right pre-emergent herbicide is crucial for effective lawn care. The ideal product depends on the specific weeds you’re targeting, your grass type’s unique sensitivities, and your yard size — which affects application rates and coverage needs. Getting this decision right ensures maximum weed prevention with minimal risk to your lawn’s health.
The type of weeds you’re trying to control
As previously mentioned, pre-emergent herbicides are effective against annual grasses like crabgrass, foxtail, and annual bluegrass, broadleaf weeds such as chickweed, henbit, and purslane, and other common weeds like spurge, oxalis, and pigweed.
Read more: How to Control Weeds by Type
Your grass type
Kentucky bluegrass, fescue, and ryegrass typically benefit from fall and early spring applications and tolerate a wider range of pre-emergent products. Warm-season grasses like Bermuda, Zoysia, and St. Augustine thrive with late winter/early spring applications before soil temperatures reach 55 to 60 degrees.
Centipede grass requires careful product selection due to its sensitivity to many herbicides. Similarly, bentgrass and fine fescues may show sensitivity to certain pre-emergent products.
My tip: Established lawns generally handle pre-emergents better than newly seeded areas. Always check your product labels for specific grass-type recommendations before application.
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The size of your lawn
Liquid pre-emergent herbicides offer precise application and immediate soil penetration upon watering. They’re ideal for small lawns where careful spot treatment is needed, particularly around landscaping features or garden beds. However, they require measuring, mixing, and specialized spraying equipment, which can be time-consuming and less practical for extensive areas.
Granular pre-emergents provide easier application through standard broadcast or drop spreaders, making them perfect for medium to large lawns where coverage efficiency matters most. These types also tend to have longer-lasting effects as they gradually release into the soil. The downside is slightly less precision around delicate landscape features and potentially uneven distribution without proper spreader calibration.
My tip: Make your final choice depending on the equipment you own, your physical capabilities, and whether you’re treating specific problem areas or providing whole-lawn protection.
FAQ about pre-emergent herbicides
Pre-emergents are less effective if applied too early. They break down before the weeds germinate, making the cell division blocker unable to prevent root cell division. While early application pre-emergent may be somewhat effective depending on how early it’s applied, it won’t be nearly as effective as it’s intended to be.
You can mow after applying pre-emergent provided you either water (rain or irrigation) the pre-emergent and allow dry time or don’t remove the grass clippings. These steps permit the pre-emergent treatment to settle into the soil. Failure to do one of these steps reduces the effectiveness of the pre-emergent. Mowing before treatment is ideal.
Yes, you should always water your lawn after applying pre-emergent. Water activates pre-emergent and binds it to the soil, creating a barrier that prevents weed roots from establishing. Most pre-emergents only need about ½ inch of water, but follow the manufacturer’s directions for accurate results.
No, you should not use pre-emergent herbicide on new grass. Pre-emergents affect new grass the same way they affect weeds and will prevent your new grass from growing roots and establishing a new lawn or filling in bare spots. However, if you sow new grass seed in the spring, you can apply pre-emergent for fall weeds in late summer.
Let us weed out your problem
Pre-emergents are great at maintaining an attractive, healthy lawn. However, pre-emergent application is time-sensitive, requires purchasing products and equipment, and closely following the manufacturer’s directions. If you’re low on time, energy, or motivation, Lawn Love has vetted, affordable local professionals who’ll tackle all your lawn care needs.
Sources:
- Premergence Herbicides: Dr. Joe Neal, North Carolina State University Extension
- Premergence Herbicides for the Home Lawn: Dr. Becky Grubbs, Texas A&M University AgriLife Extension
- Weed Control in Landscapes: Dr. Ted Whitwell, Clemson University Cooperative Extension Service
Main Photo Credit: ImageSine | Adobe Stock
