Through nearly two decades of homeownership, I’ve learned that knowing when to apply pre-emergent herbicides is crucial. These preventative treatments are the cornerstone of my lawn maintenance program.
When to apply pre-emergent herbicides mainly depends on your location. Here’s a good rule of thumb: apply them between February and April, when soil temperatures reach 55 degrees, to target summer weeds, and again between August and October, when soil temperatures drop to about 70 degrees, to target winter weeds.
In this guide, we’ll explore expert recommendations for both growing seasons. You’ll learn how to prevent summer and winter troublemakers — transforming your lawn into a neighborhood standout.
Know your weeds
Pre-emergent herbicides create a soil barrier against weed germination. With strategic applications, homeowners can combat both summer and winter weeds effectively.
Summer weeds
Summer weeds begin to sprout in spring when soil temperatures reach 55 degrees (with air temperatures at 65 degrees). They can wreak havoc on lawns during summer and either die or become dormant in the fall when soil temperatures drop below 55 degrees.
Common summer weeds you’d target with pre-emergent herbicides include crabgrass, foxtail, goosegrass, barnyard grass, spurge, purslane, lambsquarters, pigweed, yellow nutsedge, dandelion, chickweed, clover, oxalis, plantain, black medic, and knotweed.
Winter weeds
Winter weeds emerge in the fall as soil temperatures fall to 55 degrees, causing issues for lawns during the cold months. They endure frigid winter temperatures by entering dormancy and blooming in the spring, then dying back when temperatures rise too high.
Common winter weeds you’d target with pre-emergent herbicides include annual bluegrass, henbit, common chickweed, hairy bittercress, wild garlic, wild onion, annual ryegrass, Carolina geranium, shepherd’s purse, corn speedwell, lawn burweed, purple deadnettle, and winter grass.
Read more:
- Why Do Weeds Grow?
- How to Get Rid of Weeds in Flower Beds
- How to Control Annual Weeds vs. Perennial Weeds
- Guide to Cool-Season Grasses
- Guide to Warm-Season Grasses
Spring application
Dr. Becky Grubbs, a turfgrass specialist at Texas A&M University AgriLife Extension, offers a simple way to time your spring application of pre-emergents. “Apply your spring preemergence herbicide when soil temperatures reach approximately 55°F for several days,” Grubbs says.
The most accurate way to get the soil temperature is to plunge your soil thermometer 1 to 2 inches into the ground. “Even a household meat thermometer” works,” she says.
Most preemergence herbicides will last between 8 and 12 weeks. Because the pre-emergent lawn weed control effective lifespan is somewhat broad, the window of effective pre-emergent herbicide application is from early February to late April.
The more precise window in which your soil temps reach 55 degrees varies by your geographical location and climate. If you live in the South, you should apply weed preventer closer to early February, while Northerners can wait until mid to late April. Here are a few examples of when to apply spring pre-emergents.
| Location | Apply pre-emergent herbicide |
| South Florida | Early February |
| Central Texas | Mid-February |
| Southern California | Mid-February |
| Central California | Late February |
| Oregon | Early April |
| Oklahoma | Late February |
| Indiana | Early April |
| South Carolina | Mid-February |
| Maryland | Early March |
| New York | Early April |
Read more:
- How to Apply Pre-Emergent Herbicides
- 6 Best Pre-Emergent Herbicides
- Types of Pre-Emergent Herbicides
Fall application
“Fall applications can be made several months later when soil temperatures decrease to approximately 70°F for several days,” says Grubbs, of Texas A&M. Like spring applications, the most accurate way to get the soil temperature is to insert your soil thermometer 1 to 2 inches into the ground.
Depending on where you live, soil temperatures drop to 70 degrees between August and October, so that’s when you should apply your fall pre-emergent. Here is another look at some major areas of our country and when to apply fall pre-emergent lawn weed killer.
| Location | Apply pre-emergent herbicide |
| South Florida | Late October |
| Central Texas | Mid-September |
| Southern California | Mid-September |
| Central California | Early September |
| Oregon | Mid-August |
| Oklahoma | Early September |
| Indiana | Early September |
| South Carolina | Mid-September |
| Maryland | Late August |
| New York | Mid-August |
Read more:
- What Are Pre-Emergent Herbicides?
- How to Control Weeds by Type
- Is Weed and Feed Bad for Your Lawn?
- 8 Best Weed and Feeds
FAQ about pre-emergent herbicides
Applying pre-emergent herbicides too early reduces its efficacy. When applied too early, the pre-emergent breaks down before the weeds germinate, and the herbicide is unable to effectively prevent root cell division. While early application pre-emergent may prevent some weeds depending on how early it’s applied, its efficacy will be diminished.
After applying pre-emergent herbicide and watering your lawn, avoid walking on it for 24 hours. Walking on the lawn too soon can transfer the pre-emergent to unwanted areas or disrupt its application, making it less effective.
No, you should not use pre-emergent herbicide on new grass. Pre-emergent herbicides stop weed seeds from growing, making them useful for maintaining healthy lawns, but they also prevent new grass seeds from establishing roots. You can apply pre-emergent herbicide after your new grass is established, but this usually takes several months.
Get help with your emerging weed problems
Pre-emergent herbicides are essential for a beautiful, healthy lawn, but they must be applied at the correct time and require specific products, equipment, and adherence to the manufacturer’s instructions. If you’re short on time or energy, Lawn Love’s affordable, vetted, local professionals can take care of your pre-emergent and other lawn care needs.
Sources:
- Premergence Herbicides for the Home Lawn: Dr. Becky Grubbs, Texas A&M University AgriLife Extension
Main Photo Credit: plo | Adobe Stock
