Effective weed control requires perfect timing. Knowing when to apply post-emergent herbicides can make the difference between a pristine lawn and a weed-filled disappointment. These products kill sprouted weeds, but only when used correctly.
Target dandelions in spring before they flower and crabgrass when young in late spring to early summer. Apply when your lawn is actively growing, your soil is moist, and daytime temperatures range between 65 and 85 degrees. Choose a calm day with no rain forecast for at least 24 hours to prevent herbicide runoff and maximize absorption.
Weed growth stage
Any good offensive strategy involves attacking your opponent’s weakness, and the same is true of weeds. Here is their weakness: Weeds are most vulnerable to post-emergent herbicides when they are young, typically in the 2- to 4-leaf stage, before they have time to fully mature. Here’s why:
Young weeds have thinner cuticles and more active growth, allowing better herbicide absorption. For many common lawn weeds like dandelions or chickweed, this early intervention provides the highest kill rate with the lowest herbicide dose.
Mature weeds produce thicker cuticle layers that prevent them from absorbing herbicides. Dandelions, for example, become significantly harder to control once they develop their deep taproot. This often requires higher herbicide rates or multiple applications.
Flowering weeds redirect energy. When weeds reach the flowering stage, they often redirect energy to seed production rather than vegetative growth. This can reduce the movement of systemic herbicides to the roots, making control less effective. However, the plant is still metabolically active, so this stage isn’t necessarily a complete loss for treatment.
If managing weeds feels overwhelming or you don’t have time to tackle the project yourself, Lawn Love can connect you with a local professional to handle the job without any hassle.
Read more:
- How to Prevent Weeds from Growing
- How to Get Rid of Weeds in Flower Beds
- How to Control Weeds by Type
- 32 Common Types of Weeds
Weather conditions
Post-emergent herbicides work best when the weather conditions are right. Consider these factors:
Temperature: Use post-emergent herbicide when daytime temperatures are between 65 and 85 degrees Fahrenheit and soil temps range from 55 to 65 degrees. This timing keeps weeds growing, reduces stress on your lawn, and prevents the herbicide from evaporating too quickly.
When air temperatures drop below 65 degrees, weed control slows down a lot, according to Thierry Besancon of the Rutgers University Cooperative Extension Service. Weeds struggle to absorb and process herbicides in cooler weather, making them less effective.
Dry weather: Check that there’s no chance of rain for 24 to 48 hours after application. Rain can dilute or wash away the herbicide and reduce its effectiveness. Morning dew can sometimes dilute herbicides, so I suggest applying in the late morning after the dew has dried but before the afternoon heat.
Low wind: Winds, typically over 10 mph, increase the risk of herbicide drift, which can damage other plants and reduce the effectiveness of the treatment.
Moderate humidity: Apply herbicides when relative humidity is between 40 to 60%. This range helps herbicides stick and penetrate leaf surfaces while allowing proper drying. Extremely low humidity causes herbicides to crystallize before absorption, while high humidity slows evaporation, potentially leading to runoff.
Seasonal timing
Like all plants, weeds are generally picky about their growing seasons and conditions. It’s important, therefore, to know what seasons certain weeds flourish and how to attack them.
Spring
Harry Rose | Flickr | CC BY 2.0
Common weeds: Chickweed, henbit, purple deadnettle, dandelion, wild garlic, crabgrass, lambsquarters, and pigweed seedlings.
Apply herbicides when daytime temperatures consistently reach 60-75 degrees for best results. Early to mid-spring is ideal for controlling winter annuals before they set seed, while late spring works well for summer annuals just emerging. Avoid applying immediately before rain to prevent runoff and reduced efficacy.
Read more: Spring Lawn Care Tips
Summer
NY State IPM Program at Cornell University from New York | Wikimedia Commons | CC BY 2.0
Common weeds: Crabgrass, foxtail, barnyard grass, purslane, spurge, nutsedge, bindweed, and plantain.
In summer, focus on spot treatments rather than broadcast spraying when possible. Apply post-emergent herbicides in summer during early morning hours when temperatures are below 85 degrees, and there is little wind. This will reduce volatilization and drift.
Summer herbicide applications are most effective when weeds are young and not yet experiencing drought stress. For perennial weeds, wait until they’re actively growing after rains.
Read more: Summer Lawn Care Tips
Fall
New York State IPM Program at Cornell University | Flickr | CC BY 2.0
Common weeds: Dandelion, thistle, clover, ground ivy, henbit, wild violet, chickweed, and annual bluegrass.
Fall is a great time to control perennial weeds as they transport nutrients to their root systems. Apply post-emergent herbicides in the fall when daytime temperatures are between 50 to 75 degrees and before the first killing frost. Difficult perennials like dandelion and thistle are particularly vulnerable to systemic herbicides in fall as they prepare for winter dormancy.
Read more: Fall Lawn Care Tips
Lawn conditions
A weak, stressed lawn doesn’t soak up herbicide well, so it’s important to apply post-emergents when your lawn is basically healthy. Follow these rules:
Wait until your lawn is fully established: Post-emergent herbicides can be tough on young grass. If you’ve recently seeded or sodded your lawn, wait until the grass is fully matured. Sod generally takes a month to fully establish, and it takes two full growing seasons for a lawn to become fully established from seed.
Don’t apply during a drought: If your lawn is struggling from dry conditions, adding herbicide could make things worse. Plus, weeds under drought stress may not absorb the treatment properly.
Hold off if your lawn has a disease: If your grass is struggling with a disease, it’s already putting in a lot of effort to recover, and adding herbicides will only strain it further. Herbicides target weeds, but they also affect grass. Concentrate on helping your lawn bounce back before applying herbicide.
Mow a few days before applying post-emergents: Tall grass can block the herbicide from reaching the weeds, making it less effective. Mow your lawn to the right height for your grass type, and you’ll see better results. I suggest applying 2 to 3 days after mowing to allow some leaf regrowth for herbicide absorption.
Read more:
- Do You Have to Mow Your Lawn?
- How to Mow Your Overgrown Lawn
- When You Should Not Mow Your Lawn (And Other Mowing Mistakes)
Check your soil: Healthy, loose soil allows herbicide to seep into the ground where it’s needed. If your soil is compacted or has a thick layer of thatch, herbicide might not be able to reach the weed roots properly. In this case, aerate your lawn several days before applying post-emergent. This will allow your lawn to recover from the stress of aeration first.
FAQ about when to apply post-emergent herbicide
As a rule of thumb, stay off the lawn 24 to 48 hours after application to give the herbicide enough time to absorb. This also reduces the risk of exposure to you and your pets or children.
Read more: How to Apply Post-Emergent Herbicides
Apply post-emergent herbicides several weeks before you overseed the lawn. This allows the herbicide to break down, minimizing the risk to new seedlings. Weeds also compete with newly seeded grass, so eliminating them beforehand is crucial.
Generally, you’ll notice some wilting or yellowing in the first week, but the weeds might take up to 2 weeks to fully die off. Some tougher species could take as long as a month.
If you don’t see significant progress after 3 weeks, you may need to reapply, closely following the manufacturer’s instructions.
Find the best option for your lawn in our guide: 7 Best Post-Emergent Herbicides
Let the pros rid your lawn of weeds
DIY lawn care isn’t always the best option for everyone. Even if post-emergent herbicides are generally easy to use, you may have concerns about application or time limitations. In that case, a lawn weed control expert can assist you.
For a thriving lawn, Lawn Love can also connect you with a local lawn care professional for aeration, dethatching, fertilization, and mowing.
Sources
- “Establishing Lawns.” Produced by the College of Agriculture and Life Sciences at Virginia Tech University. Virginia Cooperative Extension.
- “Things to consider for a successful postemergence herbicide application.” By Liz Stahl, Phyllis Bongard, Ryan Miller, Dave Nicolai, and Debalin Sarangi. University of Minnesota Extension.
- “Weather Conditions and Herbicide Performances.” By Thierry Besancon. Rutgers Cooperative Extension.
Main Image Credit: Shutterstock
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