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When Is the Best Time to Spray Weed Killer? (Seasons, Weather & Time of Day)

backpack sprayer

Regardless of how much we fight off weeds, they make a comeback. It can feel like we’re in the fight alone and weeds have something especially against us. Weeds are a problem for gardeners across the globe, and we’ve got the option of appropriate fertilization and irrigation, but many people choose to spray weed killer.

Since that’s the case, we have to consider what time of the year is the best to spray? What about specific climates or times of the day? The best time to spray weed killer is usually spring but there are also safety issues around our home that we have to consider and emergence herbicides.

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Before You Spray

We need to understand how weed killers work before we spray. How is it that a sprayed weed killer kills the weeds but not grass other plants? Weed killers interfere in the growth of weeds by blocking protein production or destroying root formation. Herbicides aren’t as dangerous to people as pesticides, but we need to spray weed killer with caution.

Pre- and Post-Emergence Herbicides

We can use pre-emergence herbicides on an established lawn to disrupt the weed life cycle, especially for summer weeds. Pre-emergence herbicides examples are benefin and dithiopyr. Time the spraying of weed killer before weeds germinate, which is usually in early spring. Then, we can reapply the week killer eight weeks after the first spraying.

For post-emergence herbicides, we can use glyphosate chemical to kill weeds by absorbing it into the plant tissue. Glyphosate-based chemicals achieve the best results when applied to young weeds.

Safety Issues Around the Home

Not only should we protect ourselves when spraying weed killer by wearing long sleeves, gloves and goggles, as well as using a respirator and protective headgear, but we should protect others. We need to read the label to know how long we must stay out of the treated area, but before that, we need to keep pets indoors. Also, we need to keep children away from the sprayed area before and after, so they should be inside during the spraying and avoid the area for the duration specified on the label.

Tips & Tricks

Spraying by the Seasons

Although we may get attacked by weeds all-year-round, we need to strategize about when to spray weed killer to get the optimal effect. Let’s go through our strategy per season.


Weed killers work best during warmer conditions, but that doesn’t mean we can’t apply it during winter. Then, we have to do is ensure that we spray during the middle of the day during sunshine. The chemicals may lose their efficacy if allowed to freeze overnight.


Spring is the best time to spray weed killer because it’s warm, and we have a chance during early spring to apply pre-emergent to prevent them from germinating. Then, we should apply the post-emergent eight weeks later to kill the more stubborn weeds while they’re young.


If we’re going to spray in summer, we need to make sure to do it at the end of the season. By treating weeds at the end of summer, we’ll minimize the problem for next spring. We should use an herbicide that kills a variety of pest grasses and apply it in late morning or early afternoon. The spray may evaporate in the heat of full sun.


This is a great season to spray weed killer because it’s when weeds are most vulnerable. By spraying post-emergent, weeds will struggle to survive winter. We should apply two treatments in October, two weeks apart. If they don’t wilt immediately, we need to be patient until spring when we’ll see that most of them won’t return.

spraying on weeds
Image: FrankHH, Shutterstock

Spraying by Time of Day

Not only are weeds more prominent during specific seasons, but they also have peak times during the day that they flourish if we don’t take care of them.

Cold Climate

It’s pointless to spray weed killer when there’s dew on the plants because the spray will become too diluted to be effective. Harsh winters are also pointless if the ground is frozen or covered in snow. If there’s no snow and the ground isn’t frozen, then the best time is mid-day, with the sun out.

Temperate Climate

Since plants are least active from dusk until the following morning after sunrise, the ideal time to spray in a temperate climate is in the late morning or early afternoon.

Warm Climate

It’s best to treat when overnight temperatures are in the low 70’s to 60’s. Daytime temperatures should be in the mid-80’s.

Weather Conditions

hand spraying weed killer
Image: rigsbyphoto, Shutterstock

Weather has a significant role in the effectiveness of spraying weed killer.


Spraying a plant after rain has fallen will dilute the weed killer and make it less effective, so we should ideally wait 24 hours after for the plants to dry. If the region receives a lot of rain, we can opt for weed killers that are rainfast within 2-4 hours of spraying.


We need to minimize spray drift, which happens when herbicide blows to other areas of the yard during windy conditions and kills unintended plants. There’s no point in spraying when the wind is strong, so we need to wait for it to calm, which usually happens in the morning and at night.

Extreme Heat or Cold

Conditions that are extremely hot or cold won’t work because liquid evaporates before absorbing in extremely hot temperatures and freezes in extreme cold.

Other Factors to Consider

  • Replanting: It’s important for us to know when it’s safe to plant larger plants so that they’re not negatively affected, meaning we need to provide enough time for the weeds to die and the herbicide to dissipate.
  • Type of herbicide: We need to decide between granular or liquid herbicide. With granular, we don’t have to worry about drift, and it sticks to plants better when there’s moisture. Liquid is more suitable for tougher weeds due to even coating.
  • Don’t overtreat: It’s important that we follow label instructions for mixing and application so that we don’t damage the soil.
  • Treating lawn: Mowing the lawn before spraying isn’t a good idea because we need to have the weed intact to apply the weed killer.

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The best season to spray the weed killer is in spring or fall. The reason is that we should do it in warm temperatures but not when it’s extremely hot because the liquid evaporates or extremely cold so that it freezes. Spray the weed killer in early spring, then another application eight weeks later.

We can also do it in fall, with October being the best month so that the weeds don’t survive in winter. Two treatments that are two weeks apart in that month will create the optimal result.

Featured Image Credit: ninikas, Shutterstock

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