10 Common Mistakes to Avoid When Using Weed Killers

Weed killers can be powerful tools in your lawn care arsenal, but when used improperly, they can cause more harm than good. Whether you're a seasoned DIY lawn enthusiast or a weekend warrior trying to get ahead of spring weed invasions, understanding the most common weed killer mistakes to avoid can save your lawn, your time, and your money.

In this post, we’ll break down the top 10 mistakes with herbicide use and offer clear, actionable weed killer application tips to help you get the lush, weed-free lawn you’re aiming for.

1. Using the Wrong Weed Killer for the Job

One of the most common weed control errors is grabbing the first herbicide you see without checking whether it’s right for your situation. Not all weed killers are created equal — some are selective, targeting broadleaf weeds without harming your grass, while others are non-selective and will kill just about anything green.

Weed Control Dos and Don’ts:

✅ Do use a selective herbicide for controlling weeds in your lawn without damaging turfgrass.

❌ Don’t use non-selective herbicides like glyphosate unless you’re spot-treating cracks in a driveway or planning to reseed/sod an area.

Pro Tip: For lawns, selective herbicide products like Celsius and Certainty Herbicide Kit (for warm-season grasses) or Tenacity and SedgeHammer® Kit (for cool-season and Centipede lawns) are ideal when used correctly. These kits tackle both broadleaf and grassy weeds without harming your grass, and they come with a blue marker dye, so you can see where you’ve sprayed, and a surfactant to help the herbicide stick to the weed.

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tenacity-and-sedgehammer-herbicide-kit-with-surfactant-and-dye-cool-season-grass

If you do need a non-selective herbicide, why not try Mirimichi Green’s Organic Weed Control. This is an excellent alternative to glyphosate for killing your entire lawn as part of a renovation project. This product is rapidly broken down in the soil, there is no chance for environmental buildup or runoff into waterways and it will not leach through the soil to harm desirable plants.

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2. Spraying at the Wrong Time of Day

Timing is everything when it comes to safe weed killer applications. Spraying during the heat of the day, especially in summer, can increase the risk of herbicide volatility and turf injury. On the other hand, applying during the evening or when dew is present can dilute the product and reduce its effectiveness.

Weed Killer Application Tips:

✅ Apply herbicides in the early morning (after dew dries) or late afternoon when temperatures are cooler and winds are calm.

❌ Avoid spraying during high heat or windy conditions to prevent drift and damage.

3. Misjudging the Weather Forecast

Using herbicides just before or after rain can wash the product off the weed’s surface or prevent it from being absorbed. Rain can also cause runoff contaminating nearby plants, garden beds, or waterways.

Proper Weed Killer Usage:

✅ Apply when no rain is expected for at least 24 hours.

❌ Don’t spray if rain is in the immediate forecast or if the soil is already saturated.

Related: The Best Broadleaf Weed Killers for Your Lawn 

4. Overapplying Herbicide

More isn’t always better. One of the biggest mistakes with herbicide use is assuming that doubling the dose will double the effectiveness. Overapplication can lead to turf damage, environmental harm, and even herbicide resistance in some weed populations.

Avoiding Weed Killer Misuse:

✅ Always read and follow the label — the instructions are there for good reason, and your lawn will thank you.

❌ Don’t “eyeball” measurements or apply more than recommended.

Bonus Tip: Use a sprayer with clear markings, like a Yard Mastery 4-Gallon Battery Backpack Sprayer, for better control and even coverage.

yard-mastery-4-gallon-backpack-sprayer

5. Using Weed Killers on Stressed Grass

Applying herbicides to grass that’s already stressed — due to drought, heat, mowing, or disease — can push it over the edge. Herbicides are designed to target plant systems, and stressed turf may not have the resilience to bounce back after an application.

Safe Weed Killer Application Tips:

✅ Make sure your grass is healthy and actively growing before applying most post-emergent herbicides.

❌ Avoid herbicide use during drought, after scalping, or when your lawn is under disease pressure.

Related: The Best Crabgrass Weed Killers That Won’t Kill Your Turfgrass

6. Applying to the Wrong Weed Growth Stage

Herbicides are most effective when weeds are young and actively growing. Mature weeds, especially those flowering or going to seed, are much harder to control and may require multiple applications — or even a different approach altogether.

How to Properly Use Weed Killers:

✅ Treat weeds when they are small and in early growth stages (ideally before they flower).

❌ Don’t wait until weeds are large, established, or seeding — at that point, you may need to pull or dig them out.

Product Tip: Use Dimension .15% Pre-Emergent Herbicide with Fertilizer 0-0-7 or 0.38% Prodiamine Granular Pre-Emergent Herbicide 0-0-7 as a pre-emergent herbicide and fertilizer safe for warm and cool-season lawns. Apply in early spring to stop many weeds before they even sprout.

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prodiamine-38-pre-emergent-herbicide-with-fertilizer-0-0-7-50lb

7. Mixing Incompatible Products

Mixing herbicides with other lawn products (fertilizers, fungicides, and insecticides) without checking compatibility can lead to chemical reactions, ineffective treatments, or damage to your sprayer equipment.

Weed Killer Dos and Don’ts:

✅ Consult product labels and use a jar test to check compatibility before mixing products in your sprayer.

❌ Don’t mix multiple lawn products just to save time — it can backfire.

We test all our products for compatibility, and the product description states whether they can or can’t be mixed with other products. In most cases, herbicides should be used separately from other products, but can be applied on the same day. For instance, non-herbicide products such as fertilizers and biostimulants, should be applied before weed killer.

8. Ignoring Application Equipment Calibration

Another key weed killer mistake to avoid? Using uncalibrated sprayers or spreaders. Applying too much (or too little) can mean poor results and unnecessary lawn damage.

Weed Killer Application Tips:

✅ Calibrate your sprayer to ensure you’re applying the correct amount over the right area.

❌ Don’t guess how much your sprayer is putting out. Use a test area and measure your output.

Pro Tip: Always check nozzle tips for clogs and even spray patterns.

Watch this video to learn how to correctly calibrate your sprayer, choose the right nozzle to use for the task at hand, and keep it in tip-top condition: