How Long Does Roundup Take To Work

Roundup is a non-selective glyphosate-based herbicide that is known to eliminate any weed or grass effectively.

While Roundup claims that it works within a few hours after application, I observed varying outcomes across different plant species.

Quick Answer

It will take Roundup a week or so to kill grass weeds at the roots. Although initial wilting of some plants and lawn grasses occurs promptly, typically within 3 hours after application.

An important disclaimer to keep in mind is that some people view Roundup as potentially harmful to both soil health and human well-being. Keep reading to learn how to use Roundup effectively.

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What is Roundup?

Originally produced by Monsanto and acquired by Bayer in 2018, Roundup is a non-selective glyphosate-based herbicide that can eliminate any type of weed or grass and does this without you having to pull weeds by hand. Did you know Roundup is the most widely used herbicide in the United States?

This Glyphosate weed killer stays active in the soil for 3 to 249 days, making it an excellent weed control agent to have in your arsenal of weed killers. 

The Roundup weed killer product line consists of three solutions – Ready-to-Use, Ready-to-Spray, and Concentrate, each of which is suited for different needs and weeds including garden weeds.

When applied properly, most Roundup products showcase visible results within a few hours, though some may take a bit longer. Keep reading to find out the time it takes for Roundup to work, and how to speed up the process. 

spraying RoundUp on weeds

What is Glyphosate and How Does it Work? 

Glyphosate is the main ingredient in Roundup but some people tend to use the two words interchangeably. Glyphosate is not a newcomer in the herbicide space but has been used to kill weeds and unwanted plants since its inception in 1950. 

According to the Oregon State University’s National Pesticide Information Center, Glyphosate is in over 750 glyphosate herbicide-based products in the country and is also the most widely used herbicide in the U.S. 

This chemical compound is used worldwide in lawn care and agricultural industries and is generally applied by farmers via aerial or ground spraying, injections, or droplet systems.

Glyphosate is sold under several brand names including Roundup, No Grow, Weedmaster Duo, and Zero. 

How Long Does Glyphosate Take to Work? 

Glyphosate works as soon as it coats the leaves but 4 – 20 days are required to completely kill the plants including nearby plants the herbicide comes in contact with. 

Glyphosate works best when applied at the correct rates, and applying more is not better but can be detrimental to you and the environment. 

Unlike some herbicides that kill grasses and other broadleaved weeds, Glyphosate basically kills everything that’s green. 

This nonselective, systemic herbicide kills several different types of weeds and desirable plants including woody plants by entering the plant’s leaves and reaching its crown and roots using the plant’s circulatory system. 

Glyphosate then blocks the food supply to the plant, which then shrivels and dies.

spraying the lawn

Difference Between Glyphosate and Roundup

Just because Roundup contains Glyphosate, doesn’t mean they’re the same product.

The biggest difference between Glyphosate and Roundup is that the former is a chemical compound and the latter contains the chemical compound. 

Glyphosate is a pure chemical compound, meaning it doesn’t contain any other additives, but Roundup contains other ingredients apart from Glyphosate such as the surfactant POEA (polyethoxylated tallow amine), which is used to increase the efficacy of the active ingredient Glyphosate. 

Roundup vs. Glyphosate – Which One Works Faster? 

Glyphosate is the active ingredient in Roundup, but this is just one part of the chemical cocktail that makes up the herbicide.

The complete formation of Roundup is much more toxic to plant cells than Glyphosate is by itself, therefore works faster to kill weeds and plants, sometimes minutes after application or hours after application. 

Glyphosate also works quickly to eliminate plant life but not as fast or effectively as Roundup since it doesn’t contain any extra ingredients to boost its efficacy. 

How Long Does Roundup Take to Work?

The exact time period Roundup takes to work depends on a few things like the species of weeds you are trying to eliminate and the conditions including weather conditions and environmental conditions in which you use this potent herbicide. 

However, Roundup claims that it works within a few hours after application, and even though I noticed visible results within this considerably short period of time, it wasn’t the case for all plants. 

You may notice some initial wilting of most plants and lawn grasses within 3 hours after application with a garden sprayer or similar tool, but you ideally have to wait for a week or so for roundup to kill plants at the roots

How Long Does Roundup Take to Kill Grass? 

Healthy grasses including southern grasses and unwanted growth have hardy and robust root systems, hence it will take longer to kill them with Roundup. 

It will take longer to kill grasses if you use the wrong Roundup herbicide formulation. Roundup Plus Concentrate is recommended for killing grasses and delivers fast and powerful results. 

Roundup Weed and Grass Concentrate Plus Killer (Case of 12), 16...
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Roundup Weed and Grass Concentrate Plus Killer (Case of 12), 16...
  • Contains Fast Act Technology
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  • Add 6 oz. per 1 gallon of water for best results

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Roundup Concentrate Plus kills tough grasses and weeds down to the roots and is rainproof in just 30 minutes. Adding to this, it features FastAct technology, which displays visible results within just 9 hours after application. 

The best method of application of Roundup Concentrate Plus is with a Roundup sprayer.

Roundup 190314 Backpack Sprayer for Fertilizers, Herbicides, Weed...
2,163 Reviews
Roundup 190314 Backpack Sprayer for Fertilizers, Herbicides, Weed...
  • 4-Gallon sprayer meets all your spraying needs (controlling weeds and pests, growing flowers and vegetables)
  • Heavy-duty poly wand with shut-off featuring a comfort grip, lock for continuous spraying and an in-line filter
  • Viton seals and gaskets throughout the pump and shut-off for long lasting chemical resistance

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Or you can use any other good sprayer such as the Chapin garden sprayer

Chapin 20541 Made in the USA 1-gallon Lawn, Garden and...
8,994 Reviews
Chapin 20541 Made in the USA 1-gallon Lawn, Garden and...
  • FOAMING NOZZLE: ideal for heavier coverage, you actually use less product because it foams and there’s more accuracy in chemical placement because you clearly see where it’s going
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Apart from lawn grasses, Roundup Plus Concentrate can be used around flowers, shrubs, mulch beds, gravel areas, fences, and foundations. 

How Long Does Roundup Take to Kill Weeds? 

Killing weeds with Roundup takes a bit longer than grasses because all weeds aren’t created equal, some are easy to penetrate and kill, and others not so much. 

Stubborn weeds include ragweed, dandelions, and water hemp, for which Roundup takes longer to work.

There are certain weed species such as poison ivy that are extremely hard to eliminate even if you use potent Glyphosate, which is why Roundup has created special formulations for these plants. 

Roundup Concentrate Poison Ivy Killer Plus is formulated to kill over 200 types of weeds and grasses including wood brush and vines. 

Roundup Concentrate Poison Ivy Killer Plus Tough Brush Killer for...
  • BRUSH & POISON IVY KILLER: Roundup Poison Ivy Plus Tough Brush Killer kills over 200 types of weeds and grasses including Woody Brush and Vines, Poison Ivy, Poison Oak, Poison Sumac, Kudzu, and Wild Blackberry
  • FAST ACTING: Visible results in as fast as 24 hours
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Roundup poison ivy delivers results in as little as 24 hours and kills weeds at their root so you can bid adieu to them forever. Plus, it is rainproof in 30 minutes and is easy to apply with a sprayer. 

If you’re looking to kill weeds such as crabgrass, dandelions, clover, and yellow nutsedge without killing the grass in your lawn, Roundup for Lawns is a great option. 

Roundup for Lawns₄ Ready-To-Use (Southern) - All-in-One Weed...
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Roundup for Lawns₄ Ready-To-Use (Southern) - All-in-One Weed...
  • Kills weeds, not the lawn (when used as directed)
  • Kills 93 types of weeds, including dollarweed, dandelion, clover, and yellow nutsedge (see inside booklet for weed list)
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It is designed to kill over 250 unwanted plants and can be used without harming grasses such as Kentucky bluegrass, perennial ryegrass, fescue, bermudagrass, buffalograss, and zoysiagrass. Roundup for Lawns works immediately after application in as little as 3 hours. Here’s the Roundup product label and directions for use.

Lawn weeds

How to Make Roundup Work Faster? 

There are a few things you can do to make Roundup work faster.

  • Spray when the weather is better – Roundup will not work as quickly or work at all if the weather temperature within 2 days before or after spraying is above 50 degrees F.

Weeds must be healthy and not lethargic due to cold temps to absorb the Roundup herbicide Glyphosate and move it down to the growing points and roots quickly. 

In cold weather temperatures, even a lethal dose of Roundup will fail to reach the plant’s roots, which will probably make you think that the weed is resistant to Roundup. 

This is exactly the same if the weeds are under severe drought stress or the temperatures are above 90 degrees F. 

  • Keep the Water Volume Low – One of the common myths in the lawn care space is that more water equals better control, but this isn’t true! 

Tiny weeds can’t hold much moisture on their leaves, therefore use less water so that each droplet of Roundup is concentrated. A good ratio is spraying 5 to 10 gallons of Roundup per 1 acre of lawn. 

  • Spray Weeds When They’re Small – For effective and fast roundup herbicide performance, spray weeds when they are 2 to 4 inches tall. Unwanted plants of this size can absorb Roundup better than overgrown plants in your lawn. 
  • Use Flat Fan Nozzles Instead of Drift Reduction Nozzles – Flat fan nozzles deliver smaller spray droplets, resulting in better coverage and quicker weed control. 
  • Use Soft Water – Hard water containing iron, calcium, and magnesium can reduce the efficacy of glyphosate. If the water is extremely hard, Roundup can become useless as it can no longer bind to the right pathways in the plants it targets.
  • Store Roundup Properly – If Roundup is stored outside in your garage or shed in freezing or very hot conditions, it may cause the active ingredient to become ineffective. 
  • Allow a Rainfree Period – Although the Roundup label for most products states that the herbicides are rainproof between half an hour to 3 hours, I’d suggest stretching that time window to 6 hours. 
  • Apply Roundup at the Right Time of Day – For maximum and fast effectiveness, Roundup should be applied later in the day and not first thing in the morning. 

Does Roundup Need Sunlight to Work? 

Roundup doesn’t need sunlight to be effective as long as the weeds are actively growing.

But you should refrain from spraying Roundup in the evening or night as it can result in reduced herbicide control. 

Instead, it is best to spray Roundup during the day whether sunny day or cloudy day because the herbicide is easily absorbed by the plant leaves as they transport moisture during photosynthesis. 

spraying herbicide on weeds

Spraying Roundup in the evening or night will most likely not be effective given that plants and weeds do not perform photosynthesis at night. 

How Long Does it Take Roundup to Dry?

It takes approximately 30 minutes to 3 hours or a little longer for Roundup to dry. This timeframe also depends on how much Roundup you’ve sprayed. 

If you’ve soaked your lawn with Rounded, it will obviously take a bit more time to dry.  

While Roundup is wet, keep kids and pets pet of the sprayed area because getting in contact with Roundup can have severe and life-threatening effects.


Q. Can You Spray Roundup on Wet Grass? 

A. For optimal results, Roundup should be sprayed on dry foliage. Applying Roundup to wet grass will dilute the herbicide and make it less effective. There is also a good chance of herbicide runoff from already wet grass. 

Q. How Soon After Roundup Can I Plant? 

A. It is safe to plant ornamental flowers, shrubs, and trees the next day after using Roundup. But you have to wait for at least 3 days after spraying Roundup to plant edible plants and trees. 

I usually like to give Roundup time to work before planting anything because some plants take longer to absorb the herbicide than others. If you start digging, you may disturb the weeds before they are completely dead, so the live roots may sprout again. 

Q. Does Roundup Poison the Soil? 

A. For optimal results, Roundup should be sprayed on dry foliage. Applying Roundup to wet grass will dilute the herbicide and make it less effective. There is also a good chance of herbicide runoff from already wet grass. 

Here’s an informative review about Roundup for lawns:

Roundup For Lawns Before & After Review

Final Thoughts 

Roundup is a powerful herbicide that takes roughly a few hours to a few days to start working towards killing unwanted plant life. 

There are many things you can do to accelerate this process such as using Roundup at the right time of day, storing the herbicide properly, and spraying the weeds when they’re small. 

Always use precautions when using Roundup like wearing appropriate safety gloves, clothing, and eyewear, and make sure you keep kids and pets away from the treated area until the herbicide has completely dried.