Roundup is a potent herbicide made up mostly of glyphosate. It can quickly kill weeds, vines, grasses, and other unwanted plants.
Roundup is very effective when you use it correctly, but it’s also possible to mess up accidentally, especially if you’re new to gardening.
If you’ve never used the product before, you might wonder, how long does it take for Roundup to work? You also might have a few other questions before you spray. Below, I’ll answer as many as I can, so you can feel confident using this herbicide.
How Quickly Does Roundup Work?
Roundup manufacturers promise visible results within a few hours of spraying their herbicides. Of course, the exact amount of time it will take to kill grass and weeds depends on a few things, like the species of plant you’re trying to kill and the type of Roundup you spray, and the conditions in which you are using it.
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Killing Grass
Grasses of all types are hardy and robust plants, and it can take a while to kill them off completely. Though you may see a bit of yellowing within a few hours of spraying a Roundup product, it usually takes closer to twelve hours to see actual results.
You’ll still want to wait a week or two to pull the grasses out. That allows complete root death to occur and ensures the grass won’t grow back later on.
Killing Weeds
With weeds, Roundup could take as little as four hours but up to two weeks to work. The variance is because of the vast differences between weed species. Some weeds are easy to penetrate and kill, but others, like dandelions, ragweed, or water hemp, are sturdier, and it will take longer for Roundup to work.
Certain weed species, particularly vines, like poison ivy, are exceptionally hard to eliminate, even with potent glyphosate. As a result, roundup manufacturers sell special formulations for those plants, but even then, you can expect them to take several days or weeks to do their job.
When using Roundup to kill weeds around trees or other plants and shrubs in your outdoor space, you’ll need to take precautions if you want them to continue growing. Roundup will kill anything it comes into contact with so covering up any ‘keepers’ plus accurate spraying skills are required to avoid accidental loss of life. I use pieces of the tarp to cover anything I don’t want to damage or kill.
Can You Make Roundup Work Faster?
While you can’t make Roundup work faster by spraying more liberally, you can optimize your spraying conditions. By paying attention to when you spray your herbicides, you’re more likely to see fast and efficient results.
Ideally, you’ll use Roundup on a rain-free day with little or no wind, ideally in early summer or spring when the temperature is reasonably warm. Of course, a rain-free day in early spring isn’t so easy to find in some areas. As long as you have a six-hour rain-free period, though, you should be fine to spray.
To ensure that your Roundup spraying works fast, you’ll want to choose a day that’s humid as well. The high humidity encourages plant growth, and though it may sound counterintuitive, you want your weeds and unwanted grasses to be actively growing when you spray. That way, they can absorb the Roundup more efficiently.
If you notice no yellowing or other signs of plant death after ten days, you may want to reapply the Roundup. Something may have blocked the product from working effectively, like moisture on the plant’s surface or excess dust in the air.
How Roundup Works
Understanding how Roundup works can help you maximize its effectiveness so it is important to know that herbicide products containing the active ingredient Glyphosate (such as Roundup) work best when sprayed directly on the leaves and stems of growing plants.
According to a 2015 review study on glyphosate in runoff waters and the effect it has on roots, there can be some root absorption in certain crops, like beets, barley, and cotton. However, directing your sprayer toward roots usually just results in soil particles becoming bound by product, and this prevents roots from absorbing it.
How Glyphosate Kill Plants
When plants are sprayed with Glyphosate, the process of diffusion occurs. This is where the vapor that lands on leaves, stems, and woody sections of the plant get drawn in towards the plant’s nervous system.
That’s when glyphosate gets to work. This organophosphate compound begins to disrupt the plant’s enzymatic processes. Specifically, it blocks the shikimic acid pathway, which produces chorismate. Chorismate is an intermediary molecule that acts as a precursor for several amino acids plants need to survive.
To put it simply, Roundup starves plants, effectively killing them over a few hours or days. All plants and fungi have the shikimic acid pathway, so Roundup doesn’t discriminate between weeds and other species.
Humans and animals don’t have this same pathway that Roundup needs in order to be effective and, therefore, it poses less of a risk to people and pets. However, caution must always be exercised when using Roundup so always wear protective clothing, thoroughly read the manufacturer’s instructions before use and avoid the area that has been sprayed until it is safe to do so.
Best Time to Use
According to manufacturers, the best time to spray post-emergent herbicides is in spring or early summer when plants are actively growing. The temperature should be between 45 and 90 degrees Fahrenheit, and you should spray during the day rather than at dawn or dusk.
Ideal Weather Conditions For Using Roundup
In my experience, you’ll want to pay attention to other weather conditions. For example, windy days aren’t good for spraying, as the product tends to get blown everywhere except the plant leaves!
Dust in the air can also inhibit the effectiveness of herbicides. Glyphosate easily binds with soil particles and then can’t penetrate plants. So, if you’re spraying directly after kicking up dust with any sort of landscaping process, it probably won’t be very effective.
Waiting for a relatively warm and humid day with no sign of rain, little wind, and no dust is the best time to spray Roundup.
Make sure you use a clean sprayer. Using chemicals that have been allowed to hang around will be much less effective and may mean you need to do the job twice.
Length of Time to Dry
According to the manufacturer, Roundup takes from thirty minutes to three hours to dry, depending on the exact product formulation you choose and the current weather conditions.
Your bottle of Roundup should list a specific drying time on the back. Though glyphosate is of low toxicity for people and pets, you might want to keep kids and animals away until the Roundup is dry. You’ll also want to avoid watering until you’re past the stated drying time.
Spraying on Wet Grass
Spraying Roundup on wet grass isn’t a good idea. Moisture will reduce the herbicide’s effectiveness. When you spray it on wet grass, you’re essentially diluting the product. There is also a risk of runoff whereby rainwater acts as a barrier and prevents the chemicals from being absorbed by the leaves and stems. So, ideally, the grass or weeds you’re spraying will be completely dry.
Re-Planting After Use
Roundup affects all plant species, wanted and unwanted. So, you can’t plant your garden directly after spraying Roundup. How long you have to wait depends on the formula you use and the type of plants you’re trying to grow.
With most Roundup formulas, you need to wait one day before planting ornamental flowers, trees, or shrubs. For grasses, herbs, fruits, or vegetables, you need to wait three days after spraying.
If you’re using Roundup Max Control, you’ll need to wait a whole year before you attempt to plant anything in the area.
With Roundup Extended Control, you’ll need to wait four months. And, for Roundup Poison Ivy and Brush Killer, you’ll need to wait at least thirty days.
Pulling Weeds After Spraying
Roundup kills weeds and unwanted plants, but it doesn’t do all the work for you. You’ll still have to manually remove the dead weeds it leaves behind before you plant your garden.
The benefit of Roundup is that it makes weeds much easier to pull. By their very nature, hardy weeds have long, thick roots that make them tricky to remove while they’re alive. That’s why pulling weeds without Roundup often leads to them returning a few weeks later. Unless you get to the roots, weeds can grow right back.
By killing the weeds with Roundup first, you’ll easily be able to remove them, and you can rest assured they won’t return to haunt your garden.
Final Thoughts
Learning how long it takes Roundup to work is helpful before you go to use it. Roundup is a powerful herbicide that’s fairly easy to apply, but it’s not uncommon for new gardeners to accidentally inhibit its effectiveness.
If you spray at the wrong time of day, when plants are wet, or aim at the roots instead of the leaves, you probably won’t get the results you’re looking for. But, with a few careful considerations in mind, you can get rid of grasses and weeds in one easy application. Then, you can get to work planting your beautiful, weed-free garden!
