How Long Does Roundup Take to Kill Weeds?

For years, Roundup Weed and Grass Killer has been advertising that its grass and weed killer works with visible results in the first 2 hours. Lately, they have been more conservative and started to claim you will see visible results within 6 hours, so we decided to put that to the test by recording the application of some spring weeds in our flower beds and recording how they look after 2, 4, 6, and 24 hours.

We will also be recording how the weeds look after 3, 7, 14, and 21 days in a separate video, but for this article and video, we will be documenting the weeds’ appearance in the first 24 hours, so people can get a good idea of how well the product will work within the 1st day they spray the product.

The way Roundup works is that the active ingredient glyphosate blocks a specific enzyme (EPSP synthase), which is essential for plant growth. The chemical travels to the leaves, roots, and shoots, starving the plant for several days to weeks. The version we used is a non-selective herbicide, meaning it kills most vegetation it touches. There are versions that don’t kill grass (your lawn) but do kill weeds.

Because of the way it works, you will see the weeds start to break down in 4 stages:

Application & Absorption: By spraying directly onto foliage, the herbicide is absorbed and moves to the roots.
Initial Symptoms (Days 2–4): Visible wilting, yellowing, and twisting of the plant.
Necrosis (Days 7+): Leaves turn brown as the plant dies down to the roots.
Final Breakdown: The plant completely dies, with the process taking longer in colder weather.

Here is a slide show showing a variety of different weeds over the span of 2, 4, 6, and 24 hours post-Roundup application:

Having the right expectations of how and when Roundup works will help you better understand how to treat your lawn.

For example, we got a letter from our HOA stating we had a week to get our weeds under control. While spraying weeds would kill them, they would not be visibly gone within 1 week. To comply we had to pull or burn the weeds, a chemical weed killer just cant work that fast.

Roundup is a great product for those who plan to use at different points of the system, but spot spraying in weed beds, while effective, will be a long-term solution that takes weeks for great results.

As you can see, the weeds start to weaken and become limp, and some even start to lose their color but others are more hardy and need more time to break down – those look very similar at hour 24 as they did at hour 0. Regardless, given enough time Roundup will get the job done on most weeds .

To be safe with Roundup, it is very important to wear full-length pants and long-sleeve shirts along with gloves. It is also recommended not to be sprayed when windy to avoid the product from drifting into other plants or onto you.