Weeds, weeds everywhere! Gardeners are looking for the best natural weed killer to get rid of them. Roundup certainly works, but is there an equivalent natural product that is less harmful to the environment and our health. In this blog I’ll look at a variety of organic herbicides to see if they work for controlling weeds and lawn grass.
What is an Organic Herbicide?
A herbicide is a chemical that kills plants. From a chemists perspective almost every product sold for this purpose is organic, but that is not how the term is used here. In this post an organic herbicide will be one that is made from natural products, or one that is man-made, but occurs in nature. For example, acetic acid (vinegar) is found in most plant and animal cells, so it is natural, but the product we use is made in a chemical factory.
There are two basic types of herbicides; systemic and contact.
A systemic herbicide is one that is absorbed by the plant. Once inside, it causes defined chemical reactions that lead to the death of the plant. The reason these are very popular is that it affects the whole plant from the inside out. Glyphosate, the active ingredient in Roundup is a systemic, as is 2,4-D. These can be selective, affecting only some plants, or non-selective (kill all plants).
A contact herbicide is a chemical that damages plant tissue on contact. It is fast acting, but may only damage the leaves, since it never comes in contact with roots.
Pre-emergent vs Post-emergent Herbicides
There are two basic approaches to controlling weeds. Pre-emergent herbicides kill the seeds or seedlings as they germinate (ex corn gluten meal). Post-emergent herbicides are sprayed onto plants once they are growing.
For this discussion it is also important to categorize the latter into two subgroups; seedlings and older weeds. Seedlings are much easier to kill and consist of plants that have at most one true leaf. I’ll use the term older weeds for anything larger than a seedling.
When is a Weed Dead?
This seems like a simple question, but it’s not. A contact herbicide affects living plant tissue. When sprayed onto leaves it usually causes rapid death of the cells in the leaf, resulting in black or brown leaves. Many people who claim that vinegar is a good herbicide see these black leaves a few hours after spraying and declare “vinegar kills weeds.” But in a couple of weeks, the plant grows back from the roots and crown that were unaffected by the herbicide.
This video gives you a good example of this.
A weed is only dead when it is completely gone. A weed that regrows, is not dead. For this reason it is important that research studies look at the long term effect of products. Reports of what happened after only 48 hours tell us very little about the efficacy of the herbicide.
Examples of Natural Weed Killers
A number of products are claimed to kill weeds.
- Vinegar (5% acetic acid)
- Horticultural acetic acid (normally 20%)
- Plant oils (clove, lemongrass, cinnamon)
- D-limonene
- Citric acid
- Caprylic acid
- Soaps and detergents
Some commercial formulations combine one or more of the above with various plant oils, such as vegetable oil.
Some Common Myths About Organic Herbicides
The general public believes a number of myths about organic herbicides, which explains in part the interest in finding some good ones.
Organic Herbicides are Safer
This is not necessarily true. Both synthetic and organic herbicides can be safe, or they can be toxic. Some synthetic herbicides like Roundup have extremely low toxicity, even lower than vinegar. “While organic, 20% acetic acid is very toxic to the person applying the herbicide,” Windbiel-Rojas said. “It will burn one’s skin, hair and eyes so the applicator must wear more personal protective equipment than with some other herbicides. This material is also problematic to use in a public space because any bystanders could be exposed to drift during application.”
Organic Herbicides Harm the Environment Less
Not generally true. Just because a chemical occurs in nature does not mean it won’t harm the environment.
Organic Herbicides are Extracted From Natural Products
This is almost never true. Commercial products are generally produced in chemical factories in the same way as synthetic herbicides.
Organic Herbicides Don’t Contaminate Groundwater
Any chemical that dissolves in water will move along with it as it moves through the soil, on its way to rivers and lakes.
Synthetic Herbicides Contaminate Soil
Both synthetic and organic chemicals can contaminate soil, but many in both categories don’t. Roundup for example binds to clay particles very quickly and becomes inactive. Here is a little experiment I did to see how glyphosate in soil affects sprouting seeds.
Do Natural Weed Killers Work?
Here is what the science says.
This study treated both grass and weeds at a couple of locations, with various OMRI (Organic Material Review Institute) certified products including acetic acid (20%) and citric acid, as well as non-approved glyphosate (Roundup). One site also included different natural ingredients, such cinnamon oil, pelargonic acid and ammonium nonanoate. The results from one location are shown below.
Citric acid plus clove oil had almost no effect. The other natural weed killers showed immediate injury, but within a week the plants started to recover, and after three weeks they were mostly recovered. Glyphosate is slower acting because it is a systemic, but even after a month the plants were still dead.
In other studies, W. Thomas Lanini looked at acetic acid, citric acid, d-limonene, clove oil, cinnamon oil and lemongrass oil, and concluded that “while organic herbicides can burn back the tops of perennial weeds, perennial weeds recover quickly.” Seedlings are more easily killed, but older weeds are unaffected long term.
“The control ranged from better than 60% to 100% if these weeds received high volumes of these materials when they were just 12 days old. When broadleaf weeds were 26 days old, even high volumes of these materials gave at best less than 40% control.”
The effectiveness of organic herbicides is higher when used in larger amounts and when sprayed in warmer weather (best above 24° C [75° F]). This points out one of the problems using natural weed killers. It requires a lot more chemical than common synthetic herbicides. In order to compare the safety aspects of organic and synthetic, you have to take into account the higher dose which affects both safety issues and environmental issues.
Acetic acid has been studied quite a bit. Vinegar is 5% acetic acid and at that concentration it has almost no effect on older weeds, although it may kill seedlings. Horticultural acetic acid is 20%. When applied at higher doses, it can kill some perennial weeds. In one study, “optimum grass and crabgrass control occurred with 20% acetic acid applied at 100 gallons per acre (1 gallon/500 sq ft), resulting in weed control that ranged from 28 to 45%.” That is a fairly low kill rate and they only waited 7 days after spraying to see if weeds regrew.
When evaluating this kind of data it is important to look at the fine print. One report from the U.S. Department of Agriculture said, ” Canada thistle, one of the most tenacious weeds in the world, proved the most susceptible; the 5-percent concentration had a 100-percent kill rate of the perennial’s top growth.” At first glance this looks promising, until you see the words “top growth”. This means the leaves were burned off, but the plant was not dead. The quote is incorrect when it uses the words “kill rate”. Even Roundup has trouble killing this plant when it has an extensive root system.
Natural Pre-emergent Herbicides
The most common natural pre-emergent herbicide available to homeowners is corn gluten meal. Note the name – it is not corn meal.
I have discussed this product in detail in: Corn Gluten Meal – Does it Work for Weeds?
Do Natural Weed Killers Work?
None of the above mentioned products are as effective as Roundup.
That does not mean organic herbicides do not work, but a user needs to understand their limitations. Most of the above herbicides are effective on seedlings. If the herbicide is applied monthly, it will keep new weeds from getting started, but it does not prevent weed seed from germinating.
Organic herbicides will also burn down the leaves of older plants. The weed will then regrow in 2-3 weeks. If herbicide is applied at regular weekly, or monthly intervals it will eventually kill most weeds. But is this something most home owners are willing to do?
The organic herbicides are less effective on grasses and sedges than they are on broadleaf weeds, and they’re almost useless for weeds that have large underground root systems like field bindweed and Canada thistle.
Higher concentrations and higher volumes are usually more effective, but some of the products are quite expensive. They also lack residual activity and need to be applied repeatedly. Because of cost the obvious choice for homeowners is vinegar, but it is also one of the least effective.
Does household vinegar work? It kills seedlings and it burns the leaves of some older weeds. If re-applied every two weeks, it will control some kinds of weed, but not all.
Whatever weedkiller you use: spraying is not the only way to apply it. Maybe try using a brush, sponge, or cotton swab to touch your weeds with the recommended dilution. It does help to keep chemicals off areas and plants where you don’t want them.
Mulches over chemicals for unwanted plant “weed” control. Composting them helps in the sustainability effort of the system, too.
Thank you for the well presented information. I often find that problems with weeds and some pathogens are best managed to my benefit rather than spending a lot of energy going for the kill. For example, powdery mildew has been a persistent problem with my vines. For the past few seasons I have successfully applied a foliar spray of potassium bicarbonate before the powdery mildew establishes itself. As you point out, a treatment such as this, like other treatments, requires consistent reapplication throughout the growing season in order to maintain successful control.
This is fab Robert thank you. I have been trying to point all this out to the disciples of “but it’s natural so it must be better” for literally YEARS. Great to have your experiment pic to point to.
I use a garden weed burner ie gas bottle attached to a wand it has a blow torch effect/flame thrower, hold it there long enough and it will heat up the roots and destroy them as well as the top growth Note obviously use with caution and have a water hose near by, you don’t want to set the whole garden alight
They don’t work much better than organic herbicides – kill seedlings and burn foliage – do not damage roots.
What not even if you hold it there for long enough surely the searing heat must kill the roots of weeds
Why must it? The soil is a great insulator.
Ever walk on the beach on a hot sunny day? The top inch of sand is hot but below that it is still cool.
Natural weed killers definitely take more effort than chemical. One must reapply often, and not expect a permanent kill with one application. However, for a homeowner, this may well be worth not using chemicals such as Roundup. There are many reasons not to use glyphosate, and thorough research shows why.
To keep this short, I’ll just say maybe we don’t know all the consequences of these weed killers, but in my opinion, if possible, use a natural alternative and protect your eyes and skin, rather than use something with many possible long term and cumulative problems.
1) You either did not read the post about Roundup – or you are just a disbeliever.
2) We have been studying Roundup for over 50 years – we know more about it than any of the natural weed killers.
Hi Robert,
Is pelargonic acid (nonanoic acid) any more effective than acetic acid?
(I see my local council was trialling pelargonic acid due to bad public perception of roundup – and fear of litigation since there are 42,000 plaintiffs and counting who claim that glyphosate herbicides caused their cancer)
A lot depends on how much is applied and in what concentrations. Commercial products also formulate these differently. The post includes all of the references I found.
Thanks for pointing out that the surface material was burned but not anything down into the ground. Good information all the way through, thank you.
Thank you for doing the heavy lifting – and explaining so clearly. Glyphosate is the only thing that works against bindweed – which emerges after most seeds have sprouted, or transplants are in the soil. My allotment has banned glyphosate so I can’t conduct a parallel test on the effect on growing plants – although I sure would like to!
Glyphosate even has a tough time with bindweed. You need to let the plant grow, so it has enough leaves to absorb a reasonable amount of chemical. The root systems can be very extensive, and I suspect it only kills the roots near the treated green parts.
My experience with bindweed and glyphosate : it killed some bindweed and my perennial poppies The bindweed came back. I think the glyphosate also weakened several other plants near by I now mostly use glyphosate on paths and on areas where I want to kill everything
If the bindweed came back – it was not killed. Roundup won’t hard nearby plants unless you get some overspray. When I have tough weeds like this in a garden with other plants, I paint instead of spraying. Take a paintbrush and paint the leaves of the weed. This ensures no other plants get damaged.
Thank you for the post. I get tired of seeing posts from people who say vinegar and salt are great as weed killers. So many myths out there and you are tackling them one (or two) at a time.