Natural Weed Killers – Do Organic Herbicides Work?

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Robert Pavlis

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.

Natural Weed Killers - Do Organic Herbicides Work?
Natural Weed Killers – Do Organic Herbicides Work?, photo by Forest & Kim Starr

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.

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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.

YouTube video

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.”

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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.

YouTube video

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.

Effect of various organic herbicides compared with Roundup, photo by Maggie Reiter
Effect of various organic herbicides compared with Roundup, photo by Maggie Reiter

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.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

29 thoughts on “Natural Weed Killers – Do Organic Herbicides Work?”

  1. Hi I have some speedway its an ici granular product has anyone used it . I don’t have any instructions for it so don’t know the ratio to water it down. Any help?

    Reply
  2. I have never in my life used any kind of weed killer on the lawn or in the garden. I keep my garden weeded as part of the upkeep. As for the lawn, my husband hand-pulls all weeds, then he fertilizes once a year with Milorganite, and then overseeds the lawn with grass seed in the fall. Year after year the lawn has improved. He also dethatches and aerates the lawn in fall about every 2 years.
    I find if you pull the dandelions before they turn into “puff balls” they won’t seed all over your lawn. The toughest weed to deal with is crabgrass. For that my husband uses a claw weeding tool and pulls it out weekly…however, sometimes he has to resort to getting down on his hands and knees to pull it out. I admit, he looks like a crazy man out there but he’s had good success removing most weeds. A healthy lawn is the best defense against weeds.

    Reply
  3. Yes Glyphosate is very effective. That’s why there are almost no wild flowers and other plants left in our agricultural environments and bee, insect and, as a consequence, many bird and animal populations are crashing.
    Let your “weeds” grow and pull them out if they get too much.
    Monsanto and Bayer must love these posts!

    Reply
    • What a dumb comment, “That’s why there are almost no wild flowers and other plants left in our agricultural environments”. The reason there are no wildflowers in our agricultural environments is because we us those areas to grow crops!

      The single biggest drop in insect and bird populations is due to urbanization – not Roundup.

      Reply
      • “because we use those areas to grow crops’ is an even dumber comment. Do you know how many ‘weeds’ are edible? Do you know how much more productive the crops would be with wildflowers in them to create a healthy environment and actually have pollinators and healthy soil.

        Reply
        • “Do you know how much more productive the crops would be with wildflowers in them” – do you?

          Provide some links that shows the amount of harvests go up with wildflowers.

          If this actually worked, don’t you think farmers would do it?

          Reply
  4. What is your chemical solution to getting rid of aggressive perennials such as goutweed and rampion, which have rather deep rhizomes that resist even repeated applications of glyphosate? I have slightly greater success using glyphosate on rampion than goutweed, but both seem to shrug it off, and regrow quickly after an initial droop.

    Reply

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