Does Organic Weed Killer Work?

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Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil.Paul takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test.

“Synthetic” vs. “Organic” Weed Killers

Part of the struggle for gardeners is that the terminology can be confusing. The label “organic” seems to appear everywhere, but it can often be misleading.

Synthetic Herbicides

Synthetic herbicides rely on the use of lab-created chemicals. The most prevalent of these chemicals is glyphosate.

Glyphosate is an extremely effective weed killer, but it is not risk-free. A number of lawsuits have alleged–but not proven–that glyphosate can be linked to cancer, and there are claims of long-term risks to the environment as well (source).

Glyphosate is the active ingredient in many of the synthetic weed killers on the market, including Round-up. While glyphosate is not an organic herbicide, it’s important to understand how it works so you can choose the right organic alternative.

Glyphosate is a postemergence weed killer. This means that it targets weeds that have already sprouted (emerged) from the soil.

However, glyphosate doesn’t just target the weeds’ leaves and stems. It’s a systemic herbicide that moves throughout all parts of the weed, including the roots. This is how it’s able to kill both annual and perennial weeds.

Glyphosate is non-selective, which means it doesn’t kill just one type of weed. This is something it has in common with many varieties of organic herbicides (source).

Organic Herbicides

There can be some confusion over the term “organic.” Many people mistakenly believe that “organic” means “chemical-free.” However, every substance on earth, toxic or non-toxic, is composed of chemical compounds; we simply can’t exist without them.

The difference between organic and synthetic products is that an organic product is one that relies on use of chemicals found in nature, like essential oils or acids, rather than chemicals that were developed in a lab.

However, there’s another twist. Scientifically speaking, any substance that contains carbon is organic. This has led some manufacturers–not just in the herbicide market–to label their products as “organic,” when in reality, these products may contain synthetic ingredients.

This is why, if using organic products is important to you, it’s important to buy from reputable manufacturers whose ingredients are certified organic by the U.S. Department of Agriculture or the Organic Materials Review Institute.

There is also a tendency to assume that if a product is organic, then it’s non-toxic and safe. This is a misunderstanding. Organic herbicides don’t carry the same risks as synthetic ones, but you should still adhere to the manufacturer’s safety instructions, especially if you have pets or children.

How Organic Weed Killers Work

Active Ingredients

Organic weed killers rely on the following active ingredients. Don’t be alarmed by the names of these chemical compounds! All of these ingredients are naturally derived.

Acetic acid (highly concentrated vinegar)
Ammonium nonanoate
Cinnamon oil
Citric acid
Citrus oil (d-limonene)
Clove oil or clove leaf oil
Eugenol
Lemongrass oil
Pelargonic acid+fatty acids
2-Phenethyl propionate
Sodium lauryl sulfate (source)

These ingredients are often used in combination with each other and at high concentrations. A popular option for gardeners is Avenger Weed Killer (link to Amazon), an organic herbicide that uses citrus oil as its active ingredient.

It is not recommended that you try to make your own herbicide with household ingredients. For more information on homemade weed killers, see our article Do Natural Weed Killers Work?

Function

The above active ingredients don’t work equally well, but they all serve more or less the same function: burning weeds, specifically broadleaf and grassy weeds.

Like glyphosate, organic weed killers work on post-emergent weeds. Unlike glyphosate, organic herbicides are not systemic. They will burn the above-ground growth of the weed, but they won’t penetrate down to the root system.

This means that, when applied properly, organic herbicides will burn back the leaves and stems of existing weeds, but they won’t prevent new weeds from sprouting.

Furthermore, also like glyphosate, organic herbicides are non-selective. They will harm any plant they touch, so spray carefully (source).

Effectiveness of Organic Weed Killers

Several factors are involved when determining an organic weed killer’s effectiveness:

Active ingredient(s)
Concentration
Age and type of weed
Number of applications
Environmental conditions

Active Ingredients

A study performed by the University of California Cooperative Extension showed that, unfortunately, not all organic herbicides contain effective ingredients.

Researchers tested five organic herbicides in 25-square foot plots and compared the results against an untreated plot.

The plot treated with a combination of citric acid and clove oil showed no weed reduction at all.
The plots treated with citrus oil (d-limonene), ammoniated soap of fatty acids, caprylic acid (derived from coconut and palm oils), and acetic acid all showed significant weed reduction within two days.
Within 28 days, weeds in all five test plots had completely regrown (source).

Keep in mind that these were test plots designed to study which active ingredients perform best after one single application. There are still other factors to consider.

Concentration

Organic herbicides must be applied thoroughly. A couple of drops is not enough; the surface of the weed must be completely covered.

Furthermore, lower concentrations of herbicide at high spray volumes (10% concentration in 70 gallons per acre) are better than higher concentrations at low spray volumes (20% concentration in 35 gallons per acre) (source).

Age and Type of Weed

The age of the weeds you want to kill plays a significant role. The older the weeds, the harder they will be to kill.

Another study by the University of California showed that organic herbicides sprayed onto 26-day-old broadleaf weeds had very little effect. The same herbicides applied to 26-day-old grassy weeds had almost no effect at all (source).

The herbicides performed much better against broadleaf weeds that were only 12 days old. Weeds at this stage measure no more than four inches and have two to four leaves. This is by far the best time in their life cycle to apply an organic weed killer (source).

Unfortunately, the UC study demonstrated that even on very young grassy weeds, organic herbicide was not very effective. This is possibly due to those weeds’ extensive root systems.

Number of Applications

Organic weed killers don’t damage the weeds’ roots, and they don’t linger in the soil, preventing new growth. A single application of organic herbicide is simply not going to banish weeds from your garden in the long run.

However, many organic herbicides are safe enough to apply more than once. Be sure to follow all of the manufacturer’s safety and application recommendations.

Environmental Conditions

Organic herbicides perform best at temperatures above 75℉. Below that temperature, you may still see some results, but your best bet is to apply them on a warm day.

There is some evidence that shady conditions are preferable to full sun, but the temperature seems to play a bigger role (source).

Recommendations

The bottom line with organic herbicides is that they are unlikely to completely control weeds alone. They work best in combination with other non-chemical methods of weed control.
Before you purchase an organic weed killer, consult your local Extension agent or horticulturalist to find out which products might be most effective for your situation. It’s possible that they won’t recommend an herbicide at all, depending on your soil, your local guidelines, and the plants you want to grow.
If you do choose to use an organic weed killer, make sure to abide by the manufacturer’s safety instructions! “Organic” does not mean “safe”; some active ingredients, like acetic acid, can be harmful to your skin and eyes.

Non-Chemical Weed Control Options

Before You Plant

You can start controlling the weed population in your garden before you even plant. Break up the soil in your garden beds and till deeply. This can work to push weed seeds deep enough in the soil that they aren’t able to germinate.

Another option is called soil solarization. This involves covering your garden bed in a sheet of plastic to trap heat. The trapped heat creates an intolerable growing environment for weeds, killing them off.

A third method is to irrigate your empty garden beds to encourage weed growth. As weeds emerge, use deep tilling, an herbicide, or a combination of methods to kill them off before planting (source).

After Planting

Landscape fabric is an extremely effective method of weed control–it can prevent weed growth for two years or longer! Unlike the plastic sheets used in soil solarization, landscape fabric doesn’t create hot spots that will damage your desired plants, but it does stifle weeds.

Mulch is another effective option. It doesn’t take more than two inches of mulch to keep weeds at bay. Furthermore, mulch can nurture your desired plants by decreasing water evaporation, keeping soil temperatures moderate, and providing necessary minerals to your plants (source).

Finally, hoeing and hand-pulling, while strenuous, are still good ways to manage small populations of weeds. Occasional weed-pulling may not be enough; make it a part of your regular gardening routine.

Conclusion

Gardeners who wish to avoid herbicides containing glyphosate have a wide range of organic options. However, all organic herbicides, even the best ones, are limited in their effectiveness. If you choose to use an organic weed killer, plan to combine it with other methods of weed control.