Vinegar Weed Killer Myth

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Robert Pavlis

Commercial weed killers have been banned in a number of Countries including Ontario, Canada and people are now looking for natural weed killers. Does vinegar kill weeds? Is it a natural weed killer? Let’s have a closer look at this common garden myth.

bottle of vinegar — This is NOT a good natural herbicide, source: Joe Shlabotnik

Table of Contents

Vinegar Weed Killer

What happens when you pour vinegar on a weed? For most plants, the vinegar is so acidic that it damages the green leafy part of the plant. Within a day, the plant withers and the leaves go brown. It certainly looks like vinegar has killed the plant.

If you re-read the above sentence you will notice that it only talks about the green leafy part of the plant. What about the root below ground? Turns out that vinegar has almost no effect on the root system because it is a contact herbicide that is not absorbed by the plant. When vinegar reaches soil level it is quickly neutralized and converted to harmless acetate salts which have little effect on the weed.

The weed just regrows and in a week or so, the weed is back. Vinegar is not a natural weed killer.

Researchers at the USDA (United State Department of Agriculture) found that 5% and 10% concentrations killed the weeds during their first two weeks of life. Older seedlings and more mature plants were not killed.

Does Strong Vinegar Kill Weeds?

Vinegar is about 5% acetic acid, the rest being water. Pickling vinegar is about 7% acetic acid, but even this is not strong enough to kill weeds.

Growing Great Tomaotes, by Robert Pavlis

A higher concentration of acetic acid will kill some plants, but you need to use at least 20% acetic acid and 30% is even better. This is a good option from Amazon (affiliate link).

Some sources still call this vinegar, and I suggest that this is a bad idea. We all know vinegar as something that is fairly harmless, but 20% acetic acid is a dangerous chemical. This material should be called “horticultural vinegar”. The USDA has even issued a warning “WARNING: Note that vinegar with acetic acid concentrations greater than 5% may be hazardous and should be handled with appropriate precautions.” Make sure you wear gloves and eye protection when using this chemical. In this blog the term vinegar is used to refer to a product that is at most 7% acetic acid.

You will not find 20% acetic acid in the grocery store, but there are some commercial natural weed killer products on the market that contain this higher amount of acetic acid. There are also several products on the market that don’t have 20% and they are not worth buying because they are no better than pickling vinegar.

Does Vinegar Harm Lawns?

Many of the web sites that promote vinegar or the salt + vinegar mixture described below, show pictures of spraying it on lawns as in the image below.

person spraying the lawn — Spraying vinegar onto a lawn if a poor idea, source: LawnLove

Vinegar is a non-selective herbicide – it can harm all plants including the lawn. It won’t kill it, but it will turn it brown.

Will 20% Acetic Acid Kill Weeds?

20% acetic acid will kill small annual weeds but has limited effect at killing larger annual weeds. It only kills some perennial weeds and is not effective on grass weeds. The reason is fairly clear. Acetic acid does not affect the roots. The unharmed roots of large annuals and most perennials allow the plant to grow back.

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Pros and Cons of Horticultural Vinegar

The University of Maryland Extension developed this list of pros and cons for using it.

Pros

Kills some weeds rapidly, causing death in 24 hours.
Effective for killing annual broadleaf seedlings.
Vinegar biodegrades and breaks down quickly provided it is not mixed with salt.

Cons

Permanently kills only broadleaf weeds; grasses and perennials grow back.
Doesn’t harm roots.
Needs multiple applications to be effective.
Nonselective, will harm or kill your good plants as well.
Should not be sprayed on reactive metal such as aluminum, tin, iron.
Can irritate skin and trigger an allergic reaction; frequent exposure may cause chronic bronchitis, dermatitis and erosion of teeth.

Toxicity of Vinegar and Roundup

People like using vinegar because they think it is so much safer than commercial herbicides like Roundup. They are wrong.

Toxicity is measured as an LD50 value. The oral LD50 in rats for glyphosate, the active ingredient in Roundup, is 4320 mg/kg, which is less toxic than vinegar (3310 mg/kg). Low numbers are more toxic because it takes less to harm you.

Vinegar and Salt Killer Recipe

The vinegar + salt weed killer recipe consists of 1 cup table salt, 1 tablespoon dish soap and 1 gallon vinegar. Is this more effective than vinegar alone?

The soap helps the material stick to plants making it more effective. However soap also damages plants, which in this case might be a benefit.

Microbe Science for Gardeners Book, by Robert Pavlis

Table salt is sodium chloride and the sodium is quite toxic to plants. It is the main reason this mixture kills plants but there is a big problem here. Salt persists in soil where it harms any kind of plant, including the good plants you are trying to grow. Eventually, water washes it deeper into the soil layer and eventually into rivers and lakes where it causes environmental problems. The EPA has this to say, “salt can contaminate drinking water, kill or endanger wildlife, increase soil erosion, and damage private and public property”.

What About Repeated Use?

In my statement above I said that vinegar only damages the green leafy part of the plant. Let’s look at how this affects the plant. The plant has used its internally stored food reserve to grow the leaves. They are now ready to absorb sunlight and make more food. You come along and damage the leaves to the point where they are no longer useful to the plant. The plant now has to use more stored food to regrow new leaves. If you come along again and kill them off, you will weaken the plant even more. Over time this will kill the plant.

The secret to making this work is repeated applications that are timed close enough together that leaves never produce food for the plant. Go on vacation for a couple of weeks, and you can start the process all over again. Most of us are just not diligent enough to make this work.

Seedlings that do not have a good root system can be killed with a single application of vinegar.

Vinegar is also non-specific. It will damage any green plant including grass and most other living organisms.

Frequently Asked Questions

Q1? How long does it take vinegar to kill weeds?
Vinegar kills seedlings in 24 hours but it does not kill other weeds. It makes the leaves look brown in a few hours.

Q2? Is vinegar safe to use as a weed killer around pets and small children?
No! Vinegar can harm eyes and should be used away from pets and children. Horticultural vinegar is even more dangerous.

Q3? Will vinegar harm insects or other wildlife?
It can kill ants, spiders, mosquitos, amphibians and most insects. It is lethal to bees.

Q3? Will vinegar weed killer damage concrete, metal, wood, or other surfaces?
Vinegar is a weak acid but it can still damage wood, concrete and metal.

Is Vinegar Weed Killer a Myth?

So is this a myth or not? Vinegar will kill weeds and so strictly speaking it is not a myth. However, a single application will only remove the leafy parts and not kill the plant. Unfortunately most references only talk about the 5% vinegar and one application, so their recommendation won’t work – they are perpetuating a myth.

I wanted to test vinegar’s ability to kill weeds for myself. I tried it on dandelions in Home Made Weed Killer – Roundup vs Vinegar vs Salt and on other weeds in Vinegar Weed Killer – My Experieince.

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