Weed killers have been banned in a number of Countries including Ontario, Canada and people are now looking for natural week killers. Does vinegar kill weeds? Is it a natural weed killer? Let’s have a closer look at this common garden myth.
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 done the job.
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 plant below ground? Turns out that vinegar has almost no affect on the root system. When vinegar reaches soil level it is quickly neutralized so that it is no longer acidic. The acetic acid in vinegar is 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.
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.
A higher concentration of acetic acid will kill some plants, but you need to use at least 20% acetic acid.
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. 20% acetic acid should be considered to be a dangerous chemical. 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 – use pickling vinegar instead.
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 roots of large annuals, and most perennials allow the plant to grow back.
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 sun light 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 secrete 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 yet 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.
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.
1) Vinegar for Weed Control Revisited: http://www.weeds.iastate.edu/weednews/vinegar.htm
2) Vinegar as a burn herbicide: http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=195808.
3) Photo Credit: Joe Shlabotnik