Weed killers have been banned in a number of Countries including Ontario Canada. People Are looking for safe alternatives and vinegar is commonly recommended. Let’s have a closer look at this common garden myth.
What happens when you pour vinegar onto a weed? For most plants, the vinegar is so acidic that it damages the green leafy part of the plant. In a few hours to 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.
What about stronger forms of vinegar?
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 weed control products on the market that contain the higher amount of acetic acid. There are also several weed killing products on the market that don’t have 20%. They are not worth buying – use pickling vinegar instead.
How effective is 20% acetic acid?
20% acetic acid will kill small annual weeds. It has limited effect at killing larger annual weeds and it does not kill perennial weeds. The reason is fairly clear. Acetic acid does not affect the roots. the roots of large annuals, and most perennials simply grow back.
note: this section was added July 2013. Reference: http://www.weeds.iastate.edu/weednews/vinegar.htm
What about repeated use?
In my statement above I said that vinegar only damages the green leafy part of the plant and that is true. 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 the 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.
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.