Vinegar as an herbicide

I saw something recently suggesting the use of vinegar, dish soap (blue dawn), and salt as a spray on herbicide. I tested it out yesterday on some weeds growing in my driveway. It works! Well it works better on some things than others. It kills some broad leaf weeds really fast, queen ann's lace and many grasses resist it pretty well. The idea is to coat the leaves with this spray on a sunny, not too windy, day and the leaves become desiccated.

I've been researching the topic with internet searches. Unfortunately, most of the information out there is simply somebody repeating tips they heard somewhere else. I'm not finding much information from experienced people describing the pro's and con's. Apparently the salt is optional. Obviously a lot of salt in the soil is not a good thing. Just vinegar with 5% acetic acid and dish soap can be effective. Some people substitute vegetable oil for dish soap. And some people use lemon juice instead of vinegar. Some research suggests that high concentrations of vinegar or salt in the root zone kills roots also. But, I am not interested in using it that way.

What I am considering doing is using a small spray bottle to point and shoot at individual weeds that make it through the mulch near food crops. (avoiding squirting desirable plants, saturating the soil, or using massive amounts of the stuff). Looking for reasons why not?

I've done this in the past using only vinegar in a spray bottle and a section of cardboard to protect the plants from over spray. One mishap and it's all over, so be careful and watch out for breezy conditions.

I found it just as easy to pop weeds up around plants (no need to run for the spray) and save to V-spray for larger weed areas, but you may find you prefer using the spay just as well. I'm very much a take action right when I see a problem (impulsive) - so it turned out simpler to just pull for me.

I've used 5% acetic acid (vinegar) as a herbicide for probably longer than this thread has been up, but not much more than that.  I am speaking of 5% cider (clear) vinegar unless mentioned otherwise.  You need a small amount of dish detergent so that the vinegar will wet plants.  It will only burn what it can wet.  I suspect waxy leaves will not be burned.

It only burns what it comes in contact with.  The vinegar doesn't get into the roots, so the plant may be able to recover from being burned.  The decomposition products are largely carbon dioxide and water.  Last year, I was able to get 4 litre jugs for quite a bit less than $2 (CDN).  This year, it is $1.99 or $1.97.  And there is no reason for this.  A world Class acetic acid faciity in China will deliver acetic acid to the dock for $0.10 per litre (I could be misunderstanding this).  This is the "chemical" acetic acid produced probably from methanol.  It is only the product of bacterial fermentation that should be called vinegar.  The "chemical" acetic acid is all we need for herbicide use.

It burns many plants.  Some it burns white-ish, some it burns yellow-ish and some black-ish.  Alfalfa doesn't seem to be bothered by it.  I don't thin thistle was, but I could be wrong on that.  It doesn't seem to bother juniper.  There are some trees which get burned by vinegar, I don't have a list.  I know about juniper because I have grass overgrowing a juniper.

I once bought a jug of 20% acetic acid, and it was much more effective.

At one time I ran across a study from the NE continental USA about the use of acetic acid for weed control.  Among other things, they compared it to flames.  Massachusetts?

I think the most cost effective way to get higher acetic acid contents, is controlled freezing.  The acetic acid tends to be rejected to the liquid, giving more and more concentrated acetic acid.  As acetic acid is volatile, the lower temperatures help control evaporation losses.

There are industrial processes which are producing almost 100% acetic acid.  An oil patch chemicals company in Grande Prairie, was bringing in those 1300 pound tote bags (or something like that) of 53% acetic acid.  I don't know what they use it for.

I believe anything stronger than 11% acetic acid must be treated as a hazardous chemical, and it will burn human skin at that point.

To buy strong acetic acid (more than 11%), and dilute it to some other concentration is not trivial.  There is a significant heat of mixing involved.  Concentrated acetic acid is flammable, and might possibly be explosive under some conditions.

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Last year, I had too many apples (same situation this year) on one tree, and so I took about half the apples off when green (and fairly large).  I had 10 gallons of apples.  At this stage, there was very little sugar in the apples, but there was considerable malic acid (a dicarboxylic acid) present.  I had to add sugar so that yeast would have something to ferment to EtOH.  I know that I did ferment sugar to EtOH (and then the natural acetobacter on the apples fermented EtOH to acetic acid) as I have a considerable amount of "mother" now.  A 5% acetic acid solution, should have a pH of 3.  The "malic vinegar" I made, had a pH between 3.1 and 3.  Pretty close to ordinary vinegar.

I mixed some of this with a touch of dish soap, and went spraying weeds.  There are some things which seem to be killed about the same as ordinary vinegar, but by and large most things seemed to be less affected by the malic vinegar than by ordinary vinegar.  It does not appear that making herbicide from green apples is cost effective.

I will try to redo the malic vinegar in a food safe way, to see how it compares to ordinary apple cider vinegar as a food.

Hopefully in the not too distant future, I will have honey locust pods on the farm.  I believe they tend to have just slightly more sugar than sugar beets.  It might be possible to make vinegar from them.  As I understand things, if cows or horses eat honey locust pods, the seeds are not digested and will probably be viable to germinate in the manure (as nature wants for many seeds).  If sheep or goats eat the pods, apparently they digest everything, and hence we don't get honey locusts growing in their manure.  For me, if I have honey locust with cows, I want to collect the pods so they don't eat them.  But if I have sheep or goats, I wouldn't mind them eating the pods.

I tried more of the "malic vinegar" (a mixture of acetic and malic acids) on some more things.  It takes an exceptionally large amount of this to kill something like a clump of grass.  It seems to have no effect on a larger number of plants, than plain cider vinegar does (5%).  I accidentally got some on a squash vine, and I see no affect.

But, if this malic vinegar is a pH of 3 (like ordinary cider vinegar), what is causing the herbicide affect?  If it is the acetate ion, that will be much less in this malic vinegar.

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Some its from the literature:

Final Report on USDA project
EFFICACY OF ORGANIC WEED CONTROL METHODS
https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/njpmcsr12842.pdf
Latest date mentioned in paper is 2015.

Organic Herbicides: Efficacy and Safety
https://www.fortlewis.edu/Portals/89/Organic%20herbicides.pdf


Efficacy of Current Organic Postemergent Weed Control Options in Turfgrass Systems
https://turf.purdue.edu/report/2011/PDF/06_AGRY_Patton_organic%20weed%20control.pdf

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That USDA Final Report suggests that vinegar is largely a waste of time and money as a herbicide.  Solarization has been around a long time (didn't The Victory Garden on PBS talk about this 20+ years ago?).  Where I live, we can get so dry that huge grass fires are possible, I would be very leary of using flame for weed control.  Steam I could see.  Cover crops are supposed to be good, but I think it matters where you are starting from.  I am starting from 40 year old fescue pasture that has lots of weeds.  One thing I have been trying is buckwheat.  To me, buckwheat will not be an effective cover crop.  I don't know if rhubarb will establish.  This first season with comfrey, it is a fairly big plant, but will it take control of the land it is on?  Or do I need to mulch with wood chips to help it?

I'm currently at the largest mall in the city. The landscaper uses a mixture of vinegar with a little bit of dish soap when killing weeds that grow up between the pavers. He doesn't use gasoline for anything. All of his tools are cordless electric. A year ago we had to listen to a guy using a very loud hedge trimmer. Now it's almost silent.

They have put pictures of the landscaper on a placard and attached to a couple of the elevator doors, with a little explanation about what he does. Thousands of people are able to witness these practices. When he sprays weeds, a little sign goes up, explaining the harmless nature of them. Theres a kids water park and lots of people bring their pets.

As outlined above, I do use vinegar with dish soap as a surfactant.  And I have used 20% acetic acid, which is what the USDA report was considering.

For me, at the moment I get lots of green apples, which I thought could be turned into a resource.  But if I have to pay for sugar to produce EtOH for acetobacter to turn into acetic acid, there is little advantage.  Now if retailers were selling 5% acetic acid for $0.13 per gallon, instead of $2 per gallon, I think the economics of 5% vinegar would improve a lot.  But a person also needs to consider all the plants that vinegar is either not very effective at, or ineffective.

I am hoping to reduce the size of my lawn (it is 4.5 acres at the moment) by planting berry bushes.  But there are places on the lawn which do not make sense for a 6 foot wide mower to cut.  So last year I bought a Black and Decker 40V cordless mower.  It's cheap.  The frame is not square, so the thing dogtracks a bit.  It came with 2 batteries.  I had a third battery from a 40V hedge trimmer (I was trying to turn it into a sickle moiwer) that started emitting the magic smoke which comes before the machine died.  But, if a person wants to use the mower for any kind of extended mowing, you pretty much need four batteries.  A battery has to cool down before you can charge it.  And a battery must cool down once charged, before you can use it in the mower.

I have a Worx 56V hedge trimmer now.  It seems to do okay at hedges.  Eventually I'll take the B&D apart, and get another motor (with forced cooling) and try to make sickle mower, possibly robotic.  But the "tranmission" from the B&D is what I am looking for.

Sometimes I use citrus peel infused vinegar as a weed killer. I like to infuse distilled white vinegar with citrus peels  for about a year, then I use this for a weed killer and a heavy de-greaser.

Close to the house for around pavers and cracks in the sidewalk, I like boiling water, the best is boiling water from boiling pretzels (quite alkaline).

Understand the chemistry and then you can make substitutions.

The dish soap is purely there as a surfactant to cause better wetting of the leaves.  Anything that will act as a surfactant will work in this role.  That is why the citrus oils work.  Now it needs to be something that won't neutralize the acidity of the vinegar but any neutral or acid surfactant will work.

The salt is there to upset the osmotic pressure balance drawing water out.  In most cases you can replace it with epsom salts and achieve the same thing while adding good nutrients to your soil.  As it is pH neutral it won't change the acidity of the vinegar.   Eventually it will add Mg to your soil and acidify it as the sulfur enters the soil cycle.

The simple goal of this mixture is to dry the green growing part of the plant to death.  It does not kill the root.   Vinegar will kill the root but it must be used in concentrations that change the soil pH radically for it to happen.  But just sprayed on the leaves is simply destroying the current greenery by dehydrating it and the plant will put out more.  So repeated spraying to exhaust the plant will likely be necessary.

Limonene (in citrus) is a good solvent, and according to the Wikipedia, it is used in an organic herbicide.  Limonene does oxidize, but I don't see where any of the common products of oxidation have any herbicidal effect.

I hadn't thought of adding Epsom salts to vinegar, I wouldn't normally add a sodium salt to vinegar as sodium tends to be too commonly found.  I would like to disagree on what affect sulfate might have.  I believe that it you add alum (aluminum sulfate) to soil, the sulfate is reasonably soluble and may be taken up by plants.  This leaves the aluminum ion, which reacts with water to produce aluminum oxide (precipitates out) leaving behind hydronium (acid).  If you add mineral sulfur to the soil, bacteria will metabolize the sulfur, and in the process generate acid.

But, I haven't researched that enough to be positive on how acidification might be happening.

Thanks all, for the good info!!  Stacy, I've got a gallon jar of lemon peels in distilled vinegar... probably a year old!  Good to hear of your experience.  I found the idea on Erica Strauss' (now archived) website. https://nwedible.com/diy-weedkiller/  (it has tons of great info + very entertaining, especially re: PNW)

BTW, I think I 'made' approximately 20% vinegar by freezing a known amount of regular 5% distilled, then put it out to start defrosting.  I poured the liquid off as it thawed, until I had a quarter of the beginning amount.  (Acetic acid has a lower freezing point than H2O). So... at 5% vinegar, does that mean my 1/4 amount is about 20% ... maybe ? (I did the math, but it's not my strong suit :)

Also, apparently it worked great for Mark Fulford in Maine, for his carrot crop - http://sri.ciifad.cornell.edu/aboutsri/othercrops/otherSCI/usaSCI%20_Carrots_Fulford_2014.pdf

"--Approximately 1-2 days before the carrot plants were expected to emerge, the now very weedy
bed was sprayed with a mixture of water, vinegar and citrus oils. This burns down the weed
growth in full sun light and allows the emerging carrots full use of the bed with little or no competition.
--40 days into the season, a half hour of hand weeding was sufficient to clear any late-germinating
weeds from within the 4 rows of carrots. This was the only weed control time spent for the test
bed..."

I would think your vinegar should be in the vicinity of 20%, it might be 19%.  If you are into physical chemistry, using a phase diagram can help.  But that only gives the answer for infinitely slow heating and cooling.  The first solid to form is not pure water, but pure water with a small amount of acetic acid in it (and maybe other stuff).  And as more solid forms, the concentration of acetic acid in it increases.  Between that, and you "eyeballing" that 1/4 was left; there is not likely enough information to calculate what the concentration really is.

If you have a pH meter (that is reasonably well calibrated), you could measure it, and see how it compares to what a 20% acetic acid is supposed to be.  As acetic acid is a "weak acid", it's pH doesn't go down as fast the acidity goes up.  So, it might be reasonable to titrate it against a base of known strength, using an indicator which changes colour close to pH 7.

To buy "vinegar" (which normally tops out at 9% or so), you seldom need to worry about the material as a hazard.  I believe starting at about 11% acetic acid, the risk of burns to the skin from spilled acid starts to become appreciable.  So to use the 20% acetic acid/vinegar, one should exercise a little more care than just household vinegar.

Getting strong acetic acid in the eyes or mucus membranes could be painful and/or damaging.

I purchased some 30% vinegar a couple weeks ago and have been waiting for a good sunny day to use it so that I could take before and after photos. Finally two days ago we had a nice long day of sun. So I sprayed some experimental patches. I used just enough spray to wet the leaves, but did not soak the plants nor the soil. I did not add soap to the vinegar. Within hours the most sensitive plants were bleaching, or drooping. By the next morning the sprayed areas were very identifiable. This second day had nice sun until lunchtime, whereupon it clouded over. By 4 pm the sprayed plants were bleached or brown, wilted or shriveled.

The "after" photos were taken on the morning of the third day. From previous experience, the weeds will continue to brown out. If we get sun today, all the green will disappear.

I've been using this vinegar to spot kill weeds before applying fresh mulch over old mulch. I've also been using it around plants that I don't want killed or damaged, such as taro. If commercial herbicides are used within the root zone of taro, the plant gets damaged even if the leaves weren't sprayed. I've seen no damage to the taro when using vinegar even on close weeds.

I'm still experimenting with vinegar. Once I'm satisfied, I'll be retiring my weed flamer.