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Vinegar and Epsom Salt as Herbicides

A big thank you to all who attended Spring Alive! The winter storm was unfortunate but I had a great time connecting with Master Gardeners and lecturing about “good bugs”. I always enjoy attending Master Gardener events such as Spring Alive because it is a local beacon of factual, scientifically based information. Neighbors are talking to neighbors, and better techniques of growing start spreading like wildfire. It is the main reason I am part of University of Minnesota Extension, and hope it is for you too if you are a Master Gardener.

On that note, I think many Master Gardeners and extension educators often find themselves trying to stomp out quite a bit of wildfires as well. Misinformation, pseudoscience, and gardening myths are everywhere, and are as stubborn as a thistle. One of those thistles caught me by surprise, inspiring me to do this article—that a concoction of household vinegar and Epsom salts can be effective in killing weeds. 

Which to tackle first? Let us start with Epsom salts. This chemical is comprised only of magnesium and sulfate, and historically spread as a fertilizer, not a pesticide. It dissolves easily in water, and can be applied to certain deficient plants. Plants tolerate high amounts of sulfur and magnesium, and it is difficult to observe examples where these nutrients cause direct toxicity to a plant. Theoretically, you could apply so much Epsom salts that the amount of magnesium would interfere with the weed’s uptake of other essential nutrients such as calcium or potassium. This is not wise on several levels—first, because of how long magnesium can persist in the soil and second, the cost of doing so. In other words, keep the Epsom salts in the bathtub rather than use them as a pesticide.

OK, but what about vinegar? Unlike Epsom salts, horticultural vinegar has been used in organic agriculture as an herbicide for many years. It can kill weeds through simply destroying plant cells through the acetic acid in vinegar. Bear in mind this type of vinegar is not available at your local grocery store—horticultural vinegar can have up to 20% acetic acid, be hazardous to handle, and is labeled for agricultural use. Lower concentrations simply are not strong enough to effectively remove even small weeds.

Weather is also important with using horticultural vinegar. Rain can wash the product off fairly easily so reapplications may be necessary. A few companies may use something called a surfactant to help keep vinegar onto the plant leaf. A surfactant is a product added with a chemical (often pesticides) which acts similar to soap or oil. Surfactants help keep the active ingredient of the pesticide from washing or evaporating off, and thus increase its effectiveness. Surfactants are not foolproof, however.

Properly used, horticultural vinegar can act within 24 hours, with over 80% control on many common weeds. However, there are some big caveats associated with using this product. Unlike RoundUp (glyphosate), which moves within the whole plant, vinegar is a contact herbicide, which means it kills the area it touches. Anything less than 4 inches is a prime target for vinegar, and I can see it being useful if you are dealing with a bunch of pigweed germinating in one narrow area. Weed size is important because vinegar does not directly damage roots, and depending on the size of the weed, that root system can provide enough energy to keep the weed alive even after you scorch its growth aboveground—so if you are eyeing that 6 foot tall ragweed in your yard you can forget it.

Since horticultural vinegar is an acid, it can also scorch you. Read the chemical’s pesticide label (it is the law) and wear appropriate gear to minimize contact, especially with your eyes if the vinegar splashes. Also, keep this herbicide well away from any metal tools or drains to avoid any nasty chemical reactions. Do I need to say do not cook with it either?  Please do not throw it in with your salad; this is a potentially dangerous chemical.

All in all, unless you are working on a certified organic farm, horticultural vinegar has a limited use in home gardens. If you have a large area with many weed seedlings, vinegar treatment may save you some time, but it will require possible reapplication due to wet weather. Tried and true techniques such as hand weeding, mulch, or cultivation may not be exotic, but they work. If you are dealing with a particularly nasty weed that resists these techniques, do not hesitate to call your local extension office where we would be happy to assist you.

 

Works Cited:


Larson, B. 2012. Epsom Salts and Plants – Is It Worth Using Epsom Salts as a Plant Fertilizer? University of North Carolina Cooperative Extension. https://lee.ces.ncsu.edu/2012/08/epsom-salts-and-plants-is-it-worth-using-epsom-salts-as-a-plant-fertilizer/

Smith-Fiola, D. and S. Gill. 2017. Vinegar: An Alternative to Glyphosate? University of Maryland Extension. https://extension.umd.edu//sites/extension.umd.edu/files/_docs/programs/ipmnet/Vinegar-AnAlternativeToGlyphosate-UMD-Smith-Fiola-and-Gill.pdf

Wiesbrook, M. 2007. Using Vinegar as an Herbicide. University of Illinois Extension. http://hyg.ipm.illinois.edu/pastpest/200714f.html