Homemade Weed Killer - Is It Safe or Effective?

8/12/2024 8:54 pm

 

 

 

 

Homemade weed killers are the darling of the internet, but before racing off to make one of these concoctions, questions must be answered. Do these formulations work? Are they safe? Are they actually any more "natural" than commercial herbicides?

 

Almost all of the internet formulations of homemade weed killer start with a base of undiluted vinegar. (See note at bottom of article about horticultural vinegar). So our assessment starts there.

 

Vinegar.  Home strength distilled vinegar is what is typically used in these recipes since it is cheap and readily available. Distilled vinegar (and other edible vinegars) are approximately 5% acetic acid to 95% water.  

 

Vinegar of this strength is used for cooking, brining and cleaning. Since humans can safely eat some quantity of it, it would seem safe enough to use outside, right?  At least, if it is effective that is.

 

Is vinegar natural?  Only 10% of the world's vinegar production is biological currently with the other 90% chemically synthesized. While some world food laws require that it be a biological produced product through bacterial fermentation, the FDA does not require disclosure if petroleum products were used as starters for distilled vinegar. Womersley Foods A chemist would argue there is no difference in the ethyl alcohol produced naturally or via petroleum starter. Heinz advertises that it only uses a natural process with corn or apples. Heinz Ad 

 

If you are buying vinegar on the market aisle with oils and vinegars, you most likely are buying a "natural vinegar". If you stray to "cleaning vinegar," found on the cleaning product aisle (generally a 6% acetic acid), read the label closely. Most are not food safe and many contain fragrance or coloring ingredients that render them not "natural". 

 

Is vinegar effective at killing weeds? Home strength vinegar can kill young annual weeds and baby perennial weeds because it just kills the top growth.   The reason it only kills the tops is that vinegar is a "contact" killer. It is not absorbed by the plant and does not infiltrate root systems. Even pickling vinegar (7%) doesn't work better. Garden Myths

 

So while you might think you've killed other weeds with it, since it won't get to the roots, they will come back. Garden Myths (For a side by side comparison to Round Up, see Garden Professors).

 

The conclusion then is that home strength vinegar might kill young annual weeds or newly sprouted perennial weeds. But does that mean you should run out and use it?

 

Safety of Vinegar as Weed Killer. Home strength vinegar is generally safe for kids and pets, so long as it doesn't directly contact their eyes.  

 

But vinegar isn't safe for some other garden critters. Home strength vinegar can kill ants, spiders, mosquitos and amphibians. It can also be lethal to bees, so it's best not to use vinegar to kill weeds where bees forage.

 

Plus vinegar is an acid. It can damage wood and marble, so it must be used use carefully on wooden decks or near marble statuary/planters. 

 

Given vinegar's limited utility and drawbacks, under what circumstances would one use vinegar to kill weeds? I've used distilled vinegar for driveway and street crack weeds. But dandelions, crabgrass and tougher weeds only look bad for a week or two before re-emerging. My duck tool does better finishing them off! 

 

 

 

 

Now that you know just how effective vinegar is and isn't, it's time to consider whether that changes by adding other ingredients as the internet formulas suggest.

 

Vinegar and Dawn are the ingredients most often touted in homemade weed killers. Dawn is used to clean oil off of wildlife, so obviously it is an safe, more natural, effective weed killer, correct?  Um, not so fast. 

 

What is Dawn? Liquid dish "soap" is -- with an exception noted later --actually a detergent. True soap (whether bar or hand soap) is made from natural oil and fats only. While it cleans, it is very different than liquid dish detergent.  Garden Myths

 

Liquid dish detergents are specifically designed for cleaning, not horticultural use. There are many formulations of dish detergent because in addition to the chemical concoctions that allow them to effectively clean, dish detergents also often contain extra ingredients to make them appealing to users, including fragrance and essential oils. 

 

Are Dawn and other Liquid Dish Detergents "natural"?

 

Not even remotely. Liquid dish detergents are very far from "natural." Let's start with the oft recommended Dawn.

 

INGREDIENT	FUNCTION	WHAT IS IT	WHY IS IT IN THERE?
WATER	solvent	water	holds ingredients together
SODIUM LAURYL SULFATE	surfactant	cleaning agent	provides cleaning
SODIUM LAURETH SULFATE	surfactant	cleaning agent	provides cleaning
C10-16 ALKYLDIME-THYLAMINE OXIDE	surfactant	cleaning agent	boosts cleaning
ALCOHOL DENAT.	solvent	solvent*	stabilizes formula
PPG-26	viscosity adjuster	solvent*	stabilizes formula
SODIUM CHLORIDE	viscosity adjuster	salt	thickener
SODIUM HYDROXIDE	pH adjuster	mineral based pH adjuster	balances formula pH
PEI-14 PEG-24/PPG-16 COPOLYMER	cleaning agent	cleaning agent	boosts cleaning
PHENOXYETHANOL	solvent	solvent*	stabilizes formula
FRAGRANCES	fragrance	perfume	adds scent to product
METHYLISOTHIAZO-LINONE	preservative	non-formaldehyde preservative	preservative
COLORANTS, BLUE 1,YELLOW 5, RED 33	colorant	colorant	adds color to product
C9-11 PARETH-8	surfactant	cleaning agent	gently aids soil removal
STYRENE/ACRYLATES COPOLYMER	opacifier	opacifier**	makes product opaque
TETRASODIUM GLUTAMATE DIACETATE	water softener	chelant	boosts soil removal
CHLOROXYLENOL	antibacterial agent	antibacterial agent	decreases bacteria on the skin when used as a hand soap
PHENOXY-ISOPROPANOL	solvent	solvent*	enhances grease cutting
GLYCERIN	viscosity adjuster	process aid	stabilizes formula
SODIUM CUMENE-SULFONATE	process aid	process aid	stabilizes formula
PROPYLENE GLYCOL	solvent	solvent*	stabilizes formula
TERPINEOL	solvent	solvent*	enhances grease cutting

 

 

Is dish soap effective in killing weeds? Let's start off by acknowledging that there is no scientific evidence to show that dish detergents are effective or safe ingredients in a weed killing formula.  

 

Gardeners know that some plants, like English Ivy, have a waxy layer over the leaves that protects them from pests and diseases. Applying liquid detergents prior to or in conjunction with an effective weed killer may remove this protective layer and leave plants exposed to outside threats. The old formulation of RoundUp contained a surfactant that helped break down plant walls for Glysophosphate to work. So maybe the inclusion of Dawn in these formulas is for this purpose?

 

However, there are horticulture surfactants specifically designed for use prior to herbicide application which are nonionic.The ingredients of these horticulture surfactants are quite different and streamlined from the long list of ingredients found in liquid dish detergent that are primarily for cleaning.  Liquid dish detergents are ionic. Anionic Surfactants are negatively charged, and enhance foaming and other spreading properties. UGA Extension Foaming and spreading are not desirable qualities in a weed killer. They are not a proven or known safe source for use in the garden. 

 

 

Are Liquid Dish Detergents Safe for Use?

 

There are no studies to show that all the above chemical ingredients in Dawn or other liquid dish detergents are safe for the earth or how long they might linger in the ground and affect future plant growth. Nor is there a whit of scientific data to show that household vinegar combined with liquid dish detergent is an effective way to permanently kill weeds.

 

If you were to try this combo on a waxy weed, Dr. Bronner's Castille soap is a liquid soap not a detergent. While it is vegetable based potassium soap, it also contains fragrance and other ingredients like essential oils. It was not created for insecticidal use and no studies show its safety for such use, so try it at your peril if you want to make a homemade insecticidal soap. Garden Myths

 

 

 

 

 

Now we get to the most popular internet version of this so called "natural" weed killer:

 

Vinegar, Dawn and Salt.  We've all heard of "pouring salt on the wound". This is probably how one should think of this formulation: 1 gallon vinegar, 1 Tablespoon Dawn and 1 cup table salt. This concoction has all the limitations of vinegar and dish detergent discussed above plus salt! We can put the desirability of this addition to rest quickly and permanently.

 

"Table salt consists of sodium chloride. 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'”.  Garden Myths

 

Indeed, salt is more toxic to wildlife than weedkillers like RoundUp. So hopefully you've crossed that formulation off your list quickly!

 

So what about using Epsom Salt instead, as other formulations suggest?

 

 

Epsom salt consists of magnesium and sulfate and is often used as a fertilizer. Yes, you can use too much fertilizer and burn a plant, but then what happens to the ground where you have done this? It could damage soil longterm.  Habitats Matter  If used in small concentrations, alternatively, Epsom salts may simply be feeding the weeds and encouraging re-growth!  Southern Living 

 

Without any science in support, one wonders why would one randomly use epsom salts as part of a weed killing formula.  

 

A Theory As to Why Folks Are Fooled into Thinking Using "Dish Soap" in the Garden Is Okay?

 

Many of us have used "insecticidal soaps" to wash aphids or whiteflies off plants for years. However, insecticidal soap is a true soap, not a detergent.  https://www.gardenmyths.com/dish-soap-damage-your-plants/. It is made using only potassium which produces a milder, softer soap than sodium. It also uses long chain fatty acids – a special type of fat. This soap is specially made to be mild on plants. Garden Myths  

 

 

Also, the idea behind insecticidal soap is to get rid of the bugs, but not harm the plants. Obviously that use is inconsistent with the kind of liquid dish detergent one would use to kill weeds, one that breaks down cell walls! 

 

 

Conclusion

 

Homemade weed killers are still "chemicals" -- just as is everything in life is! It is important that none of these homemade products were designed to be weed killers nor have they ever been studied for safe use as such.  Mixing products, even things you think are safe, can be dangerous. Read product labels and follow their directions!  KSU Extension  Chemical herbicides work differently by getting absorbed into the plant, but labels must be read and followed for safe use.

 

If you are just trying to kill small annual weeds not near other plants, like on the driveway, using a vinegar based spray (without the dish detergent or salt) might be worthwhile as it works on contact. But anything perennial or well rooted will not be killed by contact based herbicides and there is danger to life forms, nearby plants and ground use from this supposedly natural remedy. It may in fact be worse for the environment than proper on label use of herbicides designed to act systemically.

 

Just because it is on hand doesn't make it a good choice for off label purposes! This is how folks rationalize pouring gasoline into yellow jacket nests all the time. That too, is a very baaaaaaaad idea.

 

"Gasoline should never be poured into underground nest holes. This dangerous practice creates a fire hazard, contaminates the soil, and prevents the growth of vegetation for some time. A ground application of gasoline poses greater harm to children and the environment than a yellowjacket nest." Texas A&M

 

Contributed by Liane Schleifer

 

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NOTE: Horticultural vinegar, which is sold for agricultural use, can be effective (20 to 45 % acetic acid) at weed killing. It is also hazardous if it touches eyes, skin, pets etc.  as it burns.  Permanent eye damage can occur.  It also irritates mucus membranes. It also kills ants, spiders, mosquitoes, bees, slugs, snails, frogs  and pretty much whatever.  If using it, full body protection and mask should be worn. Realistically, this has little place in a home garden environment or by nonprofessionals.  Horticulture vinegar is not a selective herbicide, so one must be very careful and intentional where it is applied.