Once again, the blogosphere is full of recipes for weed killer, lawn restorer, insecticides, etc., all containing dish detergent. Most of them call specifically for Dawn, although I recently encountered someone promoting Joy instead. The most popular herbicide recipe includes vinegar and dish detergent. Some add Epsom salts. Others add plain table salt. It’s a supposedly “organic” or “natural” alternative to a purchased product.
I’ve already written why this mixture is a horrible idea. First of all, unless you add salt, It doesn’t work—5% vinegar kills the top growth without harming the roots, allowing the weed to grow right back. (True herbicidal vinegar is 20% acetic acid, a hazardous chemical requiring careful handling.) Epsom salts only help if your soil is deficient in magnesium, in which case they’re a fertilizer, not an herbicide. And salt is a bad idea no matter how you look at it! If you spray salt on a weed, you’ll guarantee that nothing will ever grow in that spot. Furthermore, the salt easily dissolves and spreads, creating a no-grow zone downslope. There’s a reason armies salted their enemies’ fields!
So yes, it’s ineffective and could potentially ruin your soil, but what really astonishes me is that this mixture is somehow considered “natural”—and therefore safe. Here’s what Dawn contains: water, sodium lauryl sulfate, sodium laurethsulfate, c12-14-16 dimethyl amine oxide, SD alcohol, sodium chloride, PPG-26, pei-14 PEG-10/PPG-7 copolymer, cyclohexanediamine, phenoxyethanol, magnesium chloride, methylisothiazolinone, fragrance, yellow 5, blue 1. That’s natural?
How safe these chemicals are depends largely on which web site you choose to believe. Proctor & Gamble provides OSHA Safety Data Sheets for all its products. You can read them and draw your own conclusions. I clicked through to the one for Ultra Dawn Original, and found it pretty interesting.
First, they caution consumers to only use the product as directed. (Using it in an herbicide is an off-label use.) Then, there are some warnings. If you get it in your eyes, it can cause irritation, and you need to rinse well. If you swallow it, dilute it with several cups of water. Don’t let it have prolonged contact with your skin. Don’t breath it in. And apparently, it’s flammable!
Among all these ingredients, probably the most worrisome is sodium lauryl sulfate. It’s added to the detergent because it bonds with grease, allowing your dishes to come out clean. However, if it bonds with another chemical containing nitrogen (such as fertilizer!), it can form nitrosamines (a carcinogen) or nitrates (good for plants, bad for us). Sodium lauryl sulfate is currently considered safe*, but the science isn’t settled.
(Since the P&G website also proudly points out that Dawn is used to clean animals coated with petroleum, as in the case of an oil spill, they clearly don’t think it’s a significant hazard. In any case, I’d advise wearing gloves.)
What is the reasoning behind using Dawn, or another detergent, in the garden? It makes water “wetter”—helping it spread out over slippery leaf surfaces or penetrate some hydrophobic soils by reducing surface tension.
Some soils, especially those high in organic matter, may repel water, which runs off instead of soaking in. That’s because the organic particles in the soil are waxy. Wetting agents (surfactants) are able to bond with both the water and the wax (or other oils), allowing the water to soak in. (Compacted or clay soils may also have poor water penetration. In this case, the problem has nothing to do with the water’s surface tension, and wetting agents won’t help.)
So does that mean we should spray Dawn in our gardens? According to Randall Frost, Ph.D.:
The addition of ordinary washing detergent can improve the penetration of water in soil, but the effect does not last very long. Also many of these detergents contain compounds that are harmful to growing plants. They may also interfere with the life-cycles of some aquatic organisms. In high concentrations they may be poisonous.
Adding Dawn to a weed killing spray may allow the other ingredients to come in better contact with the leaves, but it could also harm adjacent plants. Adding it to a lawn could be counter-productive; you’ll get far better results from having the lawn dethatched and aerated. And if you think your homemade product is organic, read the ingredients. Whether you opt for Dawn, Joy, or another brand, the only true benefit you’ll see from spraying detergent in your garden is clean plants.
* By OSHA, the Occupational Safety and Health Administration; the National Toxicology Program, and other organizations. https://www.hunker.com/12524703/ingredients-in-dawn-dish-detergenthttps://www.hunker.com/12524703/ingredients-in-dawn-dish-detergent