Why That Homemade Insecticidal Soap Might Kill Your Garden
Don't spray plants with dish soap!
We all want an easier, cheaper way of gardening. There are plenty of websites and blogs that are willing to give you unproven remedies based upon anecdotal evidence. Some of these remedies even have some remnants of actual science in their basis but are not practical in most situations. One of the most prevalent DIY gardening “hacks” is to make homemade insecticidal soap, but I am here to tell you that it just might kill your garden.
How Insecticide Soap Works
Commercial insecticidal soap is made of the potassium salts of fatty acids. That’s a fancy way of saying that it’s a soap made from potassium hydroxide (as opposed to sodium hydroxide) and isolated fatty acid parts of oils. These oils may be palm, coconut, olive, castor, or cottonseed (Potassium Salts of Fatty Acids – General Fact Sheet, 2001). The insecticidal soap kills soft-bodied insects such as aphids by penetrating their body and breaking open their cell membranes causing them to dehydrate. This doesn’t work against insects with harder bodies such as ladybugs or bees. It also doesn’t work against caterpillars. Even though these products are rigorously tested, there are still some plants that are too sensitive and will be damaged if sprayed with insecticidal soap. These include plants with fleshy or hairy leaves that will hold the insecticide longer. Any commercial bottle should list sensitive plants, so be sure to read the bottle entirely before using it.
Why Homemade Recipes Don’t Measure Up
Most homemade recipes are liquid dish soap and water. A few also incorporate some vegetable oil to try to help it stick to the leaves longer. First off, liquid dish soap is rarely actual soap. It is typically synthetic detergent meant to cut through grease on dishes and pans. That means it is also cutting through the waxy coating on your plants, leaving them vulnerable. This is incredibly harsh on your sensitive plants, even in very low doses, and is very harmful to the soil microorganisms (Kuhnt, 1993). The recipes that incorporate oil don’t realize that the plants need to breathe just as much as the insects do. While the oil would help the solution stick to the leaves longer and can help kill the insects by suffocating them, do you really want to suffocate your plant as well? Not to mention that the sun can heat up those oils on your plants’ leaves hot enough to scorch your tender plant. It also further breaks down the waxy coating that helps protect your plant from dehydrating. While there are horticultural oils that are used in aphid control, that is more applicable to dormant fruit trees, not your vegetable or flower garden (Flint, 2014). William Habblett, a horticulturist says, “Homemade sprays are hard to ensure that you have the appropriate dilution and mix and results can vary. Some ingredients also may not be as soluble as others and the mix may not be stable. We also don’t necessarily know what the long term impact is of introducing the different chemicals from the soaps that people want to use or have available.” In case you haven’t noticed, nearly every recipe for homemade insecticidal soap is a little different from the last in percentages of soap, the addition of oil, etc. There is no regulation like in commercial products.
What About My Homemade Soap?
You would think that since synthetic detergent (dish soap) is bad, then maybe you can make your own soap that is good? Well, first off you absolutely cannot make sodium hydroxide soap for plant use. The sodium part is very detrimental to plants. Isn’t it all used up in the soap making process? Well, technically yes, but there will always be a few free-floating ions in most chemical reactions. There is always a little bit of the soap ingredients left in the finished product. What about a soap using potassium hydroxide? Shouldn’t that be exactly the same? While yes, you would be a lot closer to the same potassium salts of fatty acids, remember that the commercial product is made from the isolated fatty acids, not the whole oil. Some of the fatty acids that are isolated for use are oleic, lauric, myristic, and ricinoleic (Potassium Salts of Fatty Acids -Technical Fact Sheet, 2001). You can find these on a soapmaking oil chart. One thing these particular fatty acids have in common is that they are all long-chain fatty acids. Most of the cooking oils used in soapmaking are short-chain fatty acids and not good for plants. The same problem occurs even with the recommendation to use plain castile soap in your homemade insecticidal soap recipe. This castile soap is still made from whole oils, not isolated fatty acids, and often contains oils and additives that would be harmful to your plants.
Consider the Legalities
The last part to consider is that off-label use of dish soap as a pesticide is technically illegal, as is promoting its use. Printed right on the label it says that it is a violation of federal law to use the product in a way in which it was not intended. While the EPA probably will not bother most home gardeners who choose to make homemade insecticidal soap, those who promote its use might want to reconsider. Yes, people have been cited and fined for misuse of registered pesticides and other products.
Why is homemade insecticidal soap so often recommended when it’s bad for your plants? Well, because we all want to save money and be more self-sufficient. And even though many people have gotten lucky when their homemade recipe didn’t kill their plants, perhaps they blamed the damaged leaves on the very insects they were trying to kill instead of the killing agent? Yes, it may work; you may be one of the lucky ones with the proper dilution, but would you rather risk your garden or trust the experts?
Flint, M. L. (2014, March 11). Oils: Important Garden Pesticides. Retail Nursery and Garden Center IPM News.
Kuhnt, G. (1993). Behavior and Fate of Surfactants in Soil. Environmental Toxicology and Chemistry.
Potassium Salts of Fatty Acids -General Fact Sheet. (2001, August). Retrieved April 30, 2020, from National Pesticide Information Center.
Potassium Salts of Fatty Acids -Technical Fact Sheet. (2001, August). Retrieved April 30, 2020, from National Pesticide Information Center.
Originally published in the May/June 2021 issue of Countryside & Small Stock Journal and regularly vetted for accuracy.