Garden Myth: Dishwashing Liquid as an Insecticide

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Dishwashing liquid probably kills more plants than pests! Ill.: clipart-library.com & laidbackgardener.blog

Many gardeners regularly use dishwashing liquids (commonly called dish soap or dishwashing soap, although few contain any soap) to treat insects and spider mites on their garden and indoor plants. And this is nothing new. 

In bye-gone days, Grandma used to throw her dirty dishwater on her garden plants to control pests. And in general, soap will indeed help to control pests. Properly diluted and sprayed on insects, soap blocks their breathing pores, leading to their death by asphyxiation. The only problem is, most dishwashing liquids today no longer contain any soap. They’re detergents instead. And detergents are not much more effective against insects than a spray of plain water.

Damage to rose leaves. Photo: World Peace Roase Gardens

Dishwashing liquids were never developed for treating plants and are often phytotoxic (toxic to plants) to different degrees. They tend to melt the oils and waxes that protect the leaves and can therefore lead to serious leaf burn. Therefore, you should always test the diluted product on a leaf or two before using it on any plant. Wait 24 hours and if there is no damage, that particular bottle of dishwashing liquid (and I do mean “that bottle” and not “that brand”) can be safely used to treat the plant in question. 

But not other plants. The same dishwashing soap that causes no damage to one plant may well prove toxic to another. You always have to test each plant individually.

In addition, you can never presume your favorite brand of dishwashing liquid is going to give the same from results one bottle to the next. Manufacturers regularly change their recipes and never feel the need to inform the consumer of the change.

Ingredients of Dishwashing Liquid

What are dishwashing liquids made of? Various products, especially surfactants (detergents). As an example, here is a list of the ingredients of Palmolive Ultra Strength as listed on the Palmolive web site on December 4, 2019. 

Water
Ammonium lauryl sulfate
Ammonium laureth sulfate
Lauramidopropylamine oxide
Isodeceth-6
Poloxamère 124
Sodium chloride
SD Alcohol 3-A
Magnesium sulfate
Fragrance
Pentasodium pentetate
Dyes
Methylisothiazolinone
Benzisothiazolinone
Methylchloroisothiazolinone
Sodium bisulfite

Notice it contains are no soap at all. I don’t see much in this blend that will control insects!

A Telling Tale

Here is an anecdote that can help clarify the situation.

In the 1970s, the maker of a popular dishwashing liquid issued a press release following a flurry of complaints that their product had damaged plants. In the release, the manufacturer insisted that the product was never designed for treating plants and therefore could not be held liable for damage to plants incurred by its use. In addition—a revealing detail—the company explained that the product’s recipe varied regularly according to the price of the basic ingredients and if one bottle were harmless to plants, the next bottle could be phytotoxic.

So, use dish washing liquid to treat insect pests if you want (although I suspect you won’t find it very effective), but always do a test on every plant first … and repeat every time you buy a new bottle.

My Solution

Insecticidal soap kills insect pests, is organic and doesn’t damage plants. Pretty much the opposite of dishwashing liquid. Photo: source unknown

I no longer use dishwashing liquid to treat my plants against insects. First of all, I had a very bad experience with dishwashing soap many years ago, losing several plants (yes, I confess: I had not carried out the leaf test beforehand!) But also, I find having to test the leaves time-consuming, especially when more than one type of plant is infested. Also, waiting 24 hours for the “results to come in”—even as the pest goes to town on my plants—can be quite unnerving.

I instead use insecticidal soap. It’s an organic product developed for use on plants and the manufacturer uses the least phytotoxic ingredients possible. The recipe has been thoroughly tested for safe use (some plants do suffer from damage and these are at least listed on the label, so read it first), the result being I don’t have to do a leaf test myself. I just have to prepare the solution according to the manufacturer’s recommendations and spray the pests into oblivion.

As for those who object to the higher price of insecticidal soap (it will never be as inexpensive as a bargain brand dishwashing liquid), think of it this way. The bottle of insecticidal soap concentrate that I bought 13 years ago is still half full. And each treatment I apply probably costs less than 5 cents. I’m no spendthrift, but I don’t feel that $12 or so spent on a bottle of insecticidal soap that will probably 25 years is excessively expensive. But when you do buy insecticidal soap, do buy the concentrate, otherwise it’s true you won’t be getting your money’s worth.

Adapted from an article originally published on December 8, 2015.

4 thoughts on “Garden Myth: Dishwashing Liquid as an Insecticide

  1. If I had reason to do so, I would use it on aphid anyway. It does not take much to kill them, even without soap. However, I have not sprayed anything for aphid in many years. With proper pruning, and other cultural practices, they are not much of a problem. Phytotoxicity has not been a problem, perhaps because I so rarely do it.

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