Many gardeners regularly use dishwashing liquids (or other soaps) to treat insects and spider mites on their plants. And this is nothing new. In the old days, Grandma used to throw her dirty dishwater on her garden plants to control pests. And in general, a typical liquid dishwashing soap, one without added bleach, perfume, or antibacterial products, will indeed help to control pests: just dilute it at a rate of about 1 tsp. per quart (5 ml per liter) of water. The only problem is, it sometimes damages or kills the plant too. Here’s why:
Dishwashing liquids were never developed for treating plants and are often phytotoxic (toxic to plants) to different degrees. (By the way, this applies to organic dishwashing liquids too.) 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 that 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.
A Telling Tale
Here is an anecdote that can help clarify the situation.
In the 1970s, the maker of a popular dishing 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 was harmless to plants, the next bottle could be phytotoxic.
So, use dish washing liquid to treat insect pests if you want, but always do a test on every plant first… and repeat every time you buy a new bottle.
I no longer use dishwashing liquid to treat my plants against insects. First of all, I had a very bad experience with dishwashing soap years ago (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 is 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 9 years ago is still half full. And each treatment I apply probably costs less than 1 cent. I’m no sprendthrift, but I don’t feel that $12 or so spent on a bottle of insecticidal soap that lasts 20 years is excessively expensive. But when you do buy insecticidal soap, do buy the concentrate, otherwise it’s true you’ll be ripped off!