The advice to use dish soap to deal with garden pests and plant diseases has been around for ages. But is it a good idea? Is dish soap safe for the garden? Let’s have a look at the facts.
This is part of a series on common garden myths and misconceptions to assist beginner gardeners who wish to practice earthly-friendly garden methods.
Using Dish Soap in Gardens
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Is Dish Soap a Natural Alternative?
It seems like every household product from the kitchen has been touted as great for the garden, acting as herbicides, pesticides, fertilizers, or other simple solutions.
Vinegar. Salt. Baking soda. Dish soap. They get recommended over and over again. I see them mentioned so often in (otherwise) organic gardening discussion forums that I think I’m on the wrong page!
Dish soap, diluted in water as a spray, is frequently recommended as a ‘natural’ insecticide, killing pests in the garden. The first clue is, if it does kill pests, is it really ‘harmless’?
I used to think this stuff made sense too. But not now.
It seems the underlying message is that common household products are safe or ‘natural’ alternatives to other commercial products made to address garden problems. We’ve seen so many scary poisons on the store shelves for the last 50 decades that we’ve stopped trusting any possible solutions offered there today.
But are household products a better choice?
Let’s look at dish soap.
Joy, Palmolive, Sunlight, and Dawn in particular gets mentioned so often I wonder if Procter & Gamble has some genius grassroots advertising underway.
The logic seems to be, we use this stuff to wash our dishes and eat from them, so it can’t possibly be harmful, right?
But, what is dish soap, and what effect does it have when we spray it on plants and insects?
Dish Soap is a Detergent
Liquid dish soap is actually a detergent, not a soap. Soaps are made from natural ingredients and detergents are made from synthetic ones.
Dish soaps (really, synthetic detergents) contain a whole array of chemicals, most of which are pretty benign, but I’m not sure how it can be presented as an alternative to chemicals with such ingredients as sodium lauryl sulfate, poly-propylene glycol-26, phenoxyethanol and several others.
The Environmental Working Group shares particular concern about the ingredient methylisothiazolinone, which they say is of “high concern” for its “acute aquatic toxicity”. In other words, it gets in our water and kills stuff. [Dishwashing Products Made by Dawn | Environmental Working Group]
So, definitely not ‘chemical-free’, and certainly not harmless, right?
Let’s look at what dish soaps do.
Dish Soap is Not Designed to Target Pests
Dish soaps are designed to dissolve the food stuff leftover on our dishes including fats and oils. They also work to remove waxes.
Now, let’s think about how this would affect plants and insects.
The common advice is to dissolve a small amount of dish soap in water in a spray bottle and apply it to the unwanted pests that are chomping on our plants.
Here’s the problem.
The dish soap doesn’t know what you consider to be a pest or weed or disease. There is no specific biological interaction. That spray is just going to blindly affect everything it comes into contact with.
Yes, it may eat away at the body of the pests. But it’s also going to get any other insects who happen to be in its range.
And, completely counter-productive to the aim, it’s going to break down the plant’s own natural defence mechanisms by stripping away the naturally-occurring oils and waxes on the leaves.
So, that plant you were trying to spare is weakened too. And weakening a plant makes it vulnerable to more pests.
And does that insect have buddies lurking nearby?
So, what the heck are we doing putting dish soap in the garden?
Dish Soap is Not for Gardens
Here’s what we know.
- Dish soap is a detergent. Not a soap.
- It is synthetic, not natural.
- It is made from chemicals. And not that all chemicals are bad—that’s absurd—but the point is, the chemicals in dish soap are not designed to target specific pests or diseases.
And that’s why it’s not a harmless, natural alternative, and doesn’t really fit with the aims of an organic garden.
We might say one gardener doing this, big whoop. But there are millions of us spreading these home remedies around, and that’s how many gardeners putting this stuff in their gardens?
Using Commercial Insecticides
So, what to do?
There are better alternatives to homemade pesticides and herbicides. Not the old broad-spectrum, kills-everything sprays we grew up with, but others designed to target specific pests or diseases.
Not that I’m the poster girl for using anything like this, but the industry has come a long way. There is still a ton of harmful products out there, but, if you are determined to treat a problem, you have better options than randomly coating something in dish soap.
Iowa State University sums this up nicely on its website, saying that they provided these homemade recipes for years, but now suggest store-bought products as more effective, safer and worth the extra cost. [Control Houseplant Insect Pests Safely with Insecticidal Soap | Iowa State University]
If you are determined to go after an insect pest in your garden, do your homework first.
Here’s the process I go through:
1 Is this such a big issue that it is worthy of treatment, even at the expense of other things in the garden, or harming the environment?
2 What is the pest or disease exactly? What causes it? Can I treat the cause rather than just snuffing out the symptoms?
3 Are there products made to specifically target this problem with minimal or no residual harm?
4 Or, will this resolve itself in time?
5 Or, better still, are there greater things to put my time, money, and attention on?
Bottom line, just because it’s in the kitchen cabinet does not make it good for the garden.
~Melissa the Empress of Dirt ♛
PS: I know some of you will ask, what about using Dr. Bronner's liquid soap? Well, it is a soap, not a detergent, but again, best to use a product made for the specific purpose you are intending. Castile soaps are alkaline and not beneficial to plants.