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Dish Soap Can Damage Your Plants

Dish Soap like Sunlight or Dawn is a regular addition to home pest control remedies for the garden. You use dish soap every day and eat from the dishes you clean with it – how can it be harmful to plants? It’s time to look through the bubbles and see the truth.

Chemicals in Dawn Dish Soap by GardenMyths.com

Chemicals in Dawn Dish Soap by GardenMyths.com

Dish Soap – What is It?

Dish soap is a generic term, but it usually refers to the liquid soap products used for washing dishes. Dawn, Joy, Palmolive and Sunlight are very common brand names. It also goes by the names Dish washing liquid, washing-up liquid, dish washing soap, and dishwasher detergent.

Dish washing soap is a detergent that can include phosphate, bleach, enzymes, dyes, fragrances and rinsing aids.

Scientists distinguish between soap and detergents which, chemically, are quite different. Soaps are cleaning agents made from natural oils and fats. Detergents are cleaning agents made from synthetic chemicals called surfactants. Soap and detergents both clean, but the chemicals in the products are different.

Dish Soap is actually misnamed. It should be called Dish Detergent. You will see why this is important in a few minutes.

Dish soap works by dissolving greasy chemicals like oils, fats and waxes and it is excellent at this job. It is also a  powerful degreaser.

Insecticidal Soaps – What Are They?

Insecticidal soaps are pesticides that are used in the garden. I’ll talk more about how and why to use them below.

Insecticidal soap is a true soap, not a detergent.

A soap is made by mixing together sodium hydroxide, or potassium hydroxide with fats. The final product is something called either sodium salt of fatty acid, or potassium salt of fatty acid. This is the same ingredient found in most bars of soap, and in liquid hand soap. Chemically these are very different from detergents, although both clean things.

Insecticidal soap is a special kind of soap. It is made using only potassium which produces a milder, softer soap than sodium. It also uses long chain fatty acids – a special type of fat. This soap is specially made to be mild on plants.

Soaps will also dissolve greasy chemicals like oil, fat and wax, but they are not as good at this job as detergents. From a cleaning perspective insecticidal soap is a great soap.

Dish Soap On Plants

What happens when you spray diluted dish soap on plants? Remember dish soap is a detergent that is excellent at removing oil, grease, and wax. When you spray it on your plants, it removes the natural oils and waxes that all plants have on their leaves. These oils and waxes serve to protect the leaves.

When the protective coating is removed from the leaves, it makes it easier for pathogens to get a foothold and infect the plants.

Spraying your plants with dish soap removes their natural defenses against pests and diseases. You are setting the stage for your plants to get sick, and maybe die.

DIY Insecticidal Soap

There are many DIY home recipes for making insecticidal soap. The problem is that NONE of them are insecticidal soap. If they use dish soap – they are detergents, not soaps. If they use liquid hand soap, the fatty acid salts are made from short chain fatty acids which are phytotoxic to plants – they damage plants. You can’t make insecticidal soap using things you find around the house.

One recipe on the internet says “Use a pure liquid soap… Don’t use detergents, dish soaps, or any products with degreasers, skin moisturizers, or synthetic chemicals.  ” Soap is a synthetic chemical!

Will the homemade insecticidal soaps get rid of insects? Maybe, but they will also damage and weaken plants.

Use Insecticidal Soaps Correctly

My post, Insecticidal Soap – Use it Correctly, provides more detailed information on how to use these products. In summary, they are only effective if you spray the insects – not the plants. Insecticidal soaps are much less harmful to plants, but even they should not be used to cover the whole plant. Although, fairly safe for plants, they will harm certain plants – look at the instructions an keep away from sensitive plants.

References:

 

  1. University of Connecticut – Insecticidal Soap; http://www.ladybug.uconn.edu/factsheets/tp_05_insecticidalsoap.html
  2. Clemson Extension office – Insecticidal Soaps for Garden Pest Control; http://www.clemson.edu/extension/hgic/pests/pesticide/hgic2771.html

 

Robert Pavlis
Editor of GardenMyths.com ,
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

33 Responses to 'Dish Soap Can Damage Your Plants'

  1. Abdullahi says:

    Please i want to know in what proportion can i obtain a mixture of oil and liquid soap?

  2. William says:

    Have used Dawn, Dawn Ultra and Totally Awesome at 1/4-1oz./gal. with most waterings along with Miracle Gro and Eleanor’s as instructed to soak foliage and previously useless, highly hydrophobic, sandy soil.

    The results are gratifying: Faster growing, v. green foliage. Larger fruits, veggies and buds. Not much in the way of insects. Many bees and birds. As well, dishsoaps apparently have no ill-effects on soil microorganisms while killing soil pathogens and preventing salt-buildup at the roots.

    Almost too good to be true but we have proven this over five seasons. It seems indispensable now…

    • So it keeps insects away – but not bees?

      It does not affect the good soil microbes, but it kills the pathogen soil microbes?

      This is a real specific pesticide!!!

      sorry – the comments are not logical or supported by science.

      • docwill121 says:

        This is empirical observation [n=1(!)]; maybe others will experiment as well. The bit about beneficial vs. pathologic organisms is available online; you can Google it. 2) Insect control/abatement and keeping bees around is not mutually exclusive; gardeners have been doing this forever.

        Just thought you’d be interested. We don’t need your scientific blessings…

  3. Jay says:

    I am a firm believer that soaps and detergents really do work on many insect and fungus problems. I also think the chemical industry has invested billions of dollars to protect their product image and usage and will not be too supportive of any low cost natural remedy.

  4. Lesley says:

    I used a mixture of dishwashing liquid and water on my cactus and it’s had a bad reaction. It looks like the cactus has got scabies and been burnt were the liquid was sprayed. Please is there anything I can do to help my cactus before it dies?

  5. Anli Llego says:

    I’ve sprayed Seventh Generation dish detergent/soap (2 tsp in 32 fl oz sprayer) diluted in water mixed with 50 drops of neem oil on my petunias, lobelias, and geraniums that were being eaten up by earwigs and moth caterpillars. It worked and I wish I could upload photos of how my flower garden looks like — really gorgeous! The spray didn’t harm the predatory wasps that were hunting down the pests, too.

    • How did you measure the damage to plants? To know they were not damaged means that you were able to measure this.

      Just because the garden looks good does not mean the soap does not damage the plants. It just means a) you did not see the damage, and b) the damage was not great enough to cause other problems that you could see.

  6. sarah says:

    What would you recommend for cleaning plastic greenhouse surface where fruit trees are espaliered in front of. I need to get behind the trees to clean but don’t want to damage trees with the wrong cleaner. Thanks!

  7. Jack says:

    Rob, how about spraying highly diluted dish detergent on lawns? I have heard the enzymes help break up hard packed soil, allowing water, nutriients and oxygen to more easily access the roots. Is this another myth?

    • I doubt it works. Enzymes are proteins. Proteins are decomposed fairly quickly by microbes. They don’t stick around to loosen clay. Besides the amount of enzymes in liquid soap is very small and after dilution is almost non-existent.

      • Jack says:

        Thanks for the quick response. There are ares of my lawn that are hard as cement. I have soaked them , then used a pitch fork to loosen and aerate them, after which I immediately rake in quality top soil. Overseed them. Still no luck, surfacing KBG shoots are spindly, short , pale green and appear anemic compared to all the other KBG verdant shoots around the rest of the yard. Should I have mixed the top soil with a commercial compost?

        Thanks

        Jack

  8. CATHY says:

    Will the dish soap, if not put on the leaves, kill earwigs etc in the soil, without harming the plant. Thank you

    • I doubt it. Soap is a contact pesticide. Spraying on the soil will not contact insects very well – they are not sitting on the surface waiting for you to spray.

      • Cathy says:

        I actually put soap in my rain barrel and water the plants with a watering can, (soil only) so I assumed the soap was going through the soil, as the bubbles come out the bottoms of the containers, to kill the earwigs? Thanks

  9. andrew says:

    i was told by a master gardener in Halton that if you do a 10:1 water:dawn and then rinse it off 15 minutes later that the bugs would die and the plants would be ok. can you let me know if this information is accurate? the plants seem fine and all of the aphids are gone but i’m hesitant to do it again if it could hurt my plants.

    • That probably works to some effect.

      You say the plants are fine. But… you can’t see the minor damage being done by soap. It dissolves the oily/waxy coating that leaves have. Once this is removed in whole or in part, they are more susceptible to infection. Why not just buy the product that was made for the job – insecticidal soap.

  10. beetee43 says:

    how do I send you gardening topics I am interested in your views on ?

    • If you have a comment about a post, leave the comment on that specific post. If you have a general comment try leaving a comment on gardening FB groups. I do frequent Garden Obsessions.

      • Bernard Tyson says:

        i presume ” gardening FB groups ” means Facebook but what site ?
        my subject is the advice given ( in UK ) to cover plots ( usually with black plastic sheets ) in winter to allow them to “warm up ” but wouldn’t any heat gain , say on a sunny day in March , be lost quickly by conduction to the adjacent uncovered soil ?

        • Garden Obsessions. Some heat would be lost, but some will not. It is simple to try – put some plastic on the ground. Come back next day and measure the temperature.

        • Eugene says:

          I was told if you use a 12oz can of beer to one gallon of water and 1/4 of a tablespoon of dawn it works as a great lawn fertilizer and bug be gone?? Is this tue??

          • Why would that be true?? To be a fertilizer it needs to have reasonable amounts of NPK. Beer contains almost nothing – its mostly water, and soap is not good for plants. How can soapy water fertilize you lawn?

  11. Cameron Moir says:

    Thanks Rob, I’ve never read this before – very interesting! Cheers, Cam

  12. Very good you made it clear to distinguish between the soaps and detergents. “Add few drops of dish soap/detergent” has become a general recommendation for many products without much thought on what would be the purpose.

  13. john karanson says:

    Excellent,to the point article on dish soap.Just look at the ingriedents as i did years ago,nice to see this written about

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