Dish Soap Can Damage Your Plants

Home » Blog » Dish Soap Can Damage Your Plants

Robert Pavlis

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.

YouTube video

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.

YouTube video

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.

Microbe Science for Gardeners Book, by Robert Pavlis

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.

Is Dr. Bronner’s Castile Soaps Safe?

Castile soaps, of which Dr. Bronner’s is a popular brand, are made from vegetable oils (particularly olive, palm, and coconut) using potassium hydroxide. So they are potassium based soaps and therefore many people claim that they are the same as insecticidal soap. Most castile soaps are mixed with fragrances and essential oils for a wide range of uses but not as insecticides. This soap maybe safer than other sodium-based soap, but nobody really knows what the additives will do to plants. Since it is not labeled as a pesticide it should not be used.

Dr. Bronner's Castile Soap
Dr. Bronner’s Castile Soap, note the added peppermint oils.

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

 

If you like this post, please share .......

Robert Pavlis

I have been gardening my whole life and have a science background. 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!

141 thoughts on “Dish Soap Can Damage Your Plants”

  1. I reviewed your blog it’s really good. Thanks a lot for the information about this blog.I want more informations.

    Reply
  2. It killed my jalapenos I will never believe anything I read again I will go back to sevendust it’s safest never had no trouble with it.

    Reply
      • I believe the question is actually: Is soapy water (from used dishwater detergent) safe for watering garden plants and trees? (At least that would be my question. I live in Calif. where water conservation and re-use is going to be critical in the coming years.)

        Reply
  3. I noticed an infestation of aphids on my Caragana – 20-30 on a pod. I sprayed a diluted solution of Dawn and water and now all the leaves have curled and dropped off. Still a few aphids around…can’t believe my gorgeous little tree now looks half dead. Wish I had seen this website before I sprayed.

    Reply
    • I sometimes use soap to drown insects like aphids (the soap lets the water get into the insects’ spiracles), but I always use castile soap, which is an actual soap made as the potassium salt of whatever fatty acids are in the vegetable oils used. The most advertised brand is Dr. Bronner’s, but there are some cheaper high-quality alternatives.

      I don’t honestly know how long the carbon chains are in the fats used, but reasonable concentrations don’t bother most plants. Some plants are more sensitive though, so it’s always best to test on a small area.

      Reply

Leave a Comment