Can You Use Irish Spring Bar Soap, To Kill Bugs In A Vegetable Garden?

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

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.

Garden Myths book by Robert Pavlis

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.

Soil Science for Gardeners book by Robert Pavlis

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.

Building Natural Ponds book, by Robert Pavlis

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:

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