Black Soap or Insecticidal Soap: What’s Best for Your Plants?

There’s only one insecticide that comes into my home: insecticidal soap. It’s a habit handed down from my father, who rejected synthetic chemical insecticides (and even several biological ones) because of their toxicity. But why choose insecticidal soap? Quite simply because it’s harmless to humans and non-toxic.

That said, I only use it for my houseplants. In the vegetable garden, I prefer to let nature manage the majority of insect infestations, combining methods such as using protective netting, selecting resistant varieties or creating a diversified garden that attracts beneficial insects and natural pest predators. As for ornamental plants, they have no support from me: they have to face the pests alone. If they don’t survive, I replace them with resistant varieties.

Efficient?

You might ask, “If it’s not toxic, how can it kill pests? The answer lies in its mode of action: it’s not its toxicity that kills insects, but rather the properties of the soap it contains. The soap acts as a dehydrating and smothering agent. When it comes into contact with soft-bodied insects, it dissolves their protective layer of wax, causing them to dehydrate and die. It is therefore a physiological and mechanical effect, rather than poisoning, that makes this product effective.

Many people also use black soap in the same way. It is widely used in Europe. Its use is less common in North America, except in Quebec, where European influences, particularly from France, have contributed to its popularity.

I must confess that for a long time I had a bad impression of this product, believing it to be less effective and more aggressive for sensitive plants. However, with the onset of winter, a time when our houseplants are often more vulnerable to infestation due to the lack of light and the dry air that weakens them, I thought it was the ideal time to delve deeper into the subject and clarify these preconceived ideas.

How Black Soap and Insecticidal Soap Are Made

It goes without saying that each of these products is, as the name suggests, made from soap. At first glance, they may seem almost identical.

Black Soap

Black soap is a natural soap obtained by saponifying vegetable oils such as olive, sunflower or linseed oil, mixed with potash (potassium hydroxide) and water. This chemical process transforms the oils’ fatty acids into soap (potassium salts of fatty acids) and glycerine. Once saponification is complete, the soap can be adjusted or diluted to suit its intended use. An ecological and biodegradable product, it is versatile and used for cleaning, gardening and body care.

Insecticidal Soap

Insecticidal soap, on the other hand, is made from lighter oils, such as coconut or palm oil, chosen for their specific fatty acids that dissolve easily in water. This liquid soap is clear, fluid and formulated for diluted use.

Although the ingredients differ, the manufacturing process remains the same: saponification. It’s the oils used and their composition that give each soap its distinctive properties, adapted to particular uses.

It’s important to note that black soap, although often presented as a natural and versatile product, may contain additives or impurities depending on its manufacturing method or intended use (household, cosmetics, etc.). Some commercial black soaps may include essential oils, perfumes, colorants or other agents which, although harmless for household use, may be harmful to plants. These substances could cause leaf burn, plant stress or disrupt the biological balance of the soil. For use in the garden, I recommend choosing a pure, additive-free black soap, specifically adapted for horticultural use.

So What’s the Difference?

I have to admit that it took me a while to understand the difference between one and the other. So what’s the difference for us gardeners?

Black soap and insecticidal soap work in a similar way to eliminate soft-bodied insects such as aphids, whiteflies, mealybugs, thrips, mites and psyllids. They dissolve the waxy layer that protects their bodies, dehydrating and killing them. In addition, they form a film that blocks the spiracles, the tiny breathing holes of insects, causing them to suffocate. These soaps must be applied directly to the insects to work, as they have no prolonged effect and must be reapplied if the insects return. Their mode of action is simple and effective against many soft-bodied insects such as aphids and whiteflies.

Originally, black soap was a multi-purpose product used mainly for household cleaning, linen care, personal hygiene and leather care. Its gentle, degreasing properties made it ideal for cleaning effectively without damaging delicate surfaces or materials. It was only later that it came to be used in the garden. The oils used in black soap, rich in long fatty acids, are less effective at targeting insects, as they less easily penetrate the pests’ protective waxy layer.

Insecticidal soap, on the other hand, is specially formulated to combat harmful insects. It contains potassium salts of short fatty acids, which enhance its ability to penetrate and dissolve the protective waxy layer of insects. This composition enables it to target pests effectively while minimizing risks to plants.

To Suit Your Needs

So, insecticidal soap seems to be the best choice, doesn’t it? Well, it’s not quite that simple. The choice between insecticidal soap and black soap depends above all on your specific needs. Insecticidal soap, thanks to its targeted formulation, is often more effective in treating heavy infestations. However, black soap is sometimes considered gentler on sensitive plants, whereas insecticidal soap, with its high concentration of short fatty acids, can be more aggressive, causing burns or stress to certain fragile plants. What’s more, black soap, with its often lower cost (depending where you are), is an interesting alternative for gardeners looking for an economical solution. Ultimately, your decision will depend on your preferences, the needs of your plants and the severity of the infestation to be treated.

Black soap and insecticidal soap have a similar environmental impact, being both biodegradable and generally safe when used correctly. However, black soap, made from vegetable oils such as olive or linseed oil, can sometimes have an environmental advantage thanks to more local and sustainable ingredients, compared to insecticidal soap often formulated with oils such as coconut or palm oil, sometimes from less responsible crops. Although their effect on aquatic organisms is minimal with moderate use, it’s best to avoid using them near watercourses. Overall, both remain far superior ecological alternatives to synthetic chemical insecticides.

How to Use Them

Both products can be applied after dilution with water. For black soap, mix 30 ml of soap per liter of warm water. For insecticidal soap, follow the instructions on the label. Use a sprayer to spray leaves, stems and infested parts of plants, making sure to reach the underside of leaves where insects often hide.

For best results, apply early in the morning or at the end of the day, avoiding periods of high heat which can burn the leaves. These soaps act by direct contact and should be reapplied after a rain or every 5 to 7 days until the infestation is over. For heavy infestations, you can reduce the interval to every 3 to 5 days, but limit intensive applications to a maximum period of 2 weeks to avoid stressing the plants.

It is essential to test the product on a small section of the plant to check its tolerance. If signs of burning or stress appear, rinse the leaves immediately with clear water to minimize damage. Use targeted applications to protect beneficial insects and preserve the balance of the garden ecosystem.

Let’s Be Cautious

Above all, remember that, although these products are environmentally friendly, they are still insecticides. I advise against their use as a preventive measure; they should be reserved for situations of proven infestation. In addition, apply them in a targeted manner to minimize their impact on surrounding insects, especially beneficial ones such as pollinators, which are attracted to certain plants.

Sometimes a simple spray or shower is all that’s needed to eliminate certain pests on houseplants, or consider introducing natural predatory insects. Outdoors, encouraging diversity in your garden can attract beneficial insects and create a natural balance. Practices such as mulching and crop rotation can prevent infestations. For me, insecticides, even natural ones, are a tool of last resort. Always consider other options before using them.