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Herbicidal soap (potassium salts of fatty acids)

Herbicidal soap may be used to control moss
Photo: Jardin botanique de Montréal (Gilles Murray)

The information below is intended only as a guide. Always read product labels carefully.

Product description

Pesticides with herbicidal soap as the active ingredient consist of potassium salts of fatty acids. These salts are obtained by combining an alkali (potassium hydroxide) with fatty acids from animal or vegetable sources.

These products are available ready to use or as liquid concentrates to be diluted before use.

Target organisms 

Pesticides with herbicidal soap as the active ingredient may be used to control moss, algae, lichen and liverworts on roofs, stucco siding, wood framing, sidewalks and fences. They may also be used to remove moss from lawns.

Moss in a lawn indicates that growing conditions are not conducive to grass (shady spot, poor, acidic or compacted soil, poor drainage). Maintenance may also be a problem (overwatering, grass cut too short).

To combat moss effectively, it is important to address the cause of the problem. This means giving your lawn the best possible growing conditions and maintaining it properly.

If you cannot improve the growing conditions, it is better to consider replacing your lawn with ground cover. Moss actually makes an excellent grass substitute.

Further reading:

NB: The product label lists the plants on which the pesticide may be used, along with the target organisms. 

How it works

Herbicidal soap works on contact. The potassium salts of fatty acids cause the organisms to become dehydrated and die.


  • On structures
    • Spray on enough of the product to thoroughly wet all surfaces to be treated. For best results, the herbicide should not be applied when it is raining or snowing.
    • Moss, algae, lichen and liverworts treated during the growing season (spring/summer) should dry out within a few days. It may take longer at other times of the year for the effects of a treatment to appear.
  • On lawns
    • It is important to wet the moss and surrounding lawn before applying the herbicide. Thirty minutes after treatment, rinse the treated area. Results should appear within about a week and the dried moss may then be raked off.
    • Bare patches should be reseeded quickly to prevent other moss or weeds from growing.


The pesticide may irritate the skin and eyes, so it is best to wear gloves, long sleeves and pants and goggles when applying it.

The pesticide should be applied only to target organisms, as it is not selective. Avoid applying it on windy days, to minimize drifting.

It may leave white residue on some brick or concrete surfaces. These deposits should disappear within a few weeks.

Do not use near any body of water or wetland, or dump any pesticide or rinse your equipment there, as this will contaminate the water. Never dump pesticides down sewers.

Keep out of reach of children.

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