Will Dish Soap Kill Grass?

By Janine Soriano, Master of Science in Forestry, government environment researcher

Apart from keeping your kitchen squeaky clean, some home gardeners suggest that dish soap combined with household ammonia, beer, soda, and mouthwash can be used as a lawn conditioner. These ingredients are supposed to provide nourishment to grasses, making them greener. That said, most results are from personal accounts and needs scientific validation.

Dish soap is also used to control pest problems. By diluting it with water, some leaf-feeding insects with soft bodies can easily be washed off from plants. Lawn practitioners also recommended using it as an alternative to synthetic pesticides as it doesn’t kill beneficial lawn insects. Further studies from experts at Oklahoma State University show that flushing foliage with lemon-scented dish soap can help control sod webworms, armyworms and cutworm larvae. Additionally, some detergents have essential oils that deter insect pest migration. Examples of these are oils from tea tree, neem, eucalyptus, citronella, and peppermint.

Dish soap may kill grass in sufficient concentration and frequency. In addition, those containing bleach and ammonium content may react with chlorophyll, turning leaves lighter in color. As a result, it can hamper plant photosynthesis and lead to death.

Like humans, plants have a protective outer covering that can regulate water and other substances into the plant tissue system. The plant epidermis has an outermost layer called the cuticle. This layer is comprised of a complex chain of fatty acids and carbon molecules. Just as detergents breaks down fats and oils from kitchen wares, the same happens on plants and acts as an abrasive to the plant cuticle layer of the epidermis. This effect manifests as leaves with discoloration, burnt blotches or a wrinkly appearance.

So what if you want to kill grass?

Grass cover is an essential feature of a yard. Aside from providing green space, they cool the ambient temperature of your home. Unfortunately, grasses can become a nuisance when they creep up on your ornamentals, making them look unappealing or even threatening their survival.

There is insufficient evidence of dish soap being an effective grass or weed killer. However, you could try mixing it with other home products such as vinegar, salt, and other surfactants. Remember that elimination may take longer using this method compared to commercial herbicides.

If the purpose is to kill grass in a large area, dish soap might not be an economical solution. First, you should identify what specific brand would be best, assuming that no other chemicals are to be applied as an herbicide. Second, you should conduct an initial experiment on a small scale to determine effective dilution concentrations and frequency of spraying. Also consider the total area of the lawn and how much you’ll need on a per square feet basis. Another condition to factor in is whether freshly mowed grass would behave differently compared to non-mown.