Moss grows on your shady lawn when there’s an underlying problem such as acidic soil, or poorly drained, compacted or wet soil. Often you’ll find that there’s more than one problem. Identifying the problem is important if you want to control moss because, if you don’t fix the underlying problem, it will grow back following treatment. Moss can be difficult to eradicate, but soda ash often kills it.
Soda ash, or sodium carbonate, is an alkali chemical that can be refined from the mineral trona or sodium carbonate-bearing brines, or produced through different chemical processes. It’s also known as washing soda and can be purchased in the laundry section of many supermarkets. Soda ash has a pH of 11.4 and is often used to adjust pH in pool water or wastewater. It can also be used to adjust pH in soils.
Most lawns do best in neutral soil with a pH between 6.5 and 7.0. Mosses prefer acidic soil with a pH between 5.0 and 5.5. Many homeowners use calcium carbonate to lime their soils and raise the pH. Using soda ash will raise the pH, but it will also make the soil salty, which can reduce plant growth. Instead of using soda ash for widespread treatment of your lawn, use it for spot treatments of moss after testing the soil to see if the pH needs to be adjusted. If you use soda ash, wear protective clothing and a dust mask because the dust can irritate your eyes and mucus membranes.
Soda ash is available in a crystallized form that’s easy to work with, but you should wear rubber gloves because the crystals may irritate your skin. Make a soda crystal solution by mixing one-half cup of soda crystals with one and one-half cups of water and sprinkling the solution onto the moss. Leave overnight and rinse the next day. Be careful not to spill the solution on nearby grass or you may burn it.
Baking soda, or sodium bicarbonate, is in the same family of chemicals as soda ash and can also be used to kill moss. The main difference between soda ash and baking soda is that soda ash, or washing soda, will consume twice the acid that baking soda will. Because baking soda isn’t as caustic as soda ash, try using it instead. Soak individual patches of moss with a solution of 2 or 3 tablespoons of baking soda mixed into a quart of water to control moss. Repeat the treatment if necessary. Alternatively, sprinkle dry baking soda on the moss and let it sit overnight.