Moss on Lawns, Roofs, and Walkways
by Betty Johnson, UC Master Gardener

I can always tell when the rainy season has arrived. Moss starts showing up on my brick walkways. When I scurry around the yard, I have to be careful not to slip and fall. There's also recurring patches of moss in the bare spots of my lawn. Why does it keep coming back?

What is moss?
Moss is a primitive, non-flowering, rootless green plant. It produces a thread like growth that forms a fine-textured mat on soil, lawn or other moist surfaces. Most plants have special channels to transfer nutrients and water up the stalk, but moss doesn't. It grows vigorously during the cool, rainy seasons – fall, winter and spring – and grows little during the summer. It doesn't flower, but reproduces by spores. If you look closely at tiny moss plants, you'll discover they are actually beautiful.

Under favorable conditions in the lawn and on bare soil, moss mats can grow quite thick, competing with lawn grass and forming a barrier against water and air movement into the soil. On surfaces such as the brick or concrete walkways, or roofs, moss build-up can begin a process of decaying these surfaces.

What conditions favor moss growth?
Excessive moisture, heavy shade, restricted air movement and poor drainage favor moss growth on hard surfaces. On grass surfaces moss is additionally encouraged by compacted soil, excessive irrigation, low soil fertility, and presence of a thick thatch layer or a combination of these.

What to do!
Let's tackle moss in lawns first. When moss first appears, rake it out as soon and as often as possible as a temporary removal tactic. Realize that unless conditions change, the moss will return. A more permanent solution is to evaluate the site and make all necessary corrections to favor lawn growth, not moss growth. That might mean trimming trees or removing vegetation to improve the airflow or perhaps reducing soil compaction by aerification.

Excessive watering might also be a culprit, especially if drainage is poor or the ground is compacted. Water only when needed. Reduce fertilizing as it feeds the moss as well as the lawn. Applications of certain chemicals may be necessary for complete control. Applying sprays of copper sulfate (which will damage turf), ferrous sulfate, or ferrous ammonium sulfate at the rate of 2 to 3 ounces dissolved in 4 gallons of water per 1000 square feet will kill the moss.

Yet, this is still only a temporary response and eliminating the cause or causes is still needed to prevent reoccurrence. In all cases, dead moss should be removed by raking. If not removed, the crust can impede air and water movement to the soil and result in an even weaker lawn. Large bare spots may need to be re-seeded.

If the situation is too hard to correct, then consider redesigning that part of the landscape. Be inventive and try to figure what else might work. Somehow the excess moisture needs to be eliminated.

What to do on hard surfaces!
Elimination of moss poses a different solution on walkways and roofs. After all, replacing a roof is very expensive and the moisture trapped by moss can hasten the deterioration of a roof. To kill established moss, first remove as much of the moss as possible using a strong jet of water or a broom. Roof treatments are metal-based chemicals, usually containing copper or zinc.

Copper will cause some staining on nearby surfaces. Zinc is non-corrosive and non-staining, but more toxic to plants than copper sulfate (Lilly Miller Moss-Kil). Also available is cryptocidal soap (Safer's De-Moss Cryptocidal Soap), which contains 40% potassium salts of fatty acids. It kills existing moss growth and provides residual care.

Other treatments include using a galvanized or copper ridge, which provides effective preventive moss control on most roofs for about 10 to 15 feet below the ridge. Another treatment for long term moss control is a bare copper wire stretched horizontally across the roof every 10 feet. Be sure to carefully cleanup after using chemicals, especially, to prevent corrosion and damage to nearby plantings. On walkways, I've had success with white or biodegradable chlorine bleach, but there is always concern for nearby plants and lawn.

Well, its time I get busy and start de-mossing my own yard.

January 16, 2003