During a rainier-than-usual winter and spring, moss eagerly invaded lawns and made itself at home.
The plush, low-lying plant doesn’t get many neutral reactions. People either love it or hate it. Probably more fall on the hate-it side of the fence when it lands in their lawns. That’s why Alec Kowalewski, turfgrass expert for Oregon State University Extension Service, gets so many questions about how to get rid of the flowerless plant that dates to ancient times.
Before he gets into the answers, Kowalewski breaks some hearts.
“People have the idea that if they put product down to control it then that’s the end of the story,” he said. “There’s a misconception that once moss is killed, it won’t come back. But it will if you don’t change the environment.”
Moss thrives in moist, shady situations on compacted soil, common conditions in the western areas of Oregon. When lawn isn’t maintained properly, moss moves in.
“Moss is like a lot of weeds,” Kowalewski said. “It will do well in tough situations and doesn’t need as much nutrients as lawn does.”
Keeping a nice, dense stand of grass will keep moss at bay. To do that two things must change: soil drainage and too much shade. Grass doesn’t grow well when the soil is compacted and air and water can’t get to the roots. And no matter how many different types of grasses you try, they won’t succeed in shady areas.
“Trees and grass don’t do well together,” Kowalewski said. “By changing the environment, you can establish a healthy lawn.”
The only solution is to thin or take out trees. Once that’s done, concentrate on the soil. First get the moss out. The easiest way to do that is by renting a dethatcher and running it over the lawn. Then rake up the moss down to bare soil. Moss roots are very shallow so it doesn’t take much effort. If you don’t want to rent a dethatcher, try using just the rake.
If the soil is compacted, use an aerator – also available at rental shops – to open it up. Usually that isn’t necessary except in high traffic areas, Kowalewski said. In fact, he’s never aerated his perennial ryegrass lawn, which is what most people have in their yards.
“After removing the moss, you want to make sure you replace the area with sod or grass seed if you want to turn it back into lawn,” he said. “Spring until Memorial Day is a great time to do this as we enter into the sunnier, warmer time of year and moss growth slows down.”
A fertilizing regiment should start now, too. Using an all-purpose lawn fertilizer rather than just nitrogen is the easiest way to go. Grass likes lots of nitrogen (the N on the front of the bag), but don’t feed it only nitrogen. Like all plants, it wants the full spectrum of nutrients. The goal for fertilizing is to apply the least amount needed. The newer the lawn, the more fertilizer needed. Apply 1 pound of actual nitrogen four to six times a year in spring and fall.
It could be that the grass needs a higher pH. At very low levels, nutrients become less available to the plants and soil elements like aluminum can become toxic. Moss can handle that situation; grass can’t. Don’t just assume lime is needed. Before doing anything, test the soil, Kowalewski said. There are inexpensive test kits available at garden centers and online.
If the pH is down around 5, go ahead with a lime application. Use about 25 pounds per 1,000 square feet in the spring and another 25 pounds in fall. Buying from a farm store is less expensive than other sources. After you have applied lime take another soil test three to five years later to determine if the acidic conditions have returned over time.
Though it may seem counterintuitive, irrigation is essential for controlling moss. Why? Because you can’t have a lush lawn without it. And vigorous grass will outcompete moss. Recommendations for watering have changed, Kowalewski said. Research now shows that instead of 1 inch of water once a week, lawns should be irrigated several times a week for a total of 1 inch. Apply about 1/4 to 1/3 inch three to four times a week. During intense heat, water even more often – up to five times a week — but not any more than 1/4 inch in one application. Measure with a rain gauge, plastic cup or tuna can.
Mowing correctly helps keep lawn healthy. Kowalewski advises mowing once a week and don’t take more than a third of the length off at once. Mowing less lawn more frequently is not as hard on the plant and it will start growing laterally, taking up more space and keeping weeds from encroaching.
But back to moss. If you want to use a product to control moss, there are several available. Kowalewski recommends sulfate products such as ferrous sulfate, iron sulfate and ammonium sulfate. These are environmentally friendly options, he said. Apply by spot treating as soon as moss appears.
Sulfur products such as these will lower the soil pH, making conditions acidic. An occasional application of lime, which will raise the pH, is recommended when you are making frequent sulfur applications. Again, test soil pH before applying lime.
If you decide to use a chemical herbicide, Kowalewski recommended that gardeners choose products with soap of fatty acid or carfentrazone as an active ingredient. Read labels and follow all safety precautions when using pesticides. Remember, though, even using herbicides to kill moss won’t keep it from returning.
For more information, consult the Extension publications Practical Lawn Care for Western Oregon, Practical Lawn Establishment and Renovation, and Integrated Pest Management for Turfgrass,