CORVALLIS, Ore. – With the rainy season in full swing, it's time to count yourself in one of two camps: You either love or hate the moss that invades Pacific Northwest lawns.
But if moss is your nemesis every winter, there are some things you can do to combat this ancient plant, according to Alec Kowalewski, turfgrass specialist for the Oregon State University Extension Service. Moss is a sign of too much shade and wet soil conditions, he said.
"One of the best ways to control moss is to increase sunlight," Kowalewski said. "If you have a shaded lawn, I would suggest pruning your trees to a height of six feet and keeping the branches open for sunlight. Decide whether it's more important to you to have trees shading your lawn or a healthy stand of turfgrass. If turfgrass is a priority, thin the density of your trees to increase sunlight."
If you don't shine more light on your lawn, moss will come back every year no matter what other steps you take to control it, Kowalewski cautioned.
Consider improving the drainage of your lawn as well. Install a bioswale or rain garden to catch stormwater runoff, Kowalewski suggested. Water less frequently and more deeply during the hot days of summer.
Don't stress out your lawn either, he said: "Heal" your lawn to keep the moss at bay.
"Turfgrass, like all other living things, requires sunlight, water and air movement to photosynthesize and grow,” Kowalewski said. “If there's a problem with any one of these three things, you can expect moss and weeds to invade your grass every year.”
Here’s one way to stress out your lawn: The more grass you remove when mowing your lawn, the more likely moss will invade, Kowalewski said. Grass mowed to a height of less than two inches will open the door for plant competition, he explained.
Fertilize your lawn with 2-4 pounds of nitrogen per 1,000 square feet two to four times per year in the spring and fall to increase the density of the grass and decrease the density of moss, Kowalewski said.
Additionally, several gardening products are available for moss control. Kowalewski recommends potassium soap as well as sulfate products such as ferrous sulfate, iron sulfate and ammonium sulfate. These are all non-synthetic, 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 – but turfgrass does not like acidic conditions, Kowalewski said. So an occasional application of lime, which will raise the pH, is recommended when you are making frequent sulfur applications, Kowalewski advised.
If you decide to use a synthetic pesticide, Kowalewski recommended that gardeners choose products with carfentrazone as an active ingredient. Read labels and follow all safety precautions when using pesticides.
"It's not a long-term cure," Kowalewski said. "If you applied pesticides or other moss control products but you did not improve your turfgrass density or sunlight exposure, moss is going to come back next year. The most important factors to improve are sunlight, water and air movement."
Kowalewski holds the N.B. and Jacqueline Giustina Professorship in Turf Management within OSU's College of Agricultural Sciences.
To learn more, see the OSU Extension publication Maintaining a Healthy Lawn in Western Oregon.