Moss in the Lawn: What It's Telling You

Moss doesn't push grass out. It moves in after the grass has already given up. If you've got moss, something about that spot is bad for turf: too much shade, packed soil, standing water, acidic soil, low fertility, or several of those at once. Kill the moss and leave the cause, and it'll be back by next fall. Find the cause and fix it this fall, and the grass can take the spot back.

University of Illinois Extension lists the usual causes as excessive shade, compacted soil, poor drainage, low fertility, a soil pH that's off, and poor air circulation. Other extension services give nearly the same list. Each one leaves its own clues, so start by looking at the spot.

Read the spot before you treat it

Stand where the moss is and go through these:

• Is it under a tree, on the north side of the house, or between buildings? Then shade is the first suspect.
• Is it along a path, a dog run, or where the kids cut the corner? Then it's probably compaction.
• Does the spot stay soft and squishy a day or two after rain, or sit low? Then it's drainage.
• Is the moss spread thinly across a whole lawn that looks pale and slow? Then it's likely fertility or pH, and you need a soil test.

On a golf course we never treated a moss patch until we knew why it was there. The fix for a shaded tee and a waterlogged low spot in a fairway are completely different jobs, and the moss looks the same on both.

Shade: count the hours of sun

Shade is the cause I see most often. Grass needs direct sunlight, and the bright filtered light under a tree isn't enough.

Watch the spot on a sunny day and count the hours it gets direct sun. Penn State turfgrass specialists say that if a site gets more than three hours of direct sun a day, you can probably establish a fine fescue (creeping red, red, or Chewings fescue) as long as the soil drains well. Under three hours, they recommend giving up on grass and planting a shade ground cover like pachysandra or myrtle.

If you're right around the line, these help:

• Limb up the trees. Removing lower branches lets in morning and afternoon sun and gets air moving, which helps the surface dry faster. Hire an arborist for anything big, and see what to cut now and what to leave.
• Switch the seed. Kentucky bluegrass sold for sun won't make it in the shade. Overseed with a mix that's mostly fine fescue. More on picking seed in best grass for shaded yards.
• Mow higher. Illinois Extension recommends 3 to 4 inches for most lawn grasses. In shade, go to the top of that range so the grass has more leaf to catch light.
• Feed less. Illinois Extension puts shaded lawns at 1 to 2 pounds of actual nitrogen per 1,000 square feet a year, compared with 2 to 4 for full sun. Pushing shade grass with nitrogen makes it soft and weak.
• Keep off it. Fine fescues don't hold up to foot traffic. If that spot is also the route to the gate, put down stepping stones.

If you get less than three hours of sun and the moss looks good, think about leaving it. Illinois Extension points out that moss is an acceptable ground cover where nothing else will grow. It stays green, you never mow it, and it stops the mud.

Compaction: the screwdriver test

Push a long screwdriver into the soil when it's moist. In healthy turf it goes in easily for several inches. If you have to lean on it in the moss patch but not in the good grass a few feet away, the soil is compacted. Roots can't get down through it, water sits on top, and moss does fine in that thin wet layer.

Core aeration is the fix, and early fall is the best time for it because the grass recovers fast. Pull cores, not spikes. A spike tine pushes soil sideways and packs it even tighter. For heavy clay, plan on aerating every fall for a few years in a row. One pass won't undo ten years of foot traffic. The aeration and topdressing guide covers how deep to go and how many passes to make.

Drainage: moss loves wet feet

If the spot holds water, the grass roots are short of oxygen and moss isn't. Check the obvious things first:

• Is a downspout emptying right onto the spot?
• Is a sprinkler head overlapping it? Extension sources list overwatering as a cause of moss on its own.
• Is it the lowest point of the yard?

Redirecting a downspout is a $15 extension and an afternoon. A real low spot takes regrading or a drain. See fixing wet spots before winter. Once the irrigation is shut down, cut that zone's run time or move the head next spring.

Acidity and fertility: test, don't guess

Most cool-season lawns grow best at a soil pH of 6.0 to 7.0. Moss tolerates acidic soil better than grass does, so a lawn that has slid below 6 starts to favor moss. That doesn't mean you should spread lime because you saw moss. Illinois Extension says plainly not to lime unless a soil test shows the pH needs to come up. Plenty of mossy lawns have a pH that's fine, and liming one that's already neutral does nothing for you.

A soil test from your state's extension lab costs about the same as a bag of fertilizer, and fall is a good time to send one in. See fall soil testing and, if the number comes back low, lime for northern lawns for rates.

Low fertility works the same way. Thin, hungry grass leaves gaps, and moss fills them. The test will tell you whether potassium or phosphorus is short too, not only nitrogen.

Killing the moss you already have

You still have to get rid of what's there so new seed can reach the soil. You've got two options:

1. Rake it out. A stiff leaf rake or a dethatching rake pulls moss up easily because it has no real roots. For a small patch, this is all you need.
2. Iron-based moss killer. Products made with ferrous sulfate, ferric sulfate, or ferrous ammonium sulfate turn moss dark and kill it back while doing little harm to the grass. University of Alaska Fairbanks Extension gives a rate of 0.5 to 1.5 pounds of actual iron per 1,000 square feet. Follow the product label, since products contain different amounts of iron. Rake out the dead moss once it turns dark.

Iron has two catches. It stains concrete, pavers, and stone rust-orange, so sweep every granule off the driveway and walk. And Illinois Extension says the control is temporary, so moss comes back quickly if the conditions haven't changed. UAF also warns that most roof moss killers will kill or damage grass. Don't use leftover roof product on the lawn.

Why fall is the time to do all of this

October is when the fix comes together for cool-season lawns. Soil is still warm enough to germinate seed, nights are cool, weed pressure is low, and the trees are about to drop their leaves and let in light for weeks. Penn State makes the same point about shade seeding: the grass needs time to establish before the trees leaf out again next summer.

The fall order of work:

1. Rake out or iron-treat the moss.
2. Core aerate the patch.
3. Fix the water source (downspout, sprinkler, low spot).
4. Prune for light if shade is the problem.
5. Overseed with the right grass for the light you have.
6. Keep leaves off the new seed. A mat of wet leaves on young grass does the same damage as shade, and it sets up snow mold.

If your soil has already cooled too far to germinate seed, do steps 1 through 4 now and dormant seed in late fall or overseed early next fall. Check whether you still have time in is it too late to overseed in October.

The one-minute checklist

• Count the hours of direct sun. Under 3, plant a ground cover or keep the moss.
• Do the screwdriver test. If it's hard to push in, core aerate.
• Look for downspouts, sprinkler overlap, and low ground.
• Get a soil test before you lime.
• Shade grass gets 1 to 2 lb of nitrogen a year, no more.
• Use iron only on the lawn and keep it off concrete.
• Seed fine fescue where the sun is limited, and get it done this fall.

Sources (5)

• Managing Moss in Lawns, University of Illinois Extension
• Moss Control in Lawns, University of Alaska Fairbanks Cooperative Extension
• Choose the Right Grass Seed for Pennsylvania Lawns, Penn State
• Moss and Algae Control in Lawns, Clemson Cooperative Extension
• Moss Control in Turf, NC State Extension