Why does the moss in my lawn keep coming back?

Almost everyone who asks this has already killed the moss at least once. That is the point of the question. Ferrous sulphate blackens moss reliably and scarifying pulls it out, but both are removal, not treatment, and the bare soil left behind is the ideal moss seedbed. The useful work is the diagnosis, and it takes about twenty minutes with a spade and a bit of honesty about the site.

What you'll need

• A spade
• A screwdriver or a stiff wire
• A soil pH test kit (optional)

Step by step

1. Look at where the moss is, not just that it is there.Moss confined to one strip along a fence or under a tree is a shade problem. Moss spread evenly across an open lawn is not — that is fertility, mowing height or compaction.
2. Test for compaction with a screwdriver.Push a long screwdriver into moist soil. If it goes in 100mm with hand pressure the soil is open; if it stops at 25 to 50mm you have compaction, and that is the most under-diagnosed cause on family lawns and anywhere a trampoline or paddling pool has stood.
3. Cut a wedge of turf and look at the profile.You are looking for three things: how deep the roots go, whether there is a brown fibrous thatch layer, and whether the soil below is grey, smells sour or is visibly wet. Grey and sour means waterlogging. Roots stopping at 30mm means compaction or scalping.
4. Watch the lawn two hours after heavy rain.Standing water or a spongy squelch means drainage. Moss on a lawn that drains freely is a different problem from moss on a lawn that holds water, and the fixes share nothing.
5. Measure the sun.Check the area at 9am, 1pm and 5pm through a summer week. Under three hours of direct sun and no ordinary lawn grass will hold its own against moss, however much you feed it. This is the cause people most often refuse to accept.
6. Check your mowing height.Set the mower on the lawn and measure the blade to the ground. Under 25mm on an ordinary lawn is scalping, and scalping is a fertility problem as well as a light one, because the grass has no leaf left to feed itself with.
7. Ask when the lawn was last fed.Grass that has had no nitrogen for several years thins out and stops competing. Moss does not need much fertility and will take the space. If the answer is 'never', that is very likely one of your causes.
8. Test the pH only after the others.Acidity below about pH 5.5 does favour moss, but it is a much less common cause in UK gardens than shade and compaction, and liming a lawn that is simply compacted wastes a season. Test rather than assume.
9. Fix the cause first, then remove the moss.Aerate or drain or raise the cut or feed, and give it a season. Then kill and scarify out the moss and overseed the gaps immediately, so grass rather than moss occupies the bare soil.

Tips

• Two causes is the normal answer. A shady lawn is also usually a damp, compacted, thin one, because it dries slowly and is walked on when soft.
• If you cannot change the shade, change the surface. Shade-tolerant grass mixes help a little; a mulched bed, a woodland planting or gravel solves it permanently.
• Do the diagnosis in late winter. That is when moss is at its most visible and when you still have time to act before the growing season.

Common mistakes

• Applying moss killer every spring as a routine — It removes the symptom on a schedule and leaves the cause running. The moss returns on the same schedule, and iron-based treatments repeatedly applied also stain paving and acidify the surface slightly, which very slightly favours moss.
• Raking live moss out without killing it first — Moss regenerates from fragments. Raking a living carpet distributes it across the lawn and into the bare patches you have just made.
• Liming a lawn because moss means acid soil — It often does not. Lime applied to a lawn that is not acid raises the pH into a range that favours worms and earthworm casting, and does nothing about the compaction that was the real cause.

If it doesn't work

Questions people ask