Moss in lawns

What you are looking at

Soft green cushions between the grass plants, springy when you press them, and grass that is thin and pale wherever the moss is thickest. Worst under the trees, worst along the shaded side of the house, and worst on the line everyone walks from the back door to the shed.

Rake a handful out and there is bare soil underneath rather than grass roots.

It comes on through autumn and winter, looks its worst in February, and retreats a little when the grass starts growing again in spring. Then the same thing happens the following winter, a bit further across the lawn.

Moss is the result, not the cause

Moss has no roots to compete with grass, takes almost nothing out of the soil, and cannot force its way into thick turf. It arrives on spores, which are everywhere all the time, and it establishes in the gaps.

So a mossy lawn is a lawn where grass has stopped filling the space, and the moss is a reading of why. Kill the moss and leave the reason in place, and you have cleared the ground for the next crop of spores, which is what makes moss killer an annual purchase for so many people.

Four conditions account for nearly all of it. Shade, because grass needs several hours of direct light and moss does not. Compaction, which squeezes the air out of the soil and stops grass rooting. Poor drainage, which does the same thing with water. And close mowing on hungry ground, which starves the grass of leaf area at the same time as it lets light down to the soil.

Putting each of these right is ordinary lawn care, and it does more against moss than anything sold to kill it.

Drought belongs on the list too, working one season late. Grass burnt off in August leaves gaps, and the moss moves into them in October.

Which moss tells you which problem

Two shapes turn up in lawns and they point in different directions.

Upright moss with stiff little shoots that look like miniature conifers, often dark green to almost black, favours dry, thin, acid and starved ground. It is common on banks, on sandy soil and on old lawns that have never been fed.

Loose, feathery, trailing moss that pulls up in mats and looks like a green tangle prefers shade and damp. It is what you find under a tree, along a north wall and on ground that stays wet.

Neither identification changes the treatment much, and both are quicker to read than a soil test. What they do is tell you which condition to attack first.

The lime myth

Ask why a lawn is mossy and someone will say the soil is acid and it needs lime. That is right occasionally and wrong most of the time.

Moss grows happily from strongly acid soil right through to alkaline, so its presence says nothing about pH on its own. The great majority of mossy lawns are shaded, compacted, starved or scalped, and liming them changes nothing about the moss while raising the pH of ground that did not need it. On a lawn that was already neutral, that costs you the fine grasses and encourages coarse weeds.

Test with a cheap kit before spending anything. Where the reading comes back low, lime helps the grass grow thicker, and the moss then loses to the grass rather than to the lime.

What moss killer does and does not do

Products based on ferrous sulphate blacken moss within a few days and are satisfying to watch. What they leave behind is dead moss lying on bare soil, which has to be raked out, and bare soil takes new moss spores as readily as it takes grass seed.

Used as one step of a proper renovation, alongside scarifying, aerating, feeding and oversowing, they are worth their place. Used on their own on a lawn that is still shaded, still compacted and still mown at 15 mm, they buy a season.

The household remedies circulating online are worse than useless. Bleach, vinegar, washing soda and salt all damage grass and soil life, and none of them addresses the reason the moss is there.

What to do about it

Raise the mower to 30 to 40 mm and keep it there.

Feed the lawn in spring and again in autumn.

Test the pH before liming, and expect nothing from lime if the reading is normal.

Scarify in early autumn or mid spring, in two directions, and bin what comes out.

Aerate the compacted ground with a fork or a hollow-tine machine, and brush sharp sand and loam into the holes on clay.

Oversow the bare patches the same day you scarify.

Thin the tree canopy where shade is the problem, and fix the drainage where water sits.

Water through a bad drought, or at least leave the grass long.

Where grass cannot win, plant something that can, or keep the moss.

What it gets mistaken for

Waterlogging shows standing water, a sour smell and grass dying outright rather than thinning.

Red thread bleaches irregular patches and threads them pink, and it tells you the lawn is hungry rather than that the site is wrong.

Leatherjackets and chafer grubs eat the roots and let the turf lift like a carpet, and moss then colonises what they killed.

Algae make a slippery black-green film on the surface of bare wet soil rather than a cushion, and they say the same thing about drainage.

Liverworts form flat green scales pressed to the ground on permanently wet compacted soil, and they are a stronger drainage signal than moss is.