By the second week of July, a moss patch that looked like velvet in April can look like a tray of dried tea leaves. The edges curl inward, the green drains to olive and then to straw, and the whole thing crackles underfoot. It looks terminal. Nine times out of ten it isn't.

Mosses do not behave like the rest of the garden. They have no roots and no waxy cuticle, so they cannot hold water against the weather. When the air dries out and the temperature climbs, they simply stop working and wait. In a British summer, that is a completely normal response to a fortnight without rain.

Why moss turns brown in summer

Brown is a symptom, not a diagnosis. Three things usually cause it, and they often arrive together.

Drought stress

Moss takes in water across its whole surface, through a film that sits on the leaves and stems. That film is thinner than most gardeners imagine. On a warm, breezy day it can evaporate in a couple of hours, and photosynthesis stops almost immediately. The plant shuts down, the chlorophyll retreats, and the colour shifts through olive to tan to rust. This is desiccation tolerance, and it is one of the reasons moss has survived for hundreds of millions of years. It is not dying. It is waiting.

Heat stress

A south-facing wall, a slab of paving or a dark stone step can run several degrees hotter than the air around it, and the moss growing there cooks from below as well as above. Thin, cushion-forming species suffer first. Heat alone rarely kills moss outright, but combined with dryness it can scorch the growing tips, and those are the parts that would have produced new green growth in autumn.

Light stress, or plain bad timing

Moss that grew in shade can burn when the shade disappears. Prune a shrub hard, remove a low branch or take down a fence panel in June and you can expose a colony to full sun it has never seen. It bleaches yellow within days, then browns. Reflected glare off a wall or window does the same thing.

It is also worth remembering that some browning is genetic, not damage. Ceratodon purpureus turns a rusty brown in dry spells, and Bryum argenteum takes on a silvery-grey sheen. Both green up again after rain.

Is it dead, or just dormant?

Before you reach for the hose, work out which one you are dealing with. The two look similar at a glance and completely different up close.

• Colour: dormancy gives you olive, tan or rust. Death gives you grey-black, or a dull, uniform khaki with no variation at all.
• Texture: dormant moss is dry and brittle but intact. Soak a pinch of it in rainwater for twenty minutes and it should soften into recognisable stems and leaves. Dead moss crumbles to powder or sloughs away from the surface in a soggy mat.
• New growth: after a week or so of steady damp, living moss produces small green points at the tips of the stems. Get your nose close, or use a hand lens. Those tips are the whole point.
• Smell and touch: sour, slimy or slippery patches suggest the moss has been smothered by algae, leaf litter or standing water rather than dried out. Brown from drought is crisp. Brown from suffocation is not.

How to revive dormant moss

The instinct is to drench everything and hope. Gentle, patient work gets better results.

1. Water in the evening. Early morning is fine too, but avoid midday, when most of what you apply evaporates before it reaches the plant. Rainwater is far better than tap water, especially in hard water areas where dissolved lime leaves a crust that can block the moss's uptake.
2. Mist rather than blast. Use the mist setting on a hose or a watering can with a fine rose. Go over the patch two or three times, waiting a minute between passes so the water can actually soak in rather than run off.
3. Give it shade. A piece of horticultural fleece, a shade net or even a few cut leafy branches laid loosely overhead for a fortnight will do more for recovery than any amount of extra watering.
4. Keep it damp for two to three weeks. Recovery is slow. Moss rehydrates in minutes but rebuilds its green tissue over weeks, so a single soaking followed by another dry spell just adds stress.
5. Clear debris, but don't scrub. Lift fallen leaves, grass clippings and twigs off the surface by hand. Never rake or brush brown moss — the stems are brittle and you will remove the growing tips that were about to recover.
6. Then leave it alone. Stop walking on it. Foot traffic on dormant moss snaps stems and compacts the surface underneath.

If nothing greens up after a month of consistent moisture and shade, that patch has probably died. Cut it back and re-establish from a fresh colony, or let the surface reseed naturally from moss nearby.

Making your moss more summer-proof

Reviving a patch is one job. Reducing the chances of it happening again next July is another, and it is mostly about microclimate.

Choose species that suit the spot. Bryum argenteum, Ceratodon purpureus and Rhytidiadelphus squarrosus all cope with drying out and greening up again. Hypnum cupressiforme is reasonably tough in partial shade. Sphagnum is not a plant for a sun-baked wall, however tempting the texture.

Build a surface that holds moisture. A mix of loam, leaf mould and a little clay keeps a reservoir under the moss; pure sand or gravel drains too fast. Aim for dappled light rather than deep shade or full sun, and use the damp corners of a garden — beside a downpipe, along a north-facing wall, under a dripping gutter — as your starting points. In hot spells, one thorough evening soak twice a week beats a daily flick of the hose that only wets the top millimetre.

Keep chemicals away. Patio cleaners, weedkillers, lawn feeds and moss killers all reach moss through run-off, and the damage shows up as brown patches a week or two later. If you are treating a lawn for moss, do it in September or October, not in the heat of summer.

What to do this week

Pick one brown patch and treat it as a test case rather than trying to save the whole garden at once. Soak it in the evening with rainwater, shade it, keep it damp for a fortnight and check the stem tips with a hand lens. If green appears, you know the colony is alive and you can repeat the process elsewhere.

If you are dealing with moss in a lawn, resist scarifying or feeding while the ground is dry and hot; wait for autumn rain, when the moss will green up on its own and the lawn will recover alongside it. And if brown moss is sitting on a roof, in a gutter or against brickwork where it stays wet, deal with the underlying damp rather than the moss itself — your roof will thank you.

Most of the brown you are looking at right now is simply moss doing what moss does. Give it water, give it shade, and give it time. The autumn rains will finish the job.

Photo: maxmann / Pixabay