You can do everything by the book — water daily, press the moss in firmly, keep off it for a fortnight — and still watch the joints between your slabs turn grey and brittle by July. It is rarely bad luck. Moss is unforgiving about four things: the base it sits in, the light it gets, the species you chose, and what you do to the patio in spring. Get those right and moss becomes one of the most dependable things in the garden. Here are the mistakes behind most failures, and how to correct them.

Planting the wrong kind of moss

Moss is not one plant. The species that carpets a damp north-facing wall will sulk on a sunny, south-facing patio. For paths, two broad groups matter: cushion and upright mosses, which form tight clumps and shrug off occasional footfall, and feathery trailing mosses, which need shade and steady damp. Choose from the wrong group and no amount of watering will rescue it.

Springy turf moss (Rhytidiadelphus squarrosus) is the loose, bristly moss you see in lawns, and it takes light traffic between slabs better than most. Silver moss (Bryum argenteum) is the small grey-green cushion that appears in pavement cracks and tolerates sun and drought. Both are common, and both can be lifted from your own garden.

Match the species to the spot

• Sunny, dry, south-facing: silver moss, or fire moss (Ceratodon purpureus), which colours up in bright light.
• Damp, shaded, north-facing: feathery species such as Brachythecium and Thuidium, which will not cope with full sun.
• Not sure: use whatever is already thriving on your walls, steps or lawn edges. It has proved it likes your conditions.

One naming trap worth knowing: the "Irish moss" sold as plugs in garden centres is Sagina subulata, a flowering plant rather than a moss. It is neat and useful between slabs, but it needs watering in dry spells and will not behave like true moss.

A base that dries out by lunchtime

Most mosses have no proper roots. They absorb water and nutrients across their whole surface, which means the material in the gap matters more than anything you do above it. Joints filled with builder's sand or kiln-dried sand hold almost nothing, and cement or mortar leaches lime into the gap — most mosses prefer lean, slightly acidic conditions.

Fill joints with two parts coarse grit or sharp sand to one part sieved soil or leafmould, taken to within 5–10mm of the slab surface. Firm it, water it, and leave it a few days to settle before planting. Narrow joints under about 10mm give moss very little to grip; 15–20mm is a realistic target, and wider is easier still. If water sits in the gaps after rain, check the falls across the patio before you plant anything — moss likes damp, not stagnant.

Planting at the wrong time, or in too much of a hurry

Autumn and early spring are the windows that work. The air is cool, rain is reliable and moss grows without stress. Planting in a heatwave is a fight you will lose. When you are ready:

1. Clear weeds, grass and seed heads from the joints, and top the mix back up if it has settled.
2. Water the joints and let them drain. Damp base, not a puddle.
3. Lift moss in sheets from walls, lawn edges or old paths, keeping it damp and out of the sun. Transplant within a day or two.
4. Press a piece into each joint so it makes full contact with the mix, with no air pockets underneath. A short plank laid over the slabs and walked along firms it evenly.
5. Water with rainwater using a fine rose, then keep the joints damp for three to four weeks. Tap water in hard areas leaves a chalky film that some mosses dislike.

The painted-on slurry of blended moss and water does work on damp, shaded surfaces, but the buttermilk and yoghurt versions often attract mould, flies and worse. Plain water is enough.

Cleaning that undoes a season of growth

Spring cleaning loses more moss than any drought. Damage usually comes from one of these:

• Pressure washing. Even on a low setting it scours the joints bare. Use a hose and a soft brush instead, and brush along the joint rather than across it.
• Vinegar, bleach and patio cleaners. Run-off soaks straight into the gaps. Spot-treat only, and soak the joints with water before and after.
• Kiln-dried sand swept into the joints. It buries the moss. Brush spills off the slabs and top up joints with your moss mix.
• Stiff brushing and leaf blowers on full power. Both tear at the surface. A light rake and a gentle blow are plenty.
• Weedkiller sprayed at the joints. It cannot tell moss from bittercress.

Too much traffic, too little shade

Moss is not a joint filler for the main route across a patio. Leave the walking line clear and let moss colonise the edges, the margins by a wall, and the corners you cross only to reach the shed. Full sun through the afternoon is the other limiting factor: if a joint bakes from noon onwards, plant a sun-tolerant species, shade it with a pot or taller planting nearby, or accept that the moss will be patchy and enjoy it where it takes.

Putting a failed patch right

If the moss has gone brown and thin, work out which of the above applies before replanting. Scrape out the old material, refill with the gritty mix, and re-lay moss in autumn. Then leave it alone. Moss is slow to show willing, and a patch that looks dead in August can green up again once the rain returns. Give any replanting a full season before you judge it.

A realistic approach to moss between slabs

Work with what your garden already offers rather than fighting it. Keep leaves off the joints in autumn, since a damp mat of leaves smothers moss faster than sun does. Water with rainwater during dry spells in the first year, then let it fend for itself. Aim for moss where it wants to grow, not a uniform green carpet across every joint, and it will reward you for years. If your plans involve changing jointing, mortar or drainage, ask a qualified landscaper before you start

Photo: 652234 / Pixabay