Most growers blame the light, the substrate or the British weather when moss sulks. Often it's the water. Moss takes its moisture straight through its leaves, so whatever is dissolved in that water ends up on the plant and stays there. Two seasons of hard tap water can leave a cushion looking dusted and tired, while the same moss under a water butt looks entirely different. Here's how the two compare, and how to make the most of whichever you have.
Why moss reacts to water differently to most plants
Moss has no true roots. Rhizoids anchor it to a wall, a log or a tray of substrate, but they do very little of the drinking. Water is absorbed across the surface of the leaves and stems, often through a single layer of cells, with no root membrane deciding what gets in and what stays out.
That has two consequences. First, dissolved minerals land on the plant itself rather than being filtered somewhere underground. Second, the substrate underneath slowly changes: water hard with calcium nudges an acidic, bark-based mix towards neutral, and the species that liked it there start to fade. Because moss grows slowly, the damage creeps in rather than announcing itself.
What's actually in UK tap water
Mains water is rigorously treated and perfectly safe to drink. It's also dosed, filtered and often mineral-rich in ways that suit a bathroom tap better than a moss tray.
Chlorine and chloramine
Water companies add a disinfectant to keep supplies clean, usually chlorine, sometimes chloramine. At the low levels found in UK mains water, chlorine is unlikely to kill moss outright, and most moss problems trace back to hardness rather than to the disinfectant itself. It's still worth removing. A residual disinfectant sitting on a living surface does the plant no favours, and inside a closed terrarium it affects the springtails and micro-life working away in the substrate.
Chlorine is easy to shift. Fill a jug or watering can and leave it uncovered overnight and most of it will be gone. Chloramine is more stubborn and won't gas off; you'll need a carbon filter jug or an aquarium dechlorinator to deal with it.
Hardness and dissolved minerals
Hardness in the UK follows the geology. Much of Scotland, Wales, Cornwall and the north-west runs soft. Large parts of the south-east, East Anglia, the Midlands and anywhere sitting on chalk tend to be hard. Your kettle is the quickest test you own: thick scale means hard water, and hard water means dissolved calcium and magnesium heading for your moss.
When that water evaporates from a moss cushion, the minerals stay. Do that week after week and you get a pale crust, blocked leaf surfaces and a moss that looks grey rather than green. On terrarium glass it's worse: those chalky rings are very hard to shift without scratching.
A note on softened water
Domestic ion-exchange softeners don't remove hardness so much as swap it, trading calcium for sodium. Sodium is unkind to moss and to the surface it grows on, so if you have a softener fitted, take your moss water from an untreated tap — usually the one in the kitchen, or a bypass tap if you have one.
What rainwater gives you
Rain is close to what moss evolved alongside: soft, low in dissolved minerals, usually neutral to slightly acidic, and free of disinfectant. It also arrives cool and picks up a little dust and dissolved nitrogen on the way down, which is no bad thing in small amounts.
For acid-loving species this is near ideal. Sphagnum, most woodland mosses and anything you're trying to establish on bark or sandstone will thank you for it.
Where collected rainwater goes wrong
Rain off a roof isn't automatically clean water, and a few habits make a real difference:
- Mind the roof surface. Slate, glass and plastic shed roofs give reasonably clean water. Old tiles covered in moss and lichen give you a soup of organic debris, and roofs with lead flashing are best avoided for anything you plan to water plants with.
- Dump the first flush. The first rain after a dry spell washes dirt, pollen and bird droppings off the roof and into the butt. Proper diverters handle this automatically; a cheap workaround is to let the first few litres run to waste.
- Fit a lid and a mesh filter. Open butts collect leaves and breed midges, and water left warm and stagnant grows algae and turns sour. Cover it, filter the inlet, and use the water rather than hoarding it for months.
- Plan for frost. Butts freeze and taps snap in a cold snap. Keep a couple of lidded containers indoors or in a shed to see you through January.
Matching water to the moss you're growing
Hard water isn't universally bad. Plenty of British mosses are calcicoles that happily colonise limestone walls, mortar and concrete, and they'll cope with tap water indefinitely. The distinction is between those and the mosses most growers actually want: a dense green lawn, a Sphagnum tray, a woodland clump lifted from a shady bank.
If you're unsure what you've got, start with rainwater on everything for a season and see which patches respond. In practice, soft water rarely does harm, whereas hard water only suits a minority of species.
If you only have tap water
There's plenty you can do short of installing a butt and a diverter.
- Let it stand. Twenty-four hours in an open container clears most chlorine, though not chloramine.
- Filter it. A carbon filter jug or an aquarium dechlorinator handles both.
- Dilute. Half tap and half rainwater still cuts the mineral load substantially, and it's a sensible winter compromise when the butt is frozen.
- Reuse condensate. Water from a dehumidifier is close to distilled and fine for outdoor moss; indoors, use it with a little caution as it can pick up traces from the machine.
- Flush regularly. Every few weeks, soak the moss and its substrate in rainwater and let it drain freely. This leaches accumulated salts out of the root zone and does more good than any feed.
One more thing worth knowing: water temperature matters more than most people expect. Cold mains water splashed onto warm moss in a closed terrarium spikes the humidity, fogs the glass and gives mould a foothold. Let it reach room temperature first, and water in the morning so the surface has the day to settle.
A workable routine from here
Set up a lidded butt with a diverter if you possibly can, and use it for the fussy species, seedlings and anything in a terrarium. Keep a jug of tap water standing on the windowsill for the rest, and top it up as you use it. Once a month, give everything a proper flush with the softest water you have. Watch for white crusts on leaves and glass; they're your cue that the mineral load is building and it's time to switch sources.
If you're on a water meter or a hosepipe ban is in force, note that restrictions apply to mains supply through a hose or sprinkler, not to water you've collected in a butt, so rainwater harvesting pays off twice. If you rent, or the downpipe is shared, check with the building owner before making alterations, and if you're unsure about local rules on collection and storage, ask your water company.
Photo: viarami / Pixabay