Water Chemistry: What Your Tap Is Quietly Doing to the Moss
Marta Lindqvist · 27 July 2026 · 577 words · 3 min read
Photo by Tom Gill via Flickr, Creative Commons
A pot in a window loses water through drainage as well as evaporation, which carries dissolved minerals away. A closed vessel loses water only as vapour, and vapour leaves every dissolved solid behind. Over a year, that difference is the whole story.
Most slow terrarium decline traces back to water rather than light or humidity. Chlorine, chloramine, dissolved calcium and accumulated salts all behave differently in a closed vessel than in an open pot, because nothing ever flushes through. This guide explains hardness, pH drift, the white crust on your glass, which water sources are worth using and how to run an occasional flush without wrecking the planting.
How mosses take up water
Mosses have no roots and no waterproof cuticle worth the name. They absorb water and dissolved substances across the whole leaf surface, which is why they are used as pollution indicators and why they are unusually exposed to whatever is in your watering can.
There is no root barrier to exclude unwanted ions and no vascular system to redistribute them. What lands on the leaf goes into the cell, and what does not get used stays in the tissue.
This is also why fertiliser is nearly always a mistake. Concentrations that are trivially low for a houseplant are damaging to a bryophyte absorbing them directly across the leaf.
Chlorine, chloramine and hardness
Chlorine dissipates from standing water in about twenty-four hours, which is why the old advice to leave tap water out overnight has some basis. Chloramine, now common in municipal supplies, does not dissipate and needs either an aquarium dechlorinator or filtration to remove.
Hardness is the bigger long-term problem. Calcium and magnesium carbonates do not evaporate; they accumulate. In a vessel topped up with hard tap water for a year, the substrate slowly alkalinises and the acid-loving species stop growing without ever looking obviously sick.
Find your local hardness figure from your water supplier. Anything above roughly 150 mg/l as calcium carbonate is worth avoiding for topping up, though it remains fine for the occasional rinse of tools and gravel.
That white crust on the glass
The chalky film at the old waterline is precipitated carbonate left behind by evaporation. It is cosmetically annoying, but its real significance is as a gauge: it tells you how much mineral has been deposited into the substrate over the same period.
Remove it with a cloth dampened in dilute white vinegar, keeping the acid off the planting, then wipe with clean soft water. Never use household glass cleaner on a planted vessel; the surfactants and ammonia are far more damaging than the crust.
If crust reappears within weeks, your top-up water is the problem, not your cleaning. Switch sources rather than cleaning more often.
Which water to actually use
Rainwater is the best option where you can collect it cleanly, being naturally soft and slightly acidic. Collect from a clean surface, not a moss-covered roof that will add its own tannins and debris, and store it dark and covered.
Distilled or reverse-osmosis water is the reliable substitute and is genuinely mineral-free. Pure RO is slightly aggressive as a solvent, so many growers cut it with ten to twenty per cent tap water to restore a little buffering; in a moss vessel this is optional and mostly harmless either way.
Filter-jug water reduces chlorine and some hardness but does not remove dissolved minerals fully. It is a reasonable middle path in a soft-water area and a poor one in a hard-water area.
Flushing an established build
If a vessel has been topped up with hard water for a year or more, a controlled flush resets the substrate. Water gently with soft water until the drainage layer fills, wait ten minutes for the salts to dissolve into it, then siphon the drainage layer dry with a length of aquarium tubing.
Do it once, not repeatedly, and only in a vessel with a proper drainage layer. Flushing a build without one simply waterlogs the substrate and causes exactly the anaerobic rot you were trying to avoid.
Afterwards, expect a fortnight of unremarkable appearance while the system re-equilibrates, then a visible flush of new growth. Repeat annually at most; the goal is to prevent accumulation, not to keep the substrate sterile.
Extended notes
- Boiling water removes temporary hardness but concentrates permanent hardness, so it is not a substitute for soft water.
- Softened water from a domestic ion-exchange unit swaps calcium for sodium and is worse for moss than the hard water it replaced.
- A cheap TDS meter costs very little and tells you more about your water than any amount of guessing. Under 50 ppm is excellent for topping up.
Frequently asked
Is tap water always bad for moss terrariums?
No. In a soft-water area, dechlorinated tap water is fine indefinitely. The risk scales with hardness and with how long the vessel has been sealed.
Can I use boiled and cooled tap water?
It handles chlorine but not chloramine or permanent hardness. It is better than nothing and worse than rain or RO water.
How often should I flush a sealed build?
At most once a year, and only if it has a working drainage layer and has been topped up with hard water. Vessels kept on rainwater rarely need it.
Should I ever fertilise moss?
Essentially never. Mosses absorb nutrients directly through their leaves and burn easily. A closed system with a decomposition crew already recycles what it needs.
Source: Original guide — Mossline Care Desk
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