Marimo Moss Balls in Summer: How to Keep Yours Cool in the Australian Heat
Marimo grow in lakes that freeze over every winter. Here in Australia, December can turn a windowsill into a sauna. The good news is that keeping one happy through summer is easy once you know what heat does to it.
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The short version
- Marimo like cool water. Aim to keep it below about 25°C, and watch closely once it's warmer than that for days at a time.
- In summer, change the water every week, or more often in a heatwave.
- A few hours in the fridge is a lovely summer treat. The freezer, never.
- Keep the jar out of direct sun. A small glass jar in the sun heats up much faster than the room around it.
- Floating is usually just bubbles. A bad smell or a soft, mushy feel is the real warning sign.
Why heat is the thing to watch
In the wild, marimo live on the beds of cold, clear lakes in places like Hokkaido in Japan, Iceland and northern Europe. Lake Akan, their most famous home, freezes over in winter. Even at the height of a Japanese summer, the water down where marimo rest stays cool.
A marimo is a slow-growing green algae, and like most living things it does best near the conditions it evolved in. Cool water holds more of the oxygen and carbon dioxide it needs. Warm water holds less, and warm, still water is also where the bacteria that break down plant matter do their best work. That's why a marimo can sit happily through months of neglect in winter, then start to struggle after a few hot weeks in summer.
What temperature do marimo like?
There's no exact cut-off, but most marimo keepers go by a simple rule: cool room temperature or lower is ideal, and warmer than about 25°C for long stretches is when trouble starts. A warm afternoon won't hurt. What hurts is water that stays hot day after day.
- Comfortable: cool to mild room temperature, the kind you'd be happy to sit in.
- Keep an eye on it: warmer than about 25°C for several days. Change the water more often and find a cooler spot.
- Act now: a jar that's warm to the touch, sitting in the sun, a hot car or a room with the air-con off in a heatwave.
The easy test: pick the jar up. If the glass feels warm in your hand, the water is too warm for your marimo. Cool water should feel cool.
Finding a cool spot in summer
The spot that worked in winter might not work in January. The sun sits higher, rooms run hotter, and a sunny sill that was perfect in July can cook a small jar by mid-afternoon. A sealed glass jar in the sun works like a tiny greenhouse.
Good summer spots
A bookshelf in a shaded room. A desk away from the window. A bathroom or laundry with soft natural light. The south side of the house, which gets the least sun in Australia.
An air-conditioned room works beautifully too. If it's comfortable for you, it's comfortable for your marimo.
Spots to avoid
West-facing windowsills (afternoon sun). The top of the fridge, TV or modem, which all give off heat. Cars, verandahs and anywhere near the oven.
Marimo still need some light to photosynthesise, but less than you'd think. Bright, indirect light for part of the day is plenty, and in summer a little shade is better than a little too much sun. Too much direct sun can bleach a marimo pale or turn it brown.
The heatwave routine
When the forecast says 35°C and the house is heating up, this takes about five minutes:
- Pour off the warm waterTip it down an indoor sink, never onto the garden. (It's tempting to water the pot plants with it in summer, but jar water belongs in the drain. Here's why.)
- Rinse and give it a gentle squeezeCup your marimo under cool tap water and squeeze it softly, like a little sponge, to clear out the warm water held inside.
- Refill with cool, not icy, waterCool tap water is perfect. Skip the ice cubes: a sudden big drop in temperature is a shock of its own.
- Give it a fridge break if you likePut the jar on a middle shelf of the fridge for a few hours or overnight, then bring it back to its shady spot. Keep it well away from the freezer section.
Freezing is how marimo are destroyed when they need to be disposed of safely. A spell in the fridge cools them down, but a spell in the freezer will kill them. Keep them in the main part of the fridge, not near a freezer vent.
Signs your marimo is feeling the heat
Marimo change slowly, so the signs are easy to miss. Check yours each time you change the water.
Going pale or yellow usually too much sun
Move it somewhere shadier and give it fresh, cool water. Pale marimo often green up again over a few weeks once they're out of the sun.
Brown patches heat, light or both
Roll it so the brown side faces the light, and change the water more often. Small patches can recover. If a patch stays brown and goes soft, you can gently pull or snip it away with clean scissors.
Floating usually fine
Marimo float when tiny oxygen bubbles from photosynthesis get caught in their threads. It's a sign it's photosynthesising, and it'll usually sink again on its own. A gentle squeeze lets the bubbles out.
A bad smell or a mushy feel act now
A healthy marimo smells like clean pond water, or not much at all. A sour smell or a centre that feels soft and falls apart means part of it is breaking down. Change the water, move it somewhere cool and remove any soft, dark sections.
Wondering if floating means your marimo is happy? We looked into it in Do moss balls have feelings?
Going away over summer
Marimo are wonderfully easy to leave. They don't need feeding and they'll cope happily on their own for a couple of weeks. Over the Christmas holidays, the risk is a closed-up house with the air-con off.
- Change the water right before you leave.
- Move the jar to the coolest room, away from every window, and lower down (heat rises).
- Away more than two weeks? Pop it in the fridge before you go. It'll stay cool and happy in there, whatever the weather does at home.
When a Moonling arrives in summer
Your Moonling travels damp in a sealed bag, and it's happy in there for up to two weeks. We post orders within 1–2 business days, and in the warmer months we recommend Express so it spends the least time in a hot delivery van.
When it arrives, bring it inside straight away rather than leaving it in the sun or a hot letterbox. Rinse it in cool water, give it a fresh jar of water and let it rest somewhere shady. It's normal for a marimo to look a little different after travelling. Give it a week or two to settle in. Our quick care guide covers the rest.
Giving it as a Christmas gift? If it'll be a while before it's unwrapped, rinse the moss ball in fresh water and keep it in the fridge in its bag until the big day. Our Christmas gift guide has the order-by dates.
Questions
Tap a question to see the answer.
What temperature is too hot for a marimo moss ball?
There's no single number, but water that stays above about 25°C for days at a time is where most marimo start to struggle. A warm afternoon is fine. Hot water day after day is what does the damage.
Can I put my marimo in the fridge?
Yes. A few hours or an overnight stay in the main part of the fridge is a good way to cool a marimo down in summer. Never put it in the freezer, because freezing kills it.
Why is my moss ball floating?
Usually because tiny oxygen bubbles from photosynthesis are caught in its threads. It's a normal sign it's photosynthesising, and it'll usually sink again on its own. A gentle squeeze lets the bubbles out.
Why is my marimo turning brown?
Most often it's too much heat, too much direct sun, or water that hasn't been changed for a while. Move it somewhere cooler and shadier, change the water more often and roll it so every side gets gentle light.
Do moss balls need sunlight?
They need some light, but not direct sun. Bright, indirect light for part of the day is plenty. In summer, a little shade is safer than a little too much sun.
How often should I change the water in summer?
About once a week, and more often in a heatwave. In cooler months, every one to two weeks is enough.
A cool little companion for summer
Each Moonling is a living marimo paired with an intention crystal, ready for a shady spot on your desk.
More to read
With love, Shelby & Henning
