Five Millimetres a Year Is the Whole Hobby
The clerk at the aquarium shop called them “practically unkillable,” which is how I knew I was being sold something.
I’d come in for ammonia test strips. The aquascaping tank has been cycling for six days now — bacterial bloom finally clearing, nitrite starting to register, the Hemianthus either recovering or dying slowly enough that I can’t tell the difference. Standard water change supplies. In and out.
But next to the register, in a mason jar with no filter and no heater, sat three green spheres the size of golf balls. Velvety. Dense. Rotating slowly in the ambient light from the front window.
“Marimo,” the clerk said when she caught me staring. “Japanese moss balls. Twenty bucks for three, they’ll live forever if you don’t boil them.”
I asked about care requirements. She shrugged. Change the water every couple weeks. Keep them out of direct sunlight. Roll them around occasionally so they stay round.
That was it. No nitrogen cycle. No CO₂ injection. No four-to-six weeks of watching invisible bacteria colonize filter media before you’re allowed to add the thing you actually bought the tank for.
I bought all three.
They’re not moss. I looked this up on the drive home, which is not a responsible way to conduct research but is an honest account of how I conduct research. Aegagropila linnaei — a filamentous green algae whose cell walls contain chitin, the same structural molecule that gives insect exoskeletons their rigidity. The “moss ball” name is marketing convenience. The spherical shape isn’t genetic programming either; wild marimo form in cold, shallow lakes where gentle wave action tumbles them continuously, distributing light exposure evenly across their surface. In Lake Akan in Hokkaido, protected colonies grow to thirty centimetres across. They’ve been accumulating since before the First World War.
The growth rate is five millimetres per year.
A golf ball-sized marimo is eight to ten years old. The ones in my jar, each about four centimetres in diameter, started growing around 2018. While I was debugging Kubernetes deployments and filing flight plans and wondering what 29,680 chess games would feel like, these three spheres were photosynthesizing quietly in a Ukrainian lake, adding a few millimetres of filament per year, waiting for someone to ship them across an ocean and sell them next to the ammonia strips.
The Ainu people of Hokkaido call them torasampe — lake goblins. Icelandic fishermen call them kúluskítur, which translates roughly to “ball muck,” because they tangle in nets. There’s a three-day marimo festival at Lake Akan every October. Japan has protected them as a species since the 1920s.
All of this for an organism whose primary activity is being round.
I’ve set them up in a spare Erlenmeyer flask from the fermentation shelf — 500ml, borosilicate glass, narrow neck to minimize evaporation. Filled it with dechlorinated tap water and dropped them in. They sank, bounced once, and settled at the bottom. The afternoon light through the kitchen window made them glow slightly, the way a backlit leaf does.
I watched them for twenty minutes. Nothing happened. This was somehow exactly what I wanted.
The bonsai taught me that first year is observation — don’t intervene, don’t stress the system, let it establish itself before you start making permanent decisions. The aquascaping tank is teaching me the same lesson more painfully, with melting plants and bacterial blooms. But the marimo are different. There’s barely any system to establish. They’re photosynthesizing. They’re being round. That’s the whole hobby.
The care forums recommend rolling them gently once a week to simulate wave action and keep them spherical. If you don’t, they’ll flatten on the side that touches the bottom, growing only where light reaches. A marimo left untended for years will develop a bald spot, then a brown spot, then eventually fall apart.
So once a week, I’ll swirl the flask. That’s my intervention. That’s the extent of my agency over this system.
Common beginner mistakes, according to the forums: too much light, which encourages competing algae to colonize the surface; too much warmth, which can cause the interior to rot while the exterior stays green; tap water with chloramine, which takes longer to off-gas than chlorine. The worst mistake — and this is a genuinely alarming thing I learned today — is buying marimo that arrived with hitchhikers. In 2021, zebra mussels were discovered on commercially sold moss balls across twenty-one American states. The USGS issued a recall. Owners were advised to boil, freeze, or bleach their moss balls before disposal.
Mine appear uncontaminated. I checked. Twice.
The zebra mussel situation clarified something for me: most commercial marimo come from Ukrainian lakes, specifically the Shatskyi Lakes near the Polish border. Collecting them from Lake Akan is prohibited. So these three spheres that I’m now responsible for likely grew in a lake I’ve never heard of, in a country I’ve only seen in news coverage, following biological imperatives laid down hundreds of millions of years ago by ancestral algae that figured out photosynthesis before anything on land had lungs.
There’s nothing I need to do for them tonight. They’re in the flask. The flask is on the windowsill, out of direct sun. The water is the same temperature as the room.
Tomorrow I’ll check on the aquascaping tank — ammonia test, nitrite test, squinting at the Hemianthus to see if any leaves have uncurled. That hobby requires attention, measurement, intervention, worry.
This one doesn’t. Five millimetres a year. A decade to double in size. The marimo will be here when I get back to them, being round, being green, growing imperceptibly.