A marimo is not a moss. It is a colony of filamentous algae (Aegagropila linnaei) that grows into a dense green sphere in a few cold lakes in the northern hemisphere. In a jar, it is one of the easiest and slowest plants you can keep.
That slowness is the point. A marimo will not fill the jar, outcompete anything, or demand more light. It will sit on the substrate, grow a fraction of a centimetre a year, and give shrimp a place to rest.
Light
Marimo tolerates low light. A bright room or a desk lamp for six to eight hours a day is enough. Direct sun will heat the water and can scorch the surface of the ball; avoid it.
If the ball turns pale or yellow, the light is probably too strong or the water too warm. Move it to a cooler, shadier spot.
Temperature
Marimo prefers cooler water, roughly 15–22 °C. It tolerates room temperature but suffers in a hot room or near a radiator. In summer, a shaded spot and a little more water movement help.
Do not put a marimo in a sealed, warm jar. It will slowly lose colour and fall apart.
Water
Ordinary dechlorinated tap water is fine. Marimo does not need special minerals, though it benefits from the same water as the rest of the jar. Change part of the water as you would for shrimp.
If the ball collects detritus, gently squeeze it in a container of dechlorinated water and put it back. Do not scrub it with a brush.
Turning
In nature, wave action turns marimo balls, which keeps them spherical. In a jar, they sit still and can flatten on the bottom. Turn the ball a quarter turn every week or two, or gently roll it between your fingers. This is optional, but it keeps the shape.
Growth and splitting
A marimo grows extremely slowly. A ball the size of a golf ball may take years to reach that size. If it grows large, you can split it by pulling it apart and rolling the pieces into smaller balls. This is how cultivated marimo is propagated.
A brown or white patch is usually dead tissue. Remove it gently; the rest of the ball will continue to grow.
Floating
A marimo that floats is not necessarily sick. Small balls, or balls with trapped gas, can float. If it floats persistently and looks pale, check the temperature and light. If it floats overnight and sinks again, it is probably photosynthesis.
Real or fake?
Real marimo has a soft, velvety surface and a dense, fibrous interior. Fake “moss balls” are often made of synthetic fibre or a different algae; they are usually more uniform, brighter green, and do not smell of a lake. A real ball is slightly irregular and has a faint earthy smell.
If you buy online, look for a seller who states the species and the origin. Marimo is protected in some countries; cultivated balls are widely available.
Marimo and shrimp
Cherry shrimp like to sit on marimo and graze the surface. The ball does not provide much food, but it is a good perch and a gentle place for biofilm to collect. A marimo is a good companion for a small shrimp jar because it does not compete for light or nutrients.
The short version
- Low to medium light, six to eight hours.
- Cool water, 15–22 °C.
- Dechlorinated tap water.
- Turn it occasionally.
- Do not scrub it.
- Expect very slow growth.
A marimo is a plant for people who like to watch something without being entertained by it. In a jar, that is a virtue. The Pondlet catalogue includes a marimo for the same reason: it is the calmest thing you can add.