Marimo Moss Ball
A velvet green sphere that lives underwater and is neither moss nor a plant but a colony of algae. It sits in a jar of cool water, asks for a rinse and a water change every couple of weeks, and lives for decades. Pet-safe.
Marimo balls are the only thing in this directory that lives underwater. Each one is a colony of filamentous algae rolled into a sphere by lake currents, and they grow a few millimetres a year for a century or more. In Japan they are a protected natural treasure.
Indoors they need a jar, cool water, dim light and a rinse now and then. It is difficult to do less for a living thing.
Light
Low to medium indirect light. A north window or a desk away from the window. Direct sun heats the water and browns the ball.
Water
Every two weeks, pour out the water, rinse the ball under a cool tap, squeeze it gently to push out old water, roll it back into shape, and refill with fresh cool tap water. Tap water is fine; the chlorine does not bother algae.
Soil and potting
None. A glass jar or small aquarium, with or without gravel. Give it enough room to sit on the bottom with water above it.
Feeding
None needed. It takes what it needs from the water and light.
Propagation
Cut a ball in half, roll each half into a sphere, and hold it with thread for a few weeks until it holds its shape. Slow but reliable.
In a hot, dry home
It wants cool water, under seventy-five degrees, which is the only thing to watch here in summer. Keep the jar away from windows and heat sources; on a hot day, swap in cooler water. It does not care about humidity.
Problems and fixes
Brown patches
Likely cause: Too much light or heat, or old water
Fix: Move to a dimmer, cooler spot and change the water. Turn the brown side down.
Ball floating
Likely cause: Trapped gas from photosynthesis
Fix: Normal. It sinks again. Squeeze gently if you like.
Slimy or grey
Likely cause: Foreign algae growing on it
Fix: Rinse well and change water more often.
Marimo Moss Ball questions
Is a marimo moss ball safe for cats and dogs?
Yes. It is freshwater algae and non-toxic. The jar is the only hazard.
How often do I change the water for a marimo?
Every two weeks. Rinse the ball, squeeze it gently, and refill with cool tap water.
Why is my marimo floating?
It has trapped oxygen bubbles from photosynthesis. It is normal and it will sink again.
Plants with a similar rhythm
Haworthia
- Light
- Medium to bright indirect
- Water
- Every 2 weeks
- Pets
- Safe
