How to Grow and Care for Marimo Moss Balls, the Perfect Pet Plant for 2019

Remember pet rocks? Yeah, it’s time to upgrade. Meet the marimo moss ball.

These pet-friendly balls are often referred to as moss balls, but they’re actually not moss at all. The marimo ball is a small algae colony known as Aegagropila linnaei that’s native to the lakes of Japan and Northern Europe. “Mari” means ball, and “mo” translates to algae in Japanese.

Marimo are often given as gifts. Some Japanese families pass them down as heirlooms due to their long lifespan (we’re talking more than 100 years), and kids even get them as first “pets” because they are lively little balls that don’t require much care. 

“They’re incredibly low-maintenance plant options,” says Erin Marino, plant expert from The Sill. 

Although they’re now considered contemporary decor additions or indoor “pets,” marimo balls are part of a legend. According to Pistils Nursery in Portland, Oregon, it’s a tale of two lovers who couldn’t be together and fell into water, and their hearts turned into marimo balls. For that reason (and despite the tragic start to the story), marimo balls are said to bring good luck and love.

There are many sites with instructions for setting up a perfect marimo ball terrarium, but it’s actually quite simple. Marimo balls do best in an open (or at least breathable) container or glass bowl. Put filtered, room temperature water into your bowl, then add your marimo ball. That’s all the ball needs, though you can include rocks and whatnot for some terrarium variety.

Once you’ve set up your terrarium, refrain from placing it in the path of direct sunlight, even if it looks cute on your windowsill. Instead, find a place with low, indirect light. 

Marimo balls can even thrive in artificial light and act as natural filters, making them great additions to aquariums, as well. However, some fish love marimo balls a little too much and might peck at the balls and eventually destroy them. Keep an eye on your marimo balls when you first introduce them to your aquarium to be sure it’s the right environment before you decide to leave them there permanently.

For such a dynamic little plant, the upkeep on marimo balls is surprisingly easy.

Change the water every two to three weeks, and keep it at room temperature. After all, marimo balls are native to cool lakes, so be sure to keep your bowl in a place where it won’t get too warm.

After changing the water and before putting your ball back into the new bowl, you can gently squeeze your marimo ball to clear out any old water it’s hanging on to, and roll it around in your hands to help keep its shape and mimic the natural movement of lake tides it might receive in the wild.

“Don’t be scared to touch your marimo,” says Marino. “They’re not as slimy as you’d think they are. They’re actually quite fluffy, almost velvety.”

If your marimo ball begins to float, that’s not a problem; it’s a sign that it’s healthy.

“Marimo will spend the majority of their time at the bottom of their container, like they would in their native lake environment,” says Marino. “However, a marimo does perform photosynthesis, and makes oxygen. These oxygen bubbles may make your marimo float up to the surface of the water for a period of time.” 

Marimo balls can also live out of water for a month or more, although that’s not recommended. A week of freedom for your marimo ball should be long enough. You can place it in a jar, or it can travel easily in a sealed plastic bag. Just check on it to make sure it doesn’t start drying out.

“A yellow or brown marimo is usually a sign something is slightly off,” says Marino. “It could be receiving too much sunlight, have an infection, or its water quality could have decreased.”

She suggests running an off-colored marimo under tepid water, then replacing its container water with cool, filtered water and perhaps adding a little bit of aquarium salt, which is different from table salt. Do not add table salt. Again, make sure you don’t keep it in direct sunlight.

To propagate your marimo ball, you can cut it into smaller pieces and roll those pieces into smaller balls. Beware that the marimo ball is a very slow grower and might only expand about 5 millimeters per year. It can grow a little faster with a tiny bit of fertilizer, but don’t expect rapid growth at any point during its lifespan.