Why Moss Turns Black And What You Can Do About It

Moss grows beautifully for years, then suddenly turns black. It happens from the bottom up, and sometimes parts die altogether. The moss is close to a pond or is part of a (moss) terrarium. Do you recognize this problem?

Why Does Moss Turn Black?

There can be many reasons why moss is turning black. The first reason can be black slime mold, and the second is so-called ‘gray mold’.

If the blackening moss is close to a pond, it can be caused by black slime mold. This is affecting the area if it is wet all the time, and if water plants are creeping up the moss, they could easily help to spread this type of problem.  

Molds are one of the few enemies of mosses and often occur in closed moss terrariums (or sometimes in outside ponds). Molds are partial to the same conditions that many mosses are.

Constant moisture and shade can be a recipe for the slimy and fuzzy stuff to invade and the reason why your moss is turning black. The molds are, however, temperature dependent, and you will likely not find them growing in cooler seasons.

I have found that once the thermometer reaches 75 degrees (Fahrenheit) or above and the moisture is constant, you should keep an eye out for the attack of the molds.

What To Do When Moss Turns Black

Removing the affected blackened moss is a good precaution; monitor it carefully and ensure the water quality isn’t the problem.

Most of these attacks are easily remedied by removing the moisture, but if the problem is at the edge of your artificial water system, then removing moisture is tricky.

Before you replace anything: brown and flat is dormant, not dead, and it will green up when water reaches it. BLACK is different. Black is rot or mold and it does not come back, so cut it out and treat what is left. If you have lost a patch, terrarium grade moss is sold from a quarter of a square foot, which is enough for most repairs.

Trimming or thinning mosses growing in the water can also be preventative. Mosses may reach for and grow into the water, but that doesn’t mean it’s good for them.

Some artificial water gardens have a constant water level that natural streams or ponds do not; this consistent water line is not as forgiving as the rising and falling water lines of a natural body of water.

When mosses have direct contact with a body of water, they wick the moisture into the moss mat. This can help feed the mosses’ growth but, in certain situations, can also lead to problems.

One fix is to drain the water system faster than evaporation does on its own. The other problems are soggy soil and slime mold.

Eventually, these problems will find an equilibrium and take care of themselves, but the results may not be the desired lush moss growth right to the water’s edge.

A little moss maintenance and trimming will help in this case to stop the wicking and soothe the soul.

Related reading: A Rock’n Party! · Ken Gergle’s Photography

Pending review by Olivia Milner, practitioner-scientist with a background in sustainability and education.