How to Grow Haircap Moss: (Polytrichum commune) Care Guide

Polytrichum commune is a funky-looking moss that’s all about big hair and bold looks. 

With its characteristically tall, spiky leaves, Haircap Moss looks straight out of an 80s punk rock band, and I’m living for it. Though despite its “alternative” appearance, it’s actually very common. 

So, how does its untamed foliage fare in a terrarium setting? Turns out, pretty well if you can manage it! 

Find out how to put Haircap Moss to good use in a terrarium setting and harness its wild growth for a stunning woodland look.

Lighting

Haircap Moss is a versatile and hardy species.

It has successfully colonized a wide range of environments around the world, from forests and mountains to alpine grasslands and tundra.

So it’s not the kind of moss that will wilt at the first sight of direct light (unlike typical tropical mosses).

In fact, according to the scientific review paper, A Biological Account of Polytrichum commune by V. Zarafis, this moss “shows no preferences for open or shaded habitats.”

A moss that’s not bright indirect light or bust? Okay, I’m listening.

Zarafis goes on to say that the “light intensities in the places where it grows range from approximately 1,000 to 10,000 foot candles at mid-day.”

For reference, that’s a huge range – from super dark to super bright. So you can be pretty confident that it’s going to handle windowsills and dark bathroom positions alike.

Personally, I’ve just used Haircap Moss as I would any other terrarium moss.

Watering

You really can’t go wrong when watering this robust moss species. 

It’ll do well when maintained at a nice, consistent moisture level, but it’s also perfectly capable of “tolerating anoxic, waterlogged stations, and of growing where water availability may be restricted.” ¹

So you can likely over or underwater this moss, and it’ll still be fine!

If you need a visual guide, the leaves remain straight and sharp when nicely hydrated. When it’s a bit dry, it’ll collapse slightly into a soft wave.

Substrate & Soil

Polytrichum commune differs again from most terrarium mosses in the way in which it takes up water.

Where most mosses have a simple physiology that soaks up water like a sponge, Polytrichum mosses have mechanisms for active water uptake. ^2,3

Given that these mosses can get really quite tall (up to around 8 inches), it makes sense that they’d need water transporting from the base to the top.

Why does this matter?

All this to say, Haircap Moss will benefit from a water-retentive substrate, as it is one of the few terrarium mosses that can access moisture like a plant would.

The hair-like rhizoid fibers at the base of the moss act a little like roots and can help take up water and secure the moss in place. That said, I tend to trim a lot of the fibrous fibers off the bottom, so I’m not sure how much of a difference that makes in this case. 🤷‍♂️

Temperature & Humidity

Haircap Moss appreciates high humidity, so it naturally thrives in a closed terrarium setting. 

As for warmth, Zarafis states that “Polytrichum commune tends to grow best at an average mid-day temperature of 20° to 25°C.” [1] (That’s 68 to 77°F, for reference).

However, they do go on to note that this widespread moss grows in air temperatures up to 35°C in Australian summer and sub-zero temperatures in a variety of winter settings. 

So, it’s safe to say this hardy moss is going to do just fine wherever you live!

Again, I’ve had no issues growing this moss myself. I like to maintain a comfortable home temperature throughout the day, but it’s had to deal with some cold winter nights in the UK. 

I’m sure those in North America would encounter much of the same. 

Growth

Polytrichum commune is an acrocarpous moss, meaning it grows in a clumping pattern. 

Each of the stems radiates individually from the base in an upright, non-branching pattern, but they pack together to form a loose unit.

The thing is, this moss can be so tall it can be challenging to find the right areas to plant it in.

Honestly, it’s the size of many small terrarium plants, so it kind of needs to be treated as such. The size certainly adds an interesting visual dynamic, though. 

Propagation

Haircap Moss propagates via spores, just like any other typical moss. 

The spores are held in their characteristic “hairy caps” at the end of the long red sporocytes. It’ll all happen naturally, but there’s not much you can do on that front to actively propagate more moss for yourself.

Dividing Haircap Moss is the easier way, but it takes some elbow grease!

Unlike the fan-favorite Pincushion Moss, the fibrous underside is too tough to tear off with your bare hands (bear hands… maybe).

Some sharp scissors or secateurs will do the trick.

Unfortunately, the moss loses all structure along with the fibers, so it’ll fall apart unless you tightly hold it together.  

Varieties & Similar Plants

Several beautiful varieties of Polytrichum see some use in the terrarium trade. 

I’ve seen both Polytrichum juniperinum (Juniper Haircap Moss) and Polytrichum strictum (Bog Haircap Moss) available online. Though honestly, I can’t see any differences visually. 

Outside of the genus, the spiky nature of Polytrichum closely resembles that of Star Moss.

I tend to find that Star Mosses grow much more low and compact, and they’re a lot less common in the terrarium niche. But I’d like to experiment with them more.

Finally, If Haircap Moss is densely packed (and probably underwatered), the leaves can also fold to look a little like the windswept Mood Moss.

On second thought, based on the sheer height of this moss, it probably looks more like Mood Moss’ big brother, Greater Fork Moss.

References

1. https://www.tandfonline.com/doi/epdf/10.1080/0028825X.1971.10430234?needAccess=true
2. https://www.jstor.org/stable/43237618
3. https://www.jstor.org/stable/43237394