polytrichum is a beautiful genus of mosses. Although..looking at one, it is very very very hard to recognize them directly. Most mosses cannot be recognized without looking at its cell structure and most importantly its spore capsule. Either way...I do have polytrichum from different sources and one of the most common ones is polytrichum commune. The moss grows differently depending on the amount of light it gets. Most mosses in general grow in bright shade and I would assume polytrichum likes the same. It generally acts as a scaffolding moss for sphagnum when peat bogs are restored. On a layer of fresh peat...generally when spores and chopped up sphagnum from virgin bogs are scattered, polytrichum is the first to come up. Sphagnum uses this now support moss and the extra humidity to slowly gain control. The polytrichum does tend to hang on in few places, but most times sphagnum just overruns the polytrichum profusely.
Polytrichum so in general is just like sphagnum..it doesn't harm your plants and can be grown for the top dressing. However, it is notorious for suffocating seedlings and smaller plants. It should be avoided for plants like pygmy sundews and short rosetted dews in general. However, for plants like neps, it is perhaps among the most beautiful ground cover that you can encourage. THe moss is also different in the sense that its a very stringy moss...it doesn't compact like sphagnum and has a lot of air packets in it. This makes it ideal for nep media mixes as well. ALthough, from experience, I have found that it tends to be extra open forcing you to water the plant a lot more regularly than other mixes.