Few bryologists relish the prospect of keying out a Plagiomnium, as they can be very confusing: leaf bases decurrent or hardly decurrent, cell walls porose or not, synoicous or dioicous, shoots arcuate or ‘sometimes erect’…even friendly old P. undulatum can do the dirty on you when young, the leaves not being the usual elongated shape and not very undulate.
However, relax in the presence of Plagiomnium cuspidatum, which is pretty straightforward to identify. The leaf margin goes up from a decurrent base in an almost straight line to about half-way, completely untoothed. Then, it curves round to the sharply pointed tip, the margin now possessing strong, obvious teeth. None of the other Plagiomnium species in Britain has leaves like this.
Despite the fact that it can be found in a very wide range of base-rich habitat, occurring on soil, rock and tree bases, herb-rich turf on calcareous soils and even on mossy walls, it is not particularly common, especially in the south and east. It is always a pleasure to find.Read the Field Guide account