I remember seeing this same Shield fern (Polystichum) over 15 years ago growing near the top of a hedge bank above a small stream. From my memory it doesn’t seem to have grown much or spread into its surroundings, so just how long do individuals like this survive? If it has been in the same spot for 15 years could it have been there for the previous 15 years, or the 15 years before that? And so on.
Anyway I wanted to identify it, I know its a Polystichum because of the circular disc shaped (peltate) indusium attached to the leaf at its centre (though a bit of imagination is required at this time of year), combined with bristle pointed leaf segments. What I need to know is whether its Hard Shield Fern Polystichum aculeatum or Soft Shield Fern Polystichum setiferum.
The main diagnostic features to differentiate these two species are
1) whether the frond feels soft (setiferum) or hard (aculeatum). Obviously this is very subjective but I would say this frond feels hard.
2) if the pinnules are stalkless (sessile) = aculeatum, or stalked = setiferum. I found this quite hard as the pinnules varied greatly. I found the silhouettes in the Plant Crib to be the most useful.
3) if the lowest pinna is half as long (aculeatum) or if it is more or less the same size (setiferum) as the largest pinna. In this example the difference in length is marked.
4) Whether the angle of the base of the lower pinnules of each pinna rachis are obtuse (setiferum) or acute (aculeatum) angled. The majority of these pinnules are acute.
So overall it seems to be Hard Shield Fern P aculeatum. I didn’t find it easy to identify as a lot of the diagnostic features are subjective and there is some variability e.g. are the pinnules stalked or not? Additionally there is a hybrid between the 2 shield ferns; P bicknellii. For me the most straightforward guide to identification of these 2 and the hybrid is the Plant Crib which is available on the BSBI website.
With these similar plants of the same family I think it would be useful to have specimens or good quality photographs to compare.