Polystichum is a large genus with up to 500 or more species, distributed throughout the world. The greatest species diversity occurs in temperate Asia with more than 200 species in China (139 of which endemic) (Zhang & Barrington 2013). Three species are native in Belgium as well: Polystichum aculeatum (L.) Roth, P. lonchitis (L.) Roth and P. setiferum (Forssk.) Woynar. All are by far commonest in Wallonia but in recent times Polystichum aculeatum and P. setiferum are increasingly found in Flanders as well, often in man-made, urban habitats (brick quays, basement walls, in sewers,…) but also in young woodland. The origin of these plants is uncertain. However, they most probably originate from nearby gardens and thus may represent non-native genetic material or even related, non-native look-alikes. Additional research is required.
A useful account for species of Polystichum currently found in European gardens is provided by Jäger & al. (2008). See also: Page & Bennell (1986). A few others may also be encountered, in gardens as well as in the wild, as escapes. P. retrosopaleaceum (Kodama) Tagawa (syn.: P. aculeatum var. retrosopaleaceum Kodama) from China, Japan and Korea resembles native P. aculeatum but rachis scales are ovate-lanceolate and broadly lanceolate and pointing downward (Zhang & Barrington 2013). Further species to look out for are P. andersonii M. Hopkins from North America and P. rigens Tagawa from East Asia. The latter strongly resembles P. tsus-simense but is a taller, more vigorous plant. A very useful, illustrated overview of hardy species of Polystichum found in horticulture is available here: http://hardyfernlibrary.com/ferns/listSpecies.cfm?Genus=Polystichum.
The generic boundaries of Polystichum are controversial and its taxonomy unresolved in spite of substantial recent studies. Molecular data do not support the traditional circumscription of the genus (e.g. Lu & al. 2007). Also the placement of related genera like Cyrtomium is uncertain. The latter is sometimes included in Polystichum (e.g. Mabberley 2008) but a core group is perhaps best maintained in a separate genus (see also Zhang & Barrington 2013). It is possible to delineate a monophyletic genus Polystichum as long as Cyrtomium is excluded (Barrington 2012).
Useful information on the genus is stored at ‘The Polystichum Homepage’ at: http://www.uvm.edu/~dbarring/polystichum.htm.
1. Fronds 1-pinnate, narrowly oblong (native) === Polystichum lonchitis
1. Fronds 2(-3)-pinnate, ovate-broadly lanceolate === 2
2. Lowermost pinnae very short, less than 40% the length of the median ones. Stipe very short, often almost absent. Leaves always thick (leathery) and shiny (native) === Polystichum aculeatum
2. Lowermost pinnae longer, at least 60% the length of the median ones. Stipe always distinct. Leaves thick or thin, dull or shiny === 3
3. Stipe long, usually ca. ½ the length of the blade. Blade +/- triangular, with lowermost pinnae slightly longer than median ones, apex of blade often more or less long acuminate. Rachis scales often few, confined to stipe base and pinnae axels, blackish and linear === P. tsus-simense
3. Stipe shorter, usually less than 1/5 of the length of the blade. Blade ovate, lowermost pinnae shorter than median ones. Rachis scaly throughout, scales paler and wider === 4
4. Pinnules adnate to the rachis, not stalked. Fronds shiny and leathery, evergreen. The lowermost pinnae very obliquely inserted === P. polyblepharum
4. Pinnules distinctly stalked. Fronds dull, herbaceous, semi-evergreen. The lowermost pinnae inserted at +/- right angles (native) === P. setiferum
Barrington D.S. (2012) The fern genus Polystichum (Dryopteridaceae) in Costa Rica. Ann. Missouri Bot. Gard. 98(4): 431-446. [available online at: http://www.uvm.edu/~dbarring/publications/barrington2011-1.pdf]
Daigobo S. (1972) Taxonomical studies on the fern genus Polystichum in Japan, Ryukyu, and Taiwan. Sci. Rep. Tokyo Kyoiku Daigaku, B 15: 57-80.
Driscoll H.E. & Barrington D.S. (2007) Origin of Hawaiian Polystichum (Dryopteridaceae) in the context of a world phylogeny. Amer. J. Bot. 94: 1413-1424. [available online at: http://www.uvm.edu/~dbarring/publications/driscoll2007.pdf]
Dyce J.W. (2005) Polystichum Cultivars - Variation in the British Shield Ferns. British Pteridological Society, Special Publication No. 7: 100 pp.
Fraser-Jenkins C.R. (1997) Himalayan Ferns (A Guide to Polystichum). International Booksellers, Dehra Dun, India.
Guillot Ortiz D., Mateo Sanz G. & Rosselló Picornell A. (2006) Claves para la pteridoflora ornamental de la Comunidad Valenciana. Bouteloua 1: 25-33.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Li C.X., Lu S.G. & Barrington D.S. (2008) Phylogeny of Chinese Polystichum (Dryopteridaceae) based on chloroplast DNA sequence data (trnL-F and rps4-trnS). Journal of Plant Research 121: 19-26.
Little D.P. & Barrington D.S. (2003) Major evolutionary events in the origin and diversification of the fern genus Polystichum (Dryopteridaceae). Amer. J. Bot. 90: 508-514. [available online at: https://www.researchgate.net/profile/David_Barrington2/publication/51206...
Lu J-M., Barrington D.S. & Li D-Z. (2007) Molecular Phylogeny of the Polystichoid Ferns in Asia Based on rbcL Sequences. Syst. Bot. 32(1): 26-33. [available online at: https://www.uvm.edu/~plantbio/barringtonpapers/lu2007.pdf]
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Page C.N. & Bennell F.M. (1986) Polystichum. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 52-54. Cambridge University Press, Cambridge.
Wagner D. (1979) Systematics of Polystichum in western North America and north of Mexico. Pterodologia 1: 1-64.
Zhang L-B & Barrington D.S. (2013) Polystichum. In: Wu Z-Y, Raven P.H. & Hong D-Y (eds.), Flora of China. vol. 2–3. Beijing: Science Press and St. Louis: Missouri Botanical Garden Press: 629-713. [available online at: http://flora.huh.harvard.edu/china/mss/volume02/Flora_of_China_Volume_2_...