Adiantum aleuticum

Adiantum aleuticum (Rupr.) C.A. Paris (syn.: A. pedatum L. var. aleuticum Rupr., A. pedatum subsp. aleuticum (Rupr.) Calder & Roy L. Taylor) (N Am.) – An exceptional escape from cultivation. A small colony was discovered on an old brick wall in Assebroek (Brugge).
This species is easily distinguished from the two other species of Adiantum currently found in the wild in Belgium: its fronds are pedately branched, not pinnate. However, this population belongs to a particular dwarf form with densely set, almost imbricate pinnules, var. subpumilum (W.H. Wagner) E.R. Alverson (Alverson 2010). In this form the structure of the frond is heavily obscured. This plant is popular in horticulture where it is usually called ‘Imbricatum’ (Page & Bennell 1986). It is much hardier than A. pedatum.
This species has long been interpreted as an infraspecific variant of Adiantum pedatum. However, the two taxa are reproductively isolated and differ in an array of morphologic characteristics and should therefore be considered separate species (Paris &Windham 1988).
Recent molecular studies have shown that the A. pedatum complex is monophyletic and sister to the eastern Asian A. edentulum (Lu & al. 2011). It was shown that the clade of A. aleuticum and A. viridimontanum is nested within A. pedatum, while the Asian A. myriosorum is sister to the A. pedatum-A. aleuticum clade. The A. pedatum complex.most likely arose in East Asia from where it migrated to North America.
Although widely cultivated, this species (incl. claims of A. pedatum) has rarely been recorded as an escape in Europe, for instance in the British Isles (Grenfell 1983, Clement & Foster 1994).

Selected references

Alverson E.R. (2010) A New Status for the Dwarf Maidenhair Fern of the Pacific Northwest Coast. American Fern Journal 100(4): 230-233. [available online at:]
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
Cody W.J. (1983) Adiantum pedatum ssp. calderi, a new subspecies in northeastern North America. Rhodora 85(841): 93-96. [available online at:]
Gawler S.C. (1983) Note on Adiantum pedatum L. ssp. calderi Cody. Rhodora 85(843): 389-390. [available online at:]
Grenfell A.L. (1983) Adventive ferns - 1. B.S.B.I. News 35: 12. [available online at:]
Hoshizaki B.J. (1970) The genus Adiantum in cultivation (Polypodiaceae). Baileya 17: 97-191.
Jones J. (1986) Adiantum pedatum: another variation. Pteridologist 1(3): 96-98.
Lu J-M., Li D-Z., Lutz S., Soejima A., Yi T. & Wen J. (2011) Biogeographic disjunction between eastern Asia and North America in the Adiantum pedatum complex (Pteridaceae). Am. J. Bot. 98(10): 1680-1693. [available online at:]
Nakato N. & Kato M. (2005) Cytogeography of the Adiantum pedatum complex (Pteridaceae, subfamily Adiantoideae). Acta Phytotax. Geobot. 56: 85-96.
Olsen S. (1980) Adiantum pedatum var. subpumilum. Hort. Northwest 7(2): 29-30.
Page C.N. & Bennell F.M. (1986) Adiantum. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 20-23. Cambridge University Press, Cambridge.
Paris C.A. (1991) Adiantum viridimontanum, a new maidenhair fern in eastern North America. Rhodora 93: 105-122. [available online at:]
Paris C.A. (1993) Adiantum. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 2. Oxford University Press, New York-Oxford: 125-130. [available online at:]
Paris C.A. & Windham M.D. (1988) A biosystematic investigation of the Adiantum pedatum complex in eastern North America. Syst. Bot. 13: 240-255.
Rabe E.W. & Haufler C.H. (1992) Incipient polyploid speciation in the maidenhair fern (Adiantum pedatum; Adiantaceae). Amer. J. Bot. 79(6): 701-707.
Seibert Z. (1988) Adiantum pedatum L. var. subpumilum W.H.Wagner var. nova. Skalnicky 19(3): 82-84.
Wagner W.H., jr. & Boydston K.E. (1978) A dwarf coastal variety of maidenhair fern, Adiantum pedatum. Canad. J. Bot. 56(15): 1726-1729.
Zachary Theis E.W. & Hoess C. (2016) Identifying a Cryptic Adiantum Population through DNA Barcoding. American Fern Journal 106(2): 135-142.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith