Equisetum hyemale var. affine
Equisetum hyemale L. subsp. affine (Engelm.) Calder et R.L. Taylor [syn.: E. hyemale L. var. affine (Engelm.) Eaton E. robustum A. Braun var. affine Engelm., E. praealtum Raf.] (N and C-Am.) – A rare but much increasing garden escape, relic of cultivation or throw-out, known since 2007 when it was observed in Oostende. Recorded in abundance (nearly monospecific) on damp wasteland in Zwijndrecht in 2011. Since then seen in several different, widely scattered localities (see here for an up-to-date overview of Belgian records: http://waarnemingen.be/soort/view/21033). Much cultivated in recent times and possibly overlooked elsewhere. Populations of Equisetum hyemale from aberrant habitats are likely to be this. However, subsp. affine has also been observed in more natural, often remote habitats where human influence is less likely. It seems to spread also by spores, not only clonally.
This taxon has been given specific rank but is often hardly distinguished from Equisetum hyemale s.str. It is in fact the New World counterpart of Old World subsp. hyemale. Plants are often coarser and taller (sometimes up to 250 cm tall with stems up to 15 mm diameter) and, of course, often occur in Belgium in ordinary man-made habitats (waste lands, derelict gardens). It is however sometimes planted in nature-like habitats (pond margins). According to Hauke (1963) this taxon “(…) exceeds var. hyemale in stem size and number of ridges.”
Based on molecular results (Christenhusz & al. 2019), the North American accessions of E. hyemale form a clade that appears to be derived from within the Old World samples. Because of their geographical disjunction, the taxa are perhaps better maintained at the species level. The two species differ morphologically in height, stem colour and width and numbers of ridges on the sheath teeth. The correct name at species rank for the American species is E. praealtum Raf.
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Equisetum hyemale var. affine |
Equisetum hyemale |
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Selected literature
Christenhusz M.J.M., Bangiolo L., Chase M.W., Fay M.F., Husby C., Witkus M. & Viruel J. (2019) Phylogenetics, classification and typification of extant horsetails (Equisetum, Equisetaceae. Botanical Journal of the Linnean Society 189: 311–352.
Hauke R.L. (1963) A taxonomic monograph of Equisetum subgenus Hippochaete. Beih. Nova Hedwigia 8: 1-123.
Saintenoy-Simon J. (2005) Groupe Flore Bruxelloise. Rapport des excursions de l’année 2004. Adoxa 46-47: 35-68.
Van der Meijden R., Holverda W.J. & Duistermaat L.H. (1996) Nieuwe vondsten van zeldzame planten in 1993, 1994 en (ten dele) 1995. Gorteria 22: 57-81.