Cotoneaster x watereri Exell, Gard. Chron., Ser. III.83: 44, 1928.
Section Densiflori, series Salicifolii x Section Frigidi, series Frigidi
Origin: an artificial hybrid, presumably of Cotoneaster frigidus Wallich ex Lindl. and C. salicifolius Franch. (Fryer & Hylmö 2009). According to Roloff & Bärtels (2006) other species are probably also involved: initially Cotoneaster frigidus, C. henryanus (C.K. Schneider) Rehder & E.H. Wilson and C. salicifolius, later also C. sargentii G. Klotz and C. floccosus (Rehder & E.H. Wilson) Flinck & B. Hylmö.
Presence in western Europe: Naturalized in Great Britain (Stace 2010).
Cultivation in Belgium and the Netherlands: “rare” (De Koning & van den Broek 2009).
Comparative taxonomy: Species included in Cotoneaster frigidus by Dickoré & Kasperek (2010). According to these authors most (all?) Central European records of Cotoneaster xwatereri probably belong to either C. frigidus or C. salicifolius, its putative parents. This hybrid is unknown in the wild and hence omitted from the Flora of China (Lingdi & Brach 2003).
Illustrations: Roloff & Bärtels (2006), De Koning & van den Broek (2009), Stace (2010). See also: http://www.british-wild-flowers.co.uk/C-Flowers/Cotoneaster,%20Waterer%27s.htm.
Cotoneaster xwatereri was probably first recorded in the wild in Belgium on a brick quay wall of river Leie in the city center of Kortrijk in 2000 (Verloove 2001). Subsequently it was recorded on several different occasions and in widely scattered localities but usually in small number (often single individuals) and probably nowhere genuinely naturalized. Recent records are available from Gent (Groene Vallei, since 2001), Kontich (Molenbos), Menen (old city walls, since 2006), Mons, Soignies, etc.
In Belgium, Cotoneaster xwatereri always seems to be confined to half-shady to shady habitats. It has been recorded on old city walls (quays, ruins), spontaneous woodland in urban areas, along railway tracks and in open (natural) woodland.
Cotoneaster xwatereri is an exceedingly variable species and rather poorly delimited. It is readily distinguished from most other garden escapes in Belgium by its robust, erect growth (more than 4 m tall and often reaching 8 m at maturity), its large, (semi-) evergreen leaves and multi-flowered inflorescences with white spreading petals. However, some of its cultivars are hardly told apart from Cotoneaster salicifolius (and related micro-species). As a rule, Cotoneaster xwatereri has longer and wider leaves (up to 12 cm long and 3 cm wide) that longer remain more or less hairy beneath (often readily glabrescent in C. salicifolius) and larger berries (7-9 mm vs. 5-6 mm). Cotoneaster xwatereri always has distinctly reddish leaf veins in winter. The existence of Cotoneaster henryanus – a species that “comes close to some variants of C. xwatereri that are closer to C. salicifolius than to C. frigidus” (Stace 2010) – further blurs the species boundaries within this complex. Dickoré & Kasperek (2010) include all European records of Cotoneaster xwatereri in C. frigidus, its other putative parent. All Belgian collections differ from the latter in having moderately to (more rarely) strongly bullate leaves (i.e. with veins deeply impressed on upper leaf surface) and in being (semi-) evergreen. Genuine Cotoneaster frigidus (a usually deciduous species with flat and dull upper leaf surfaces) has not been reliably recorded in Belgium so far.
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C. xwatereri |
C. salicifolius |
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Leaves up to 12 cm long and 3 cm wide, moderately bullate (rarely not bullate). Leaf veins distinctly reddish in winter. |
Leaves usually smaller, up to 9 cm long and 2,5 cm wide (often much less), strongly bullate. Leaf veins not distinctly reddish in winter. |
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(Semi-) evergreen |
Evergreen |
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Berry usually ca. 7-9 mm |
Berry usually ca. 5-6 mm |
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Literature
De Koning J. & van Den Broek (2009) Nederlandse Dendrologie (14th ed.). K.N.N.V.: 547 p.
Dickoré W.B. & Kasperek G. (2010) Species of Cotoneaster (Rosaceae, Maloideae) indigenous to, naturalising or commonly cultivated in Central Europe. Willdenowia 40: 13-45 [available online at: http://user.uni-frankfurt.de/~kasperek/papers/dickore_kasperek_2010.pdf].
Fryer J. & Hylmö B. (2009) Cotoneasters. A comprehensive guide to shrubs for flowers, fruit, and foliage. Timber Press, Portland-London: 344 p.
Lingdi L. & Brach A.R. (2003) Cotoneaster. In: Wu Z.Y. & Raven P.H. (eds.), Flora of China, vol. 9. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis: 85-108 [available online at: http://flora.huh.harvard.edu/china/PDF/PDF09/Cotoneaster.PDF].
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Stace C. (2010) New Flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Verloove F. (2001) Conyza bilbaoana J. Rémy, Cotoneaster x watereri Exell en Erigeron karvinskianus DC., nieuw voor de Belgische flora in Kortrijk. Dumortiera 78: 24-27.