Potentilla rivalis Nutt. ex Torr. & A. Gray (N-Am.) – An exceptional alien, known from a single locality in Aalter (Kraenepoel), where it was discovered in fairly large amounts on the exposed banks of a fishpond in 2001 (Hoste 2006 a, b). The species was possibly introduced long before (claims of ‘Potentilla norvegica’ from the very same locality in 1969 were most likely also ascribable to P. rivalis; Hoste 2006 a, b). The number of plants visible varies from none (when deeply submerged) to thousands, depending upon the height of the water level. In this unique locality Potentilla rivalis seems firmly established but an expansion to nearby suitable habitats has not been observed (yet).
Potentilla rivalis is much reminiscent of P. norvegica and possibly overlooked. It has petals that are much shorter than the sepals (ca. ½ as long), smooth and smaller achenes and often less stamens (usually less than 10).
In Europe it has been recorded from a similar locality (exposed bank of a pond) in the British Isles (Clement 1980). In both the Belgian and British locality it was suggested to have been introduced as a contaminant in maize and pheasant food respectively.
Potentilla rivalis is a very polymorphic species and here accepted in a wide sense. Segregate species like P. millegrana Engelm. ex Lehm. and P. pentandra Engelm. seem to be poorly distinguished (compare with Clement 1980, Hoste 2006a) and are best considered conspecific (see also: http://www.tropicos.org/Home.aspx).
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Selected literature:
Clement E.J. (1980) Potentilla rivalis Nutt. ex Torrey & Gray new to Britain. Watsonia 13: 49. [available online at: http://archive.bsbi.org.uk/Wats13p49.pdf]
Hoste I. (2006a) Potentilla rivalis in Aalter (Oost-Vlaanderen), nieuw voor de Belgische flora: een cultuurhistorisch relict van de karperteelt? Dumortiera 88: 6-12.
Hoste I. (2006b) Potentilla rivalis. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 709-710.
Kurtto A., Lampinen R. & Junikka L. (eds.) (2004) Atlas Florae Europaeae. 13. Rosaceae (Spiraea to Fragaria, excl. Rubus). The Committee for Mapping the Flora of Europe & Societas Biologica Fennica Vanamo, Helsinki: 320 p.