Eleocharis obtusa (Willd.) Schult. (N-Am.) – A very rare but locally well-naturalized and apparently increasing alien. First seen in 2006 in a military base in Elsenborn (initially confused with native Eleocharis ovata), along with E. austriaca (Lambinon & Mause 2010). It grows along muddy tracks, depressions, ditches, usually disturbed by tank manoeuvres. It accompanies rare native species like Juncus tenageia L. f. Recently confirmed and reported from several other locations in and near the camp site. In 2010 furthermore discovered in nature reserve Klein Schietveld in Brasschaat (former military training area for tanks): few specimens in temporarily wet, sandy tracks. In this area additional populations were discovered subsequently and in 2014 it was also recorded in a nearby reserve (pond margin, Den Uitlegger). In 2015, E. obtusa was recorded for the first time in a third area in Belgium: several plants were found growing in a dried-out shallow pond in Loppem near Brugge.
Aberrant populations of native Eleocharis ovata should be carefully checked, especially those in more or less disturbed places (military bases,…). However, populations from a military camp in Marche-en-Famenne definitely belong with Eleocharis ovata. As a rule and in addition to the characters mentioned in the key, Eleocharis obtusa is a much coarser species with thicker and longer stems and inflorescences. Moreover its stems are usually erect while they are more or less decumbent in Eleocharis ovata. Elsewhere in Europe, Eleocharis obtusa is well naturalized in Italy (initially in rice fields but by now predominantly in more natural habitats). A very closely related North American species, Eleocharis engelmannii Steud., is known from a military base in Germany (Plieninger 2001).
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Herbarium specimen: Eleocharis obtusa
Selected literature:
Lambinon J. & Mause R. (2010) Deux Eleocharis (Cyperaceae) nouveaux pour la flore belge: E. austriaca et E. obtusa au Camp d’Elsenborn (Haute-Ardenne). Dumortiera 98: 1-5.
Larson B.M.H. & Catling P.M. (1996) The separation of Eleocharis obtusa and Eleocharis ovata (Cyperaceae) in eastern Canada. Canad. J. Bot. 74: 238-242. [available online at: http://brendonlarson.com/wp-content/uploads/2013/04/Larson-and-Catling-1996-Eleocharis.pdf
Plieninger W. (2001) Ein Fund von Eleocharis engelmannii Steudel bei Heilbronn. Ber. Bot. Arbeitsgem. Südwestdeutschland 1: 47-52. [available online at: http://www.botanik-sw.de/BAS/media/texte/Bericht_1.pdf]
Verloove F. (2015) Eleocharis engelmannii and E. obtusa (Cyperaceae), two recent acquisitions from sect. Ovatae in Belgium. Dumortiera 107: 25-30. [available online at: http://www.br.fgov.be/DUMORTIERA/DUM_107/Dum_107_25-30_Eleocharis%20Ovat... ]