Trifolium michelianum Savi (S-Eur., W-As.) – An exceptional and ephemeral alien. Recorded only once on a demolition site of a former horse artillery in Gent in 1994, obviously from a long-buried seed bank (Verloove & Heyneman 2012).
Trifolium michelianum was initially confused with T. nigrescens. Both are indeed superficially similar. However, the former has calyx teeth that are 2-4 times as long as the tube (in Trifolium nigrescens about as long as the tube). Trifolium hybridum subsp. elegans is also similar but perennial and also has much shorter calyx teeth.
Selected literature:
Calabuig E.L. & Gómez Gutiérrez J.M. (1976) Nueva cita de Trifolium michelianum Savi para la Península Ibérica. Lagascalia 6(1): 3-6. [available online at: http://bibdigital.rjb.csic.es/Imagenes/P0422_06/P0422_06_003.pdf]
Caldarella O., La Rosa A., Cusimano D., Romano S. & Gianguzzi L. (2013) Distribution, ecology and conservation survey on Trifolium michelianum Savi (Fabaceae) in Sicily (Italy). Plant Biosyst. DOI:10.1080/11263504.2013.790852.
Verloove F. & Heyneman G. (2012) Merkwaardige plantencollecties van twee antropogene zaadbanken in Gent (Oost-Vlaanderen, België). Dumortiera 100: 19-24. [available online at: http://www.br.fgov.be/DUMORTIERA/DUM_100/Dum%20100_19-24_Zaadbanken%20Gent-Verloove%20en%20Heyneman.pdf]