Trifolium nigrescens Viv. (Medit.) – A very rare and probably ephemeral alien. A small population was discovered in a dry, sun-exposed and regularly disturbed lawn in a camping in De Panne in 2016 and subsequently also in a few other camping sites in coastal areas (Koksijde). In 2017 also observed in an inland camping site (Berlare). Previous claims were probably all erroneous and referable to the closely similar T. michelianum (see also there).
Trifolium nigrescens superficially looks like native T. repens (both have white-flowered heads with distinctly pedicellate flowers). However, the former obviously is an annual while Trifolium repens is a creeping perennial that roots at nodes. T. nigrescens also has smaller flower heads.
The Belgian records of Trifolium nigrescens are ascribable to the widespread subsp. nigrescens.
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Selected literature:
Ertekin A.S. & Akbayin H. (2000) A new variety of Trifolium nigrescens Viv. (Fabaceae) from Turkey. Israel J. Pl. Sci. 48(1): 71-73.
Keith E.L. (2013) Trifolium nigrescens (Fabaceae), new to the Texas flora. Phytoneuron 2013-32: 1-6. [available online at: http://phytoneuron.net/2013Phytoneuron/32PhytoN-Trifoliumnigrescens.pdf]