Sinapis alba L. (Medit.) – A rather rare and ephemeral alien. Cultivated as a forage crop (mustard) since quite a long time in Belgium and frequently escaping or found as a relic on arable land.
Two more or less distinct taxa of debated taxonomic rank are usually separated:
1 Siliqua hispid. Leaves pinnately lobed === subsp. alba
Siliqua nearly glabrous. Leaves deeply pinnately lobed, nearly laciniate === subsp. dissecta
Subsp. dissecta (Lag.) Bonnier (syn.: S. dissecta Lag.) is a more weedy taxon and more often occurred in cultivated fields (predominantly in flax). It is much rarer than subsp. alba and was probably last seen in a flax field in Sart Moulin in 1951. According to Tison & de Foucault (2014) it may as well be extinct now and recent claims are referable to laciniate leaved forms of subsp. alba. The latter is still frequently seen, especially along motorways and in port areas (mainly from spilled seeds). It is also regularly seen on dumps or in urban areas, possibly as a birdseed alien.
Selected literature:
Schnedler W. (1977) Pflanzen, von denen in der mitteleuropaischen Literatur selten oder gar keine Abbildungen zu finden sind: Folge 4.2: Drei Senf-Arten: Sinapis alba L., Brassica juncea (L.) Czern. und Brassica nigra (L.) Koch. Gött. Flor. Rundbr. 11(4): 92-95.
Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.