As traditionally circumscribed Melilotus is a genus of ca. 20 species. Most are native in Eurasia, some occur in North and East-Africa. The generic limits of Melilotus are not uncontested. Recent molecular studies demonstrated that it is in fact nested in Trigonella (Steele & al. 2010). However, morphologically, most species are very distinct and pending further studies Melilotus is here accepted in its traditional sense. In their recent monograph of French Leguminosae Coulot & Rabaute (2013) merged both genera.
The original distribution area for a lot of species is questionable: many are weeds of arable land or occur on disturbed sites and have spread far beyond their original range. In Belgium only one species is probably native, Melilotus altissimus Thuill. It has much increased in the past decades, at least in Flanders (Ronse 2006a), and part of its populations is probably non-native. Residence status of Melilotus officinalis is uncertain. It was surely present before 1800 (Verloove 2006) and might well be an archaeophyte rather than a neophyte.
Flower colour is an important feature but should be noted when fresh. In pressed specimens yellow corollas readily turn whitish. In the herbarium specimens of Melilotus albus and M. officinalis are not easily told apart! Corolla measurements are also preferably noted before desiccation. The degree of dentation of stipules does not warrant a high taxonomic weight (contrary to, for instance, Lawalrée 1961 and Sales & Hedge 2000). Even in Melilotus sulcatus, a species with – according to most authors – unambiguously deeply dentate to laciniate stipules, these can be nearly entire and potentially do not differ from those that are told to be entire or merely shallowly toothed (for instance M. indicus). Moreover, on a single specimen the degree of dentation of stipules often varies a lot. Stevenson (1969) and Sales & Hedge (2000) refer to the stipules on the upper half of the plant whereas according to Lawalrée (1961) the stipules of the middle cauline leaves should be taken into account. It readily becomes obvious that stipule dentation does not offer a reliable identification feature; it is therefore not taken into account in the present treatment.
1. Corolla white === 2
1. Corolla yellow === 3
2. Pedicels at least as long as the corolla, ca. 2-4(-5) mm long. Corolla 3-3,5 mm long. Inflorescence usually very lax === Melilotus wolgicus
2. Pedicels shorter than the corolla, ca. 1-1,5 mm long. Corolla 4-5 mm long. Inflorescence usually rather dense === M. albus
3. Ovary and young pod pubescent. Corolla golden-yellow (native) === M. altissimus
3. Ovary and young pod glabrous. Corolla pure to pale yellow === 4
4. Pod with distinct concentric veins === 5
4. Pod reticulate veined === 6
5. Concentric veins ca. 7-17 in number, closely and regularly spaced. Standard shorter than keel === M. sulcatus
5. Concentric veins ca. 4-7 in number, irregular and not crowded. Standard at least as long as keel === M. infestus
6. Small annual, usually 10-40 cm tall. Corolla ca. 2-3 mm. Pod 1,5-3 mm long. Pedicels 0,5-0,8 mm long. Seeds minutely tuberculate === M. indicus
6. Taller biennial, usually ca. (30-) 40-120 cm tall. Corolla ca. 4-7 mm. Pod 3-5 mm long. Pedicels 1-2 mm long. Seeds smooth === M. officinalis
Additional alien: Melilotus elegans Salzm. ex Seringe (Medit., grain alien?) and M. messanensis (L.) All. (syn.: M. siculus (Turra) B.D. Jackson) (Medit., wool alien).
Coulot P. & Rabaute P. (2013) Monographie des Leguminosae de France. Tome 3 Tribu des Trifolieae. Bull. Soc. Bot. Centre-Ouest N.S. 40: 760 p.
Di H, Duan Z, Luo K, Zhang D, Wu F, Zhang J, et al. (2015) Interspecific Phylogenic Relationships within Genus Melilotus Based on Nuclear and Chloroplast DNA. PLoS ONE 10(7): e0132596. doi:10.1371/journal.pone.0132596
Hansen A. (1968) Melilotus. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 2. Cambridge University Press, Cambridge: 148-150.
Jauzein P. (1995) Flore des champs cultivés. INRA, Paris: 898 p.
Kita F. (1965) Studies of the genus Melilotus (Sweet Clover) with special reference to inter-relationship among species from a cytological point of view. J. Fac. Agric. Hokkaido Univ. 54(2): 23-122.
Lawalrée A (1961) Papilionaceae. In: Robyns W. (ed.), Flore Générale de Belgique, vol. 4, fasc. 1. Jardin Botanique de l’Etat, Bruxelles: 9-134.
Ronse A. (2006a) Melilotus altissimus. 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: 584.
Sales F. & Hedge I.C. (1993) Melilotus Miller (Leguminosae): typification and nomenclature. Anales Jard. Bot. Madrid 51: 171-175.
Sales F. & Hedge I.C. (2000) Melilotus. In: Talavera S. & al. (eds.), Flora Iberica, vol. 7(II). Real Jardín Botánico, Madrid: 720-731.
Sano Y. & Kita F. (1976) Analysis of the genus Melilotus by the methods of numerical taxonomy. J. Fac. Agric. Hokkaido Univ. 9: 286-302.
Schulz O.E. (1901) Monographie der Gattung Melilotus. Bot. Jahrb. 29: 660-753.
Steele K.P., Ickert-Bond S.M., Zarre S. & Wojciechowski F. (2010) Phylogeny and character evolution in Medicago (Leguminosae): Evidence from analyses of plastid trnK/matK and nuclear GA3ox1 sequences. Am. J. Bot. 97(7): 1142-1155.
Stevenson G.A. (1969) An agronomic and taxonomic overview of the genus Melilotus Mill. Can. J. Plant Sc. 49: 1-20. [available on line at: http://pubs.aic.ca/doi/pdf/10.4141/cjps69-001]
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p.