Several recent molecular phylogenetic studies have considerably changed the relationships within the Lilioid monocots. Many genera formerly included in Liliaceae are now accommodated in various other families (see APG II 2003, APG III 2009 for an overview). The Belgian representatives of Liliaceae s.l. as per Lambinon & Verloove (2012), native as well as non-native, are now segregated as follows:
Asparagaceae (incl. Agavaceae, Hyacinthaceae, Ruscaceae): Agave, Anthericum (only native taxa), Asparagus (only native taxa), Camassia, Chionodoxa, Chlorophytum, Convallaria (only native taxa), Danae, Honorius, Hosta, Hyacinthoides, Hyacinthus, Loncomelos, Maianthemum, Muscari, Ornithogalum, Polygonatum, Ruscus, Scilla, Yucca;
Colchicaceae: Colchicum (only native taxa; see however Londo & Londo-Eeken 2006);
Liliaceae s.str.: Erythronium, Fritillaria (only native taxa), Gagea, Lilium, Tulipa;
Melanthiaceae: Paris (only native taxa), Veratrum;
Nartheciaceae: Narthecium (only native taxa);
Xanthorrhoeaceae (incl. Asphodelaceae, Hemerocallidaceae): Asphodelus, Hemerocallis, Kniphofia.
Several additional taxa of Liliaceae are in cultivation and might occur as garden escapes or -throw-outs, or as relics of cultivation. An extensive account of Liliaceae (sensu lato) in cultivation in Europe is provided by Walters & al. (1986). See also Jäger & al. (2008). Stace (2010) gives a key for the identification of the subspontaneous species of Liliaceae in the British Isles.
Besides taxa from other genera, at least one species from the native genus Fritillaria L. should be mentioned, F. imperialis L., a native of temperate Asia. It is a common garden plant with leaves in several whorls and broadly bell-shaped, red to yellowish flowers in umbels. It has been claimed once as a garden escape but this requires confirmation.
1 Flowers solitary and pendent. Tepals purplish-pink (rarely white), conspicuously reflexed at base. Leaves purple-spotted === Erythronium
Plants never combining the above features === 2
2 Flowers usually solitary (exceptionally 2-3; in the latter case, tepals never yellowish-greenish and small) === 3
Flowers usually 2-several in an umbel, raceme or panicle (if flowers solitary, than tepals always yellowish-greenish and small) === 4
3 Flowers pendent. Tepals chequered light and dark purple and cream (rarely entirely white). Leaves 4-9 mm wide. Stigmas free. Capsule subglobose (native) === Fritillaria
Flowers erect. Tepals variously colored, most often yellow or red. Leaves usually wider, up to 80 mm wide. Stigmas connate with the ovary. Capsule oblong === Tulipa
4 Inflorescence (sub-)umbellate, subtended by one or more spathes. Tepals yellowish-greenish, rarely exceeding 15 mm. Plants small, up to 25 cm tall === Gagea
Inflorescence a raceme, not subtended by spathes. Tepals orange or purplish, spotted, at least 25 mm long. Plants more robust, 30-130 cm tall === Lilium
Given the importance of lilies and related genera for the horticultural trade there is a vast literature on the family. Useful additional information is available on Wikipedia at: https://en.wikipedia.org/wiki/Liliaceae.
APG II (2003) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants. APG II. Bot. J. Linn. Soc. 141: 399-436. [available online at: http://www.biodiversitas.org.br/florabr/apg2.pdf]
APG III (2009) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants. APG III. Bot. J. Linn. Soc. 161(2): 105-121. [available online at: https://academic.oup.com/botlinnean/article/161/2/105/2418337]
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Lambinon J. & Verloove F. (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Londo G. & Londo-Eeken I. (2006) Tijlozen in Nederland, wild en verwilderd: Colchicum autumnale L., C. tenorei Hort. en C. byzantinum Ker-Gawl. Gorteria 32: 149-159. [available online at: http://natuurtijdschriften.nl/download?type=document&docid=537437]
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Walters S.M., Brady A., Brickell C.D., Cullen J., Green P.S., Lewis J., Matthews V.A., Webb D.A., Yeo P.F. & Alexander J.C.M. (eds.) (1986) Liliaceae. In: The European Garden Flora, vol. 1. Cambridge University Press: 117-271.