The circumscription of the Orobanchaceae family dramatically changed lately as a result of molecular phylogenetic studies (e.g. Schneeweiss & al. 2004). In its current circumscription it also includes semi-parasites that were formerly accommodated in the Scrophulariaceae family.
Eight genera are native to Belgium: Euphrasia, Lathraea, Melampyrum, Odontites, Orobanche, Pedicularis, Phelipanche and Rhinanthus (Lambinon & Verloove 2012). Representatives of some of these, however, have an ambiguous residence status. Lathraea clandestina L. is known since quite a long time from Belgium but its area is very disjunct. Moreover, it is often found near castles and estates and is spreading in recent times. It may be an old introduction rather than a genuine native species (see also Atkinson 1996). Species of Rhinanthus are increasingly sown these days, for instance near retention basins and for other revegetation purposes. Non-native variants may be involved and their identity should be critically (re-) assessed. Alien look-alikes may also be involved, e.g. R. rumelicus Velen., a species found in wild flower seed mixtures in the British Isles (Clement 2001).
1 Total parasite without chlorophyll, wholly yellow, brown, red or purplish (Orobanchaceae s.str.) === 2
Partial or non-parasite with obvious green coloring === 4
2 At least middle flowers distinctly pedicellate. Calyx with dents shorter than tube, all dents +/- equal (native) === Lathraea
Most or all flowers sessile. Calyx deeply divided to base or almost so, with unequal dents === 3
3 Each flower subtended by a bract and two lateral bracteoles (these adnate to calyx and reminiscent of the calyx teeth). Calyx deeply divided but not to base. Pollen tricolpate (three grooved) === Phelipanche
Each flower subtended by a bract, without bracteoles. Calyx divided to base. Pollen inaperturate === Orobanche
4 Leaves all or mostly alternate, deeply divided. Calyx usually 5-lobed, the lobes toothed (native) === Pedicularis
Leaves all or mostly opposite, entire or at most toothed. Calyx 4-lobed with entire lobes === 5
5 Calyx tube inflated, especially at maturity (native) === Rhinanthus
Calyx tube not inflated === 6
6 Plant glandular hairy and sticky. Seeds minute, ca. 0.5 mm across, smooth or very finely reticulate === Parentucellia
Plant non-glandular (rarely a few glands in the inflorescence present). Seeds larger, at least 1 mm across, ridged or striate === 7
7 Corolla lips distinctly lobed: the lower with three distinctly emarginated lobes, the upper with two diverging lobes. Corolla small, rarely exceeding 10 mm in length (except sometimes in E. officinalis), ground color usually white (native) === Euphrasia
Corolla lips less deeply lobed or entire. Corolla often longer or, if less than 10 mm long, never with ground color white === 8
8 Mouth of corolla partially closed by boss-like swelling. Capsule with 1-4 seeds === Melampyrum
Mouth of corolla not partially closed. Capsule with more numerous seeds === 9
APG IV (2016) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Bot. J. Linn. Soc. [available online at: http://onlinelibrary.wiley.com/doi/10.1111/boj.12385/full]
Atkinson M.D. (1996) The distribution and naturalisation of Lathraea clandestina L. (Orobanchaceae) in the British Isles. Watsonia 21(1): 119-128. [available online at: http://archive.bsbi.org.uk/Wats21p119.pdf]
Clement E.J. (2001) Rhinanthus rumelicus Velen. new to Britain (v.c. 11). BSBI News 86: 51-53. [available online at: http://archive.bsbi.org.uk/BSBINews86.pdf]
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (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.
Scheunert A., Fleischmann A., Olano-Marín C., Bräuchler C. & Heubl G. (2012) Phylogeny of tribe Rhinantheae (Orobanchaceae) with a focus on biogeography, cytology and re-examination of generic concepts. Taxon 61(6): 1269-1285. [available online at: https://www.researchgate.net/publication/281691099_Phylogeny_of_tribe_Rh...
Schneeweiss G.M., Colwell A., Park J.M., Jang C.G. & Stuessy T.F. (2004) Phylogeny of holoparasitic Orobanche (Orobanchaceae) inferred from nuclear ITS sequences. Mol. Phylogenet. Evol. 30(2): 465-478. [available online at: https://www.researchgate.net/publication/8926181_Phylogeny_of_holoparasi...