In August 2011 I came across a dense population of a species of Lathyrus that I know since at least 25 years. It grows by a disused railway track and in adjacent grassland near Kortemark (prov. West-Flanders, Belgium; IFBL D1.26.41). It has always been referred to as Lathyrus latifolius L. and is mentioned as such on waarnemingen.be as well (see for instance http://waarnemingen.be/waarneming/view/55774177). Indeed, these plants are much reminiscent of the latter but also show some resemblance with native Lathyrus sylvestris L. A more profound study now has demonstrated that they belong to a related, non-native species, Lathyrus heterophyllus L. This short note aims at drawing the attention of Belgian botanists to this potentially overlooked species of Lathyrus. Distinguishing features are briefly discussed and photographs are presented. A more detailed account will be published in a future issue of Dumortiera.
Lathyrus heterophyllus is obviously most closely related to L. latifolius and L. sylvestris and would key out to these species in the flora of Belgium (Lambinon & al. 2004; couplet 12). In fact, it combines features of both: its corollas are relatively small (at most 20 mm) and share the typical pinkish-greenish colour of Lathyrus sylvestris (usually fuchsia in L. latifolius), while vegetative characters are like in L. latifolius: leaves up to 40 mm wide and stipules at least ½ as wide as the stem. Moreover, the calyx teeth are obviously unequal with the longest at least as long as or slightly longer than the calyx tube (all teeth shorter than the tube and subequal in Lathyrus sylvestris). The peduncle is much longer than the subtending leaf, like in Lathyrus latifolius.
In typical Lathyrus heterophyllus (var. heterophyllus) at least the upper leaves usually have 2-3 pairs of leaflets while L. latifolius and L. sylvestris always have a single pair. However, in northwestern Europe (the British Isles, see Stace 2010; but also in Scandinavia as seen in BR!) a variety with only one pair of leaflets seems to be the usual plant. It is distinguished as var. unijugus W.D.J. Koch. The population from Kortemark also seems to pertain to that taxon.
Confusion is furthermore likely with a peculiar form of Lathyrus sylvestris that probably has not been recorded yet in Belgium: subsp. platyphyllos (Retz.) Vollm. This taxon has leaves up to 40 mm wide but otherwise corresponds with typical Lathyrus sylvestris (stipules, calyx teeth, etc.) (see Jäger & Werner 2005).
In Kortemark Lathyrus heterophyllus has been known for at least 25 years and is abundantly naturalised. It grows on the sidings of the former railway track towards Ieper (now a cycling track), close to the (now demolished) Bonduelle-plant and extends to adjacent grassland. It now occupies at least 1,5 ha and locally occurs in nearly monospecific stands. Its vector of introduction is uncertain. According to some authorities it is a garden escape outside its native distribution range (see for instance Clement & Foster 1994) but this seems rather unlikely: it is not very garden-worthy and, moreover, it is not mentioned in European garden floras (Hibberd 1995; Jäger & al. 2008), although it might have been confused with Lathyrus latifolius and/or L. sylvestris. Likewise, it is rather unlikely that is should be associated with the industrial activities of the former Bonduelle factory (vegetable processing). Lathyrus heterophyllus might have been introduced (unintentionally) by railway traffic or perhaps even on the occasion of the construction of the railway track, more than a century ago.
Lathyrus heterophyllus has a very disjunct distribution area in large parts of Europe. According to Ball (1968) it naturally occurs in Austria, the Czech Republic, France, Germany, Switserland, Spain, Italy (incl. Sardegna), Portugal, Poland and Sweden. Some of these claims are surely erroneous (see for instance those from the Iberian Peninsula; Gallego 1999) and others possibly refer to non-native populations. It is uncertain whether this highly disjunct pattern is genuine or rather a result of a poor knowledge on the species. Belgian botanists are encouraged to watch out for this indistinct species: it combines distinguishing features of two rather frequent, closely resembling species and might easily pass unrecorded…!
Finally, two other look-alikes of Lathyrus latifolius should be looked for. Lathyrus rotundifolius Willd. is distinguished by leaves that are hardly longer than wide and L. undulatus Boiss. has distinctly wavy leaf margins. Both are cultivated as ornamentals in Europe (Hibberd 1995, Jäger & al. 2008) and may occur as escapes in Belgium.
Ball P.W. (1968) Lathyrus. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 2. Cambridge University Press, Cambridge: 136-143.
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
Gallego M.J. (1999) Lathyrus. In: Talavera S. & al. (eds.), Flora Iberica, vol. 7(I). Real Jardín Botánico, Madrid: 423-489.
Hibberd F.K. (1995) Lathyrus. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 518-521.
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.
Jäger E.J. & Werner K. (eds.) (2005) Rothmaler Band 4. Exkursionsflora von Deutschland. Gefässpflanzen: Kritischer Band. Springer Verlag, Berlin: 880 p.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Filip Verloove, September 2011