Petrorhagia (Ser.) Link is an Old World genus with ca. 33 species, ranging from the Canary Islands to Kashmir. One species, Petrorhagia prolifera (L.) P.W. Ball & Heywood, is native in Belgium although it often occurs in highly artificial habitats (by railway tracks, in gravel pits, on coalmine heaps, etc.) where it is most likely introduced. A second species, Petrorhagia saxifraga (L.) Link, has been recorded in Belgium as an ephemeral escape from cultivation. It is rather different in being a mat-forming perennial with flowers in lax cymes or solitary (vs. annual with flowers in compact heads, enclosed by brown scarious bracts). On these grounds the species related to Petrorhagia prolifera are sometimes accommodated in a separate genus, Kohlrauschia Kunth (Mabberley 2008). Future molecular phylogenetic studies should clarify the relationships between these genera but they are here maintained in their traditional sense.
Petrorhagia prolifera itself is a member of a taxonomically difficult species-complex (Ball & Heywood 1964, Thomas 1983, Thomas & Murray 1983, Rabeler 1985). In addition to Petrorhagia prolifera proper, at least two additional species have been recorded as adventives beyond their native distribution range: P. dubia (Rafin.) G. López & Romo (syn.: P. velutina (Guss.) P.W. Ball & Heywood) and P. nanteuilii (Burnat) P.W. Ball & Heywood (see for instance Rabeler 1985, Clement 2005, Rabeler & Hartman 2005). The latter was recently also reported for the first time in Belgium (Verloove 2006), based on a single, probably ephemeral record in the port of Antwerpen in 1992 (roadside of Zomerweg between Churchill- and Zesde Havendok).
In general appearance these three species look exceedingly similar and they occur in identical habitats (dry, open, sun-exposed places, often on gravely or sandy substrates). All have weedy tendencies. A recent thorough herbarium revision yielded no additional collections but mature seeds are required for an accurate identification (and these were often lacking). Botanists are encouraged to look out for Petrorhagia prolifera and its look-alikes, especially in anthropogenic habitats (port areas, railway stations, coalmining areas, gravel and sand pits, etc.). It may well be that Petrorhagia nanteuilii (and possibly also P. dubia) are overlooked in Belgium. These three species are best told apart as follows:
1 Mature seeds granulose-reticulate, without tubercles or papillae. Leaf sheath ca. as long as wide. Midstem internodes either glabrous or scabrous (native) === Petrorhagia prolifera
1 Mature seeds distinctly tuberculate or papillate. Leaf sheath longer than wide. Midstem internodes glabrous or stipitate glandular === 2
2 Mature seeds tuberculate. Leaf sheath ca. 1,5-2x as long as wide. Midstem internodes usually glabrous to stipitate glandular === P. nanteuilii
2 Mature seeds with papillae that are longer than wide (cylindrical). Leaf sheath 2-3x as long as wide. Midstem internodes usually stipitate glandular, more rarely glabrous === P. dubia
Seed characters are easily assessed with a hand lens which enables identification in the field (see SEM-photos).
Petrorhagia nanteuilii obviously occupies an intermediate position between P. dubia and P. prolifera and the latter probably are its progenitors. Mature seeds doubtlessly provide the best feature to separate these species. For convenience seed samples of all three are here provided.
Voucher specimens can be sent to the author at:
National Botanic Garden of Belgium
Domein van Bouchout
Ball P. W. & Heywood V.H. (1964) A revision of the genus Petrorhagia. Bull. Brit. Mus. (Nat. Hist.), Bot. 3: 121-172.
Clement E.J. (2005) Petrorhagia dubia established in S. Hants (V.C. 11). BSBI News 100: 46.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Rabeler R.K. (1985) Petrorhagia (Caryophyllaceae) in North America. Sida 11: 6-44.
Thomas S.M. (1983) A taxonomic clarification of Petrorhagia section Kohlrauschia(Caryophyllaceae). Bot. Journ. Linn. Soc. 87(1): 55-75.
Thomas S.M. & Murray B.G. (1983) Chromosome studies in species and hybrids of Petrorhagia sect. Kohlrauschia (Caryophyllaceae). Pl. Syst. Evol. 141: 243-255.
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p.