Alan C. Smith : Sempervivum tectorum (Linn.)

Sp. Plant. 464 (1753). This is an extremely variable species with a wide distribution in Europe, from the Pyrenees and central France to the Apennines and northern Balkan regions. The variation of this species has led botanists of former years to subdivide S. tectorum into subspecies, varietates and formae; one example: Rouy and Camus (Flore de France 7), subdivided the French tectorum into 40 varietates and formae, grouped under three sub-species: S. rupestre, S. calcareum and S. arvernense. Much of this variation is purely phenotypic; of the genetically determined variations resulting from hybridization within its own forms, both in the wild and cultivated plants. All these forms of S. tectorum will hybridize with other species, the resulting offspring to a large extent being fertile, will often cross back with their parents and give rise to an  infinity of forms, a mongrel race. The four species of the Alpes that intercross with S. tectorum, as well as each other, are S. arachnoideum, S. montanum, S. grandiflorum and S. wulfenii. The diagnosis of the  species tectorum remains easy, even though it is quite futile to try to attempt naming the endless and indefinite forms and crosses. The general features common to most forms are: rosettes large and open, 3 to 8 cm in diameter, dark green occasionally pale green, sometimes red-brown or purple and frequently tipped at leaf apex with darker colouring. Leaves are oblong-lanceolate to obovate, sharply acuminate, very fleshy, red or white at base, glabrous on face and back (rarely pubescent), margins fringed with conspicuous white cilia. Stolons are stout up to 4 cm long. Flowering-stems stout, 20 to 40 cm high, very pubescent; lower stem-leaves glabrous apart from cilia, the upper ones are pubescent. Inflorescence is usually large and flattish, comprising of 40 to 100 flowers; petals dull pink or purple, anthers orange-brown.

We have restricted the varietates of S. tectorum to just the three out of the four varietates that Dr. Lloyd Praeger recognized as worthy of distinction; the fourth, namely S. tectorum var. calcareum (Jordan) Cariot & St. Lager, is now regarded as a separate species: Flora Europaea, 1 (1964).

S. tectorum var. tectorum (Linn. sensu stricto). Common Houseleek.

This is the best-known of the cultivated Sempervivums, having been extensively grown in most of Europe, especially on the roofs of cottages (originally as a charm against lightning). The origin is obscure as it is nowhere indigenous, but occasionally naturalized. Cultivation of this variety has been from early times – Greek and Latin writers of botany knew it and it was illustrated in the manuscript of Dioscorides “Materia Medica”  (early sixth century). This Linnean type is distinguished by its very large, flattish rosettes up to 18 cm in diameter. The rosette leaves are obovate-lanceolate, glabrous and of a bright green, whitish (not red) at the base with apices usually well tipped purple-brown.

Flower-stems are 30 to 50 cm high. An unusual feature is that the inner whorl of stamens are mostly sterile, and often replaced by carpels.

S. tectorum var. alpinum (Griseb. & Schenk) Praeger.

(S. boutignyanum Billot & Grenier; S. arvernense form boutignyanum Rouy & Camus). This variety was originally described by Grisebach & Schenk in Linnaea 25, 600 (1852). It is a small and neat-growing plant of the mountains, found in the Pyrenees, and especially common in the northern and central Alps; Istria. Rosettes 2 to 6 cm in diameter with green leaves that are always reddish at the base, with or without a purple-brown leaf tips. Flower-stems 10 to 30 cm high, inflorescence is compact and flattish.

S. tectorum var. glaucum (Tenore) Praeger.

(S. acuminatum Schott; S. schottii Baker in Gard. Chron. 1874, ii, 103 (1874) not to be confused with the hybrid S. x schottii (montanum x tectorum) of Lehm. & Schnittsp. In Ber. Offenb. Verein Naturk. 1 (1860) 35, tab. 2.

The variety glaucum was described by Tenore in Index Seminum Hort. Neap. 1830, 13 (1830). In nature glaucum comes from the southern and eastern Alps; Istria. The rosettes are rather large 5 to 10 cm in diameter, leaves glaucous or subglaucus, whitish (not red) at base and without purple-brown tips.

Flower-stems up to 60 cm high. Although there appears to be many variations of glaucum in existence - we have a richly glaucus plant which answers very well to the description. (Tenore & Praeger).