Houseleek

Growing to 15 cm (6 in) tall by 50 cm (20 in) broad, it is a rosette-forming succulent evergreen perennial, spreading by offsets. It has grey-green, tufted, sessile leaves, 4–10 cm (2–4 in) in diameter, which are often suffused with rose-red. In summer it bears clusters of reddish-purple flowers, in multiples of 8–16, on hairy erect flat-topped stems. The species is highly variable, in part because hundreds of cultivars have been propagated, sold, and traded for nearly 200 years. Sempervivum tectorum was described in 1753 by Linnaeus, who noted that its leaves are ciliate, that is, fringed with hairs.

The plant has been traditionally thought to protect against thunderstorms, and grown on house roofs for that reason, which is why it is called House Leek. Many of its popular names in different languages reflect an association with the Roman thunder-god Jupiter, notably the Latin barba Jovis (Jupiter's beard), referred to in the Floridus traditionally attributed to Aemilius Macer, and its French derivative joubarbe, which has in turn given rise to jubard and jo-barb in English; or with the Norse thunder-god Thor as in German Donnerbart. It is also called simply thunder-plant. Anglo-Saxon þunorwyrt may have either meaning. However, the association with Jupiter has also been derived from a resemblance between the flowers and the god's beard; in modern times, it has also been called St. George's beard.

Other common names, such as Anglo-Saxon singrēne, Modern English sigrim, sil-green, etc. and aye-green, refer to its longevity. William Fernie tells a tale in support of this:

History relates that a botanist tried hard for eighteen months to dry a plant of the House Leek for his herbarium, but failed in this object. He afterwards restored it to its first site when it grew again as if nothing had interfered with its ordinary life. It has been believed to protect more generally against decay and against witchcraft. …

Sempervivum tectorum is one of several houseleek species to be cultivated. It is valued as groundcover for hot, dry places. It has gained the Royal Horticultural Society's Award of Garden Merit. Sempervivum tectorum multiplies horizontally by propagation of underground roots. Propagation occurs in the spring, when budding occurs. From each mother plant up to four or, on good days, ten new ones can be harvested. They come off easily when taking care not to break the roots. Seed propagation works in spring.