The Sempervivum genus comprises nearly forty species of succulent perennials distributed across the northern hemisphere (Europe, North Africa, West Asia).
They form low cushions of dense rosettes composed of fleshy coloured leaves, from green to orange, sometimes hairy or covered with white cobwebby threads. For example, Sempervivum arachnoideum covers itself with what looks like a real spider web! This adaptation helps the plant to protect itself from the heat of the sun and also to capture morning dew.
Each rosette is monocarpic(*) and, before reaching maturity and flowering, produces several other similar rosettes. When mature, the rosette spreads out and produces a stem bearing a bunch of flowers.
Sempervivum are essential plants in rock gardens and have the added advantages of being decorative all year round, easy to grow and famously tough! You can use them in several settings: in a raised bed, troughs, pots, scree, cracks, walls or alpine greenhouses.
Although the Sempervivum produce splendid flowers, they are grown more for their structured foliage and over the years they grow into superb colonies.
L. William Curtis (1778-1781)
"In the middle of winter, when most other rockery plants are leafless, the Sempervivum take on remarkable reddish or copper tones, under the influence of the cold, and offer plant lovers a pleasant moment of contemplation during the rarer moments of sunshine."
For dry, sunny gardens. The Sempervivum are easy to grow. They enjoy full sun and a dry, poor soil that is very well-drained in winter. A gravel mulch can be used to bring out their architectural interest and will provide protection against too much damp in the winter. The species or hybrids with silky foliage can suffer from the cold when there is too much damp. It is better to grow (or overwinter) them in an alpine greenhouse or cold frame.
When growing in pots, the mixture should comprise equal parts of coarse sand (filter sand, 2-5 mm gravel, pozzolan) and garden soil. You can also add some lime to the mix in the form of crushed shells, lithothamnium, or chalk powder.
(*) “Monocarpic” is a botanical term to designate plants which only flower and bear fruit once before dying. Conversely, a polycarpic perennial is one that flowers several times during its lifetime.
Bibliographical source: Alpine Garden Society, Revue horticole (Vol 70 - 1898)