Sempervivum
Sempervivum is a genus of low-growing, cold-hardy rosette succulents in the family Crassulaceae, native to the mountains of Europe, North Africa and western Asia. Commonly known as houseleeks or hens and chicks, they form tight, geometric rosettes of fleshy leaves that multiply into dense mats and shrug off frost, snow and hard winters that would kill most other succulents. The Latin name means roughly "always alive", a fitting nod to their toughness and their long history on cottage roofs and stone walls.
Description
Sempervivum species share an unmistakable form: a symmetrical rosette of thick, often pointed leaves arranged in a tight spiral. Rosettes range from under a centimetre to well over 15 cm across depending on species and cultivar, and colours span green, red, purple, bronze and near-black, frequently shifting with the seasons — many are richest in colour under the cold, bright conditions of late winter and spring. Some forms carry fine hairs along the leaf margins, while the so-called cobweb houseleeks are laced across the rosette with white webbing.
Each rosette is monocarpic: it flowers once, then dies. Mature rosettes send up a stout stem topped with a cluster of star-shaped flowers, usually pink, red, purple, yellow or greenish. By the time a rosette blooms, however, it has almost always surrounded itself with a ring of offsets, so the plant as a whole carries on unbroken.
Distribution
The genus is centred on the mountain ranges of southern and central Europe — the Alps, Pyrenees, Carpathians, Apennines and Balkans — extending into North Africa and across to the Caucasus and mountains of western Asia. Plants grow on rocky slopes, cliff ledges, scree and thin alpine soils, often in exposed positions with intense sun, sharp drainage and bitterly cold winters. This alpine heritage is exactly what makes them so hardy in cultivation.
Notable species
- Sempervivum tectorum — the common houseleek, the classic broad green rosette long grown on rooftops and walls.
- Sempervivum arachnoideum — the cobweb houseleek, small rosettes veiled in white webbing.
- Sempervivum montanum — a compact alpine species with resinous, aromatic foliage.
- Sempervivum calcareum — grey-green rosettes tipped in deep red-brown.
- Sempervivum ciliosum — a hairy, dome-forming species from the Balkans that appreciates extra winter dryness.
The genus also includes the former Jovibarba group (the "rollers", which offset by loose balls that roll away to root), now often folded into Sempervivum or kept as a close relative depending on the classification followed.
Cultivation
Houseleeks are among the easiest succulents to grow outdoors in temperate climates, and among the few that positively want a cold winter. Give them full sun, a gritty, fast-draining mix, and a shallow container, trough, wall crevice or rock garden pocket. They tolerate heat and drought once established, but resent sitting wet — winter wet, not winter cold, is what kills them. See Watering for general technique.
Feeding should be minimal; lean conditions keep rosettes tight and colourful, while rich soil and heavy feeding produce loose, floppy growth. In very hot climates a little afternoon shade prevents scorching, but in most gardens they thrive on neglect in the sunniest, driest spot available. Because they are so hardy, they are excellent container and green-roof plants where tender succulents cannot survive the winter.
Propagation
Propagation could hardly be simpler. Each rosette produces numerous offsets — the "chicks" — on short runners around its base. These can be detached once they have a few roots of their own and pressed into gritty soil, where they establish quickly; see Propagation — offsets. Species also come readily from seed, though seed-raised plants of hybrids will not come true. When a rosette flowers and dies, simply remove the spent stem and let the surrounding offsets fill the gap.
Hobby and cultivar notes
Sempervivum is a hugely popular collector's genus, with many hundreds of named cultivars selected for rosette size, colour and hairiness. Because colour shifts dramatically with season, light and temperature, the same cultivar can look strikingly different through the year, which is part of their charm. They combine beautifully in shallow bowls, alpine troughs, living walls and rockeries, and mix well with other hardy succulents such as Sedum. Growers often display a single cultivar per pot to appreciate its geometry, or crowd many together for a tapestry of contrasting colours.
Common problems
- Rot — from wet, poorly drained soil, especially in winter; the base of the rosette turns soft and brown.
- Etiolation — too little light makes rosettes open up, pale and lose their tight form and colour.
- Pests — vine weevil larvae (which chew the roots), mealybugs and, in damp still conditions, occasional fungal spotting. See Pests and diseases.
See also
- Sempervivum tectorum · Sempervivum arachnoideum — popular species
- Sedum — a frequent companion in hardy succulent plantings
- Soil and potting mix · Watering · Propagation — offsets · Repotting