Sempervivum arachnoideum
| Light | Full sun to bright light; tolerates some shade but colours and webbing are best in strong light |
|---|---|
| Water | Sparingly; drought-tolerant once established, keep drier in winter |
| Soil | Gritty, sharply-draining mineral mix (see Soil and potting mix) |
| Temperature | Very cold-hardy; survives hard frost, roughly USDA zones 5–8 |
| Propagation | Offsets (primary); also seed |
| Toxicity | Non-toxic to cats and dogs |
Sempervivum arachnoideum is a low, mat-forming alpine succulent from the mountains of southern and central Europe, instantly recognised by the fine white threads that web across the tops of its tiny rosettes like a cobweb. This dense hairy veiling gives the plant its common name, the cobweb houseleek, and makes it one of the most charming and beginner-friendly members of the genus Sempervivum.
Description
Sempervivum arachnoideum forms tight, rounded rosettes usually only 1–3 cm across — among the smallest in the genus — each made of many fleshy, incurved leaves. From the tip of every leaf grow fine white hairs that stretch to neighbouring leaf tips, spinning a web of silvery threads over the whole rosette. The webbing is densest in strong light and in the compact forms, where a rosette can look wrapped in cotton wool.
The rosettes are typically green flushed with red or bronze, the colour deepening in sun, cold and drought. Plants spread quickly by short stolons into low, ground-hugging mats of many offsets (the "chicks" of the familiar hen-and-chicks). Like all houseleeks each rosette is monocarpic: after several years it may send up a short stem topped with star-shaped, pink to rose-red flowers, then die — but by that time it has usually surrounded itself with a ring of young offsets that carry on.
Distribution and habitat
The species is native to the higher mountains of Europe, including the Alps, Pyrenees, Apennines and Carpathians. It grows on rocky ledges, screes, crevices and thin alpine turf, often at considerable altitude, rooting into gritty pockets among the rock. This is a plant adapted to lean soils, intense mountain sun, and long, hard winters under snow — which is exactly why it is so tough and cold-hardy in cultivation.
Cultivation
Sempervivum arachnoideum is one of the easiest succulents to grow and an excellent choice for beginners, rock gardens, alpine troughs, green roofs and outdoor containers. Give it full sun and a very free-draining, gritty mix — good drainage matters far more than rich soil. Water when the mix has dried, then let it dry again; established plants are highly drought-tolerant and resent sitting wet.
Its great strength is cold-hardiness: it shrugs off hard frost and is happiest grown outdoors year-round in temperate climates, needing a cold, dry winter rest rather than protection. The main risk is the opposite of the usual houseplant problem — too much shade, too much water, or a stagnant mix will cause the rosettes to stretch, loosen and rot. In damp winter climates a pane of cover or an open, gritty planting keeps the crowns dry. See Watering and Repotting for general technique.
Propagation
Propagation could hardly be simpler. Each plant produces a steady supply of offsets on short stolons; simply detach a "chick" — with or without roots — and press it into gritty mix, where it roots in a week or two. This is the standard and most reliable method. The species also comes readily from seed, though seedlings vary and are slower than offsets. See Propagation — offsets for a full walkthrough.
Cultivars
S. arachnoideum and its hybrids have given rise to a large number of named selections prized for the density of their webbing, rosette size and colour, ranging from tiny cotton-wool balls to larger red-and-silver forms. It also crosses freely with other houseleeks, and its cobwebbing appears in countless garden hybrids. See the Sempervivum genus page for an overview.
Common problems
- Rot — caused by wet, poorly-drained conditions, especially in a mild damp winter; crowns soften and collapse.
- Etiolation — too little light makes rosettes open up, pale and stretch, losing their tight webbed form.
- Pests — mealybugs can hide among the dense hairs and in the leaf axils; vine weevil grubs may attack roots in outdoor pots. See Pests and diseases.
- Confusing webbing with cobweb pests — the white threads are normal foliage, not spider mite webbing; check for actual mites (fine mobile webbing, bronzed tissue) before treating.
See also
- Sempervivum — the genus overview
- Propagation — offsets · Soil and potting mix · Watering · Repotting