Overview
Sempervivum arachnoideum is a mat-forming succulent perennial in the Crassulaceae family, characterised by dense rosettes 0.5–1.5 inches (1.5–4 cm) in diameter covered with fine white cobweb-like hairs stretching between the leaf tips. Individual rosettes are composed of 50–80 tightly packed, fleshy, lance-shaped leaves 0.3–0.5 inch (8–12 mm) long, green to grey-green, often flushed reddish-purple in cold weather or high light. The cobweb filaments are produced by glandular trichomes at the leaf tips and are most prominent on new, tightly closed rosettes — older, open rosettes may show sparser webbing. Plants spread by stolons to form dense mats 6–12 inches (15–30 cm) across composed of the mother rosette surrounded by numerous offsets. Rosettes are monocarpic: after 3–5 years, an individual rosette elongates into a 4–6 inch (10–15 cm) flower stalk bearing 5–15 star-shaped pink to rose-red flowers 0.6 inch (15 mm) across, then dies. The surrounding offsets continue the colony. The species is native to alpine and subalpine rocky habitats in the European mountains and tolerates temperatures to -30°F (-34°C) when established in well-drained sites. Root rot occurs rapidly in waterlogged soil — the primary cause of failure in cultivation. Susceptible to mealybugs and vine weevil larvae in container culture.
Native Range
Sempervivum arachnoideum is native to the mountains of central and southern Europe, including the Alps, Pyrenees, Apennines, and Carpathians, growing on rocky outcrops, cliff faces, and scree at elevations of 2,000–10,000 feet (600–3,000 m). It occurs from France and Spain east to Austria, Slovenia, and the western Balkans.Suggested Uses
Planted in rock gardens, crevice gardens, green roofs (minimum 2 inches / 5 cm substrate depth), and stone walls. Effective as a container specimen in shallow pans and troughs — minimum 2 inches (5 cm) deep with drainage. Tolerates extreme cold, wind, and poor soil. Combine with Sedum, Jovibarba, and alpine plants for textural contrast in mineral-based plantings.How to Identify
Appearance
Size & Dimensions
Height1" - 6"
Width/Spread6" - 1'
Reaches mature size in approximately 2 years
Bloom Information
Flowering rosettes elongate in June–July, producing a hairy, 4–6 inch (10–15 cm) stalk bearing a flat-topped cyme of 5–15 star-shaped flowers. Individual flowers are 0.6 inch (15 mm) across with 8–12 narrow pink to rose-red petals. Bloom lasts 2–3 weeks. The flowering rosette dies after seed set; surrounding offsets are unaffected.Detailed Descriptions
Flower Description
Pink to rose-redFoliage Description
Green to grey-green with white cobweb filaments, reddish in cold/high lightGrowing Conditions
Sun Requirements
Requires 6-12 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Plant in full sun in sharply drained, gritty soil with a pH of 6.0–7.5. A mix of 50% mineral grit (pumice, perlite, or coarse sand) and 50% standard potting soil works well. Space rosettes 3–4 inches (8–10 cm) apart; offsets will fill gaps within 1–2 seasons. Water sparingly — allow soil to dry completely between waterings. In the Pacific Northwest, the primary concern is winter wet: ensure planting sites drain freely and consider raised beds, rock gardens, or containers with drainage holes. No fertilisation required in mineral-rich soil; in poor substrates, a single application of dilute balanced fertiliser in early spring is sufficient. Remove dead flowering rosettes when dry to maintain appearance.Pruning
No pruning required. Detach and remove dead monocarpic rosettes after flowering. Separate offsets from congested mats every 2–3 years if desired for propagation — simply pull or cut offsets with a short section of stolon and replant on the soil surface.Pruning Schedule
J
F
M
A
M
J
J
A
S
O
N
D
summer
Maintenance Level
very lowContainer Growing
✓ Suitable for container growing
Minimum container size: 1 gallons