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Sempervivum arachnoideum

The Cobweb Houseleek

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Sempervivum arachnoideum

The Cobweb Houseleek

Availability: In stock

Packet Size:10mg
Average Seed Count:200 Seeds


Always an interesting plant, Sempervivum arachnoideum is an exotic and interesting variation which forms small green rosettes of fleshy leaves, the tip of each leaf connected to another by a network of silvery filaments that resemble a spider’s web.
Growing in very tight rosette forming mats, each rosette may have up to 50 to 60 leaves. These drought-tolerant, evergreen, perennial succulents are easy to grow, tough, undemanding, and easy to propagate. Just as with all sempervivum, the mother ‘hen’ will produce many offsets or ‘chicks’. In the S. arachnoideum variety, the mother hen is not usually larger than the chicks so there is a more uniform size across all the rosettes. After a few years, she will send up a short spike of starry shaped, magenta pink flowers in summer.

Sempervivum arachnoideum are hardy to at least minus 12°C (10°F). They require a well-drained soil and a position in full sun to light shade. They will tolerate full shade and in warmer climates prefer protection from too much sun.
In their native environment, high in the mountains of Europe, the cobwebs protect the plants from the intensely bright light but excess moisture can damage them in winter as there is a risk of rotting. Container plants benefit from being brought into the shelter of the greenhouse or cold frame in winter.

Sempervivum successfully complement other rock-garden plants, use them in alpine gardens, troughs and pots, tucked into crevices of a stone wall or path. They can be used as a groundcover between flagstones or for edging a pathway where they can be can be grown around rocks or anchored into crevices of walls. They are especially suitable for green roofs or living walls. Planted in containers with other succulents they make wonderful components for miniature or fairy gardens.

  • Awarded the RHS Award of Garden Merit
    Sempervivum arachnoideum was awarded the Royal Horticultural Society’s Award of Garden Merit (AGM) in 1993. The award was reconfirmed in 2008.

Sowing: Sow indoors all year, or outdoors in February to July
Fill small pots or trays with a light and well-aerated seed compost. Tap the container to settle the compost, but do not firm the mixture down. Stand the pots in water, moisten thoroughly and drain. Seeds should be scattered very lightly over the surface.
If possible, place in a propagator otherwise, secure a polythene bag around the pot or cover the container with glass or and place in a warm shaded place. Many people make use of a warm place such as the airing cupboard, or near the kitchen boiler. Care should be taken to prevent the pots drying out from below. Some people stand the containers on a tray of damp sand, so that they do not dry out.
The majority of seeds germinate best at temperatures of 18 to 20°C (65 to 70°F). Most seedlings may appear within 15 to 30 days others may take a little longer. Stratification after sowing by chilling in the fridge at 4°C (39°F) for a few weeks, or outdoors during the winter, will enhance germination.

Be careful to keep the top of the compost damp but not wet. As soon as the first seeds have germinated, remove the plastic or raise the lid slightly to permit some circulation of air. From now on, the tiny seedlings need to be in a good light, but must be protected from direct sun. Shading from all but winter sun is desirable for the first few months.
The first sign of germination is the appearance of a pair of tiny succulent seed leaves (cotyledons) and this is soon followed by the development of the new rosette. Almost certainly there will be far too many seedlings germinating to grow them all but after a few months the most promising ones can be transplanted and grown on.
Transplant seedlings when large enough to handle into 7.5cm (3in) pots. Grow on in a cold frame or cold greenhouse and plant out the following spring 15cm (6in) apart in full sun. For a green roof or groundcover effect, space plants closer together.

Soil requirements:
Houseleeks will grow in almost any type of soil provided that it is well drained and in a sunny position. For growing in containers use compost such as John Innes No.1 or No.2, with 25% sharp sand or horticultural grit for added drainage. Best results are probably obtained by adding slow release fertiliser granules to the compost at the rate recommended for alpine plants. A top dressing of horticultural grit around the neck of each plant provides an attractive background and prevents the plant from being splashed with mud during heavy rain.

Sempervivum need to be grown in full sun to fully exhibit their different colours. Out of full sun many cultivars tend to end up as a similar green colour. They look good in containers and in combination with other succulents such as sedum, echeveria or stonecrop. You can also display them with “steppables” such as mosses, ajuga, or thyme.
The lifespan of Sempervivum is about three years. A young hen starts producing the season following its development. It will produce many chicks to carry on during its productive years. Flower stalks usually develop during its third year of life. Sempervivum is monocarpic; each rosette can only ever flower once and then die.
Nature planned it so that there will be sufficient room for the chicks to develop and grow.
It is a good idea to detach some of the offsets for planting elsewhere or spread them out so that they have room to root and grow. Even plants growing in open ground benefit from being dug up and re-planted occasionally. They are remarkably tolerant of root damage and often seem to grow better if most of the old roots are removed when re-potting.

There are approximately fifty species and over 6,000 named cultivars of Sempervivum, in the family Crassulaceae. Sempervivum arachnoideum is native to the mountains of Europe from the Pyrenees to the Carpathians.
They are native to Europe, through the mountains of Iberia, the Alps, Carpathians, Balkan mountains, Turkey, the Armenian mountains, in the north-eastern part of the Sahara Desert, from Morocco to Iran and the Caucasus. Their ability to store water in their thick leaves allows them to live on sunny rocks and stony places in the montane, subalpine and alpine belts.
Sempervivum are related to Jovibarba and Rosularia and are distinguished from each other by their flowers.

Named by Linnaeus (1757) The name Sempervivum has its origin in the Latin semper, meaning 'always', and vivus, meaning 'living', because this perennial plant keeps its leaves in winter and is very resistant to difficult conditions of growth.
The species name arachnoideum derives from the ancient Greek arachnion meaning 'spider', and oides meaning 'like' or 'resemble' and refers to the plants a network of silvery filaments that resembling spider webs.
One of the common names is houseleek. Sempervivums are traditionally grown on roofs between thatching, tiles or shingles. The plant is sometimes still planted in roofs since it is supposed to give protection against lightning, thunderbolts and fire to any house that it grows on. It is also said to preserve thatch. There is some justification to this belief because the leaves contain a great deal of water and do not burn easily - if there are many of the plants growing on the roof then they will tend to put out the fire before it can take hold properly.
They were also known by the delightful name "welcome-home-husband-however-drunk-you-be" because of the way the rosettes roll off the roof.

Sempervivum and Sedum are considered ‘Old World Treasures’ and are associated with mythology and were considered sacred to Jupiter in Roman and Thor in Nordic mythology. Both gods presided over the air, governing thunder and lightening, the winds and rains.
During early centuries in Scandinavian countries, the Jovibarba varieties were called Thor’s Helper’ and were believed to drive off demons and guard homes if planted on roofs. Thor is portrayed with a mane of red hair. The flower of this sempervivum was said to resemble the beard of the God.
The Romans called them 'Beard of Jupiter' and planted them on roofs to guard against lightning.

Additional Information

Additional Information

Packet Size 10mg
Average Seed Count 200 Seeds
Seeds per gram 20,000 per gram
Family Crassulaceae
Genus Sempervivum
Species arachnoideum
Synonym Sempervivum arvernense, Sempervivum densum
Common Name The Cobweb Houseleek
Other Common Names St. Patrick's Cabbage. Welcome home husband, however drunk you be
Hardiness Hardy Perennial
Hardy -12°C (10°F)
Flowers Vibrant magenta flowers.
Natural Flower Time Only once in around three years.
Foliage Succulent green rosettes with cobweb threads.
Height 8cm (3in)
Spread 15cm (6in)
Aspect Full sun, or partial shade
Soil They will grow in almost any type of soil provided that it is well drained
Time to Sow Sow indoors at any time of year.
Germination Most seedlings appear within 15 to 30 days others may take a little longer.
Notes Seeds germinate best at temperatures of 18 to 20°C (65 to 70°F).

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