Sempervivum arachnoideum subsp. tomentosum is an ornamental mat-forming, evergreen perennial succulent with rosettes up to 4 cm in diameter, of fleshy, obovate, mid-green to red leaves, densely covered in white, cobweb-like hairs. The star-shaped, pink flowers up to 1.5 cm in diameter are borne in flat cymes on stems up to 15 cm tall.
Scientific Name: Sempervivum arachnoideum subsp. tomentosum (C.B.Lehm. & Schnittsp.) Schinz & Thell.
Synonyms: Sempervivum tomentosum, Sempervivum laggeri, Sempervivum arachnoideum var. laggeri, Sempervivum webbianum.
Common Names: Woolly Cobweb Houseleek, Cobweb Hen and Chicks.
How to grow and maintain Sempervivum arachnoideum subsp. tomentosum:
It thrives best in full sun to light shade. In indoor, an east or west-facing window where they receive four to six hours of sunlight is ideal.
It needs excellent drainage. Poor, sandy soil would be just fine. You could work some peat into heavier soil, to lighten them and improve drainage.
Water regularly during the summer and spring. keep the soil evenly moist but not soggy. You can allow the topsoil to become slightly dry between each watering. Reduce water in the winter.
It prefers an average summer temperature 65 degrees Fahrenheit – 70 degrees Fahrenheit / 18 degrees Celsius – 21 degrees Celsius. In winter, some varieties can withstand temperatures down to freezing.
Fertilize with a controlled-release fertilizer at the beginning of the season or weekly with a weak liquid solution. Use a balanced 20-20-20 fertilizer at 1/4 strength on mature plants, and a fertilizer with less nitrogen on young plants.
Re-pot as needed, preferably during the warm season. To re-pot, a succulent, make sure the soil is dry before repotting, then gently remove the pot. Knock away the old soil from the roots, making sure to remove any rotted or dead roots in the process. Treat any cuts with a fungicide. Place the plant in its new pot and backfill with potting soil, spreading the roots out as you re-pot. Leave the plant dry for a week or so, then begin to water lightly to reduce the risk of root rot.
Pests and Diseases:
It has no serious pest or disease problems. Cobweb Houseleek can get vine weevil and may be subject to rust.
It can be easily propagated by seed sown in spring or root offsets in spring. Sempervivum earned their famous name “Hen and Chicks” from their growth habit. The mother plant, or hen, sends off numerous offsets, which will cluster around her base like chicks. These offsets can be easily re-potted, or the plants can be left to form a clumping mat.