Sempervivums are often called “Hens and Chicks” because of the way they grow, with small rosettes (the chicks) that “sprout” around a larger, central rosette (the hen). One of the most unique and interesting sempervivums is Sempervivum arachnoideum, known also by its common name of “Cobweb Houseleek.” I can remember the first time I ever saw this plant. I was visiting a nursery on an early summer morning, the dew was heavy, and when it had settled on top of this little plant and accentuated the cobwebbing, I was certain that it had been the work of a spider sometime during the night. However, despite its species name of arachnoideum, spiders have nothing to do with the cobwebs that cover the top of this sempervivum. The “cobweb” is actually made up of specialized “leaf hairs,” or more technically, trichomes, that emanate from the tips of the leaves. The trichomes form a thick hairy mat at the center of the plant. Then, as the rosette grows, the leaves stretch the trichomes into longer and longer strands until, at the outer edges of the plant, many of them will tear.
Trichomes are common in almost all plants and vary widely in appearance. They serve many purposes, including insulating the plant from cold and frost, helping to reduce plant evaporation, protecting the plant from wind and heat, and also serving as a defense mechanism against many herbivorous insects.