P. These succulents are mostly of the easiest possible cultivation in rock gardens, dry walls etc., given a sunny position and perfect drainage. They flower in summer. Sempervivum tectorum is the houscleek often grown on tiled roofs. It was known to the Greeks and the Romans and was originally grown on roofs in the belief that it warded off lightning. It is a very variable species, numerous forms being described under a variety of names. Sempervivums generally, like sedums (only more so) are still in a state of confusion as regards names but several attempts have been made to sort this out, and one well-known specialist in rock garden plants offers approximately 70 kinds which are correctly named. Sempervivum tectorum typicum is a very good form with purplish flowers consisting of 12 to 16 petals, purplish or whitish with red lines. They are borne on stems about 8 in. tall. The glabrous green leaves usually have purplish-brown tips. S. arachnoideum is the cobweb houseleek. This again is very variable. The bright rose-red flowers have very wide petals and are borne on 3—4 in. stems. Although the colour of the flowers varies, as does the pointed foliage which may be deep red, deep green or a mixture of the two, the cobweb of white vegetable hairs extending from tip to tip of the leaves (very like a spider’s web) is confined to this species. It is accordingly very easy to identify. Note that in very wet, sunless weather, the amount of cobweb diminishes. Sempervivums are easily increased by division in spring.

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