Davide,
What a GORGEOUS publication. You should be commended for gathering such wonderful information and glorious photographs.
If I had one comment it would be to your comments on the cultivars of S. calcareum. I grow both 'extra' and 'Pink Pearl' and think both clones are well worth growing and sufficiently distinct from other clones. The symmetry of 'Extra' makes it one of my favorites. However, as you note, there are NO bad calcareums!
In the US calcareum blooms very rarely but I have used it as a parent in some of my earliest crosses as it produced lovely fluffy pollen. The cross of 'Cleveland Morgan' (predominantly montanum breeding) X S. calcareum gave 'Greenwich Time', which is a velvety purple in spring but turns to a velvety version of calcareum later in the season. It is nearly sterile, unfortunately. Because it was so easy to make hybrids with calcareum pollen I'm thinking that the complete sterility of the hybrids may have prevented the widespread occurrence of such events in nature. 'Greenwich Time' is certainly the exception in fertility among the sempervivum hybrids. The flowers of the hybrid are very close in color to calcareum too.
Kevin
