They used to be divided into Sempervivum and Jovibarba. The name of Jovibarba has been dropped and all are now under the Genus of Sempervivum.
The Jovibarba used to consisted of species: heuffelii, globiferum (consisting of a number of subspecies), now these are Sempervivum heuffelii and such. These species and subspecies also have a different form of bloom than what we all considered as sempervivum. Can you see the difference in the flowers?
heuffelii, which have rosettes that look very similar to sempervivum, but do not produce offsets on stolons like sempervivum. They reproduce by forming new rosettes from the same crown as the parent rosette.
globiferum, and many of it's subspecies, reproduce by offsets formed on very thin brittle stolons that break away from the parent rosette and usually roll away from the parent. They can even settle on their tops, but will send roots out and pull them into position for rooting into the ground.
These are the ones that go by the common name of 'Rollers'. They are so much fun to have in the garden. Little travelers.
A few of the roller types.
Hope this explanation made sense?
