Pure Sax longifolia seldom or never forms offsets, and is of course monocarpic. Seed is the only practical means of increase. Tissue culture might be a possibility but I never tried it when I had a lab set up. Germination is as for other silver saxes. They have a cold and or Ga3 requirement to germinate. Seed is very fine and should be Sown on top of a pre gritted pot. Much of what is in cultivation under this name is actually hybrids of longifolia to other silvers. All are easy wonderful garden plants. Bob Stewart Arrowhead Alpines