Michael,
Like you I find the Semps easier to cross than the Jovibarba type flowers. With them I have resorted mainly to self pollinating them as I find it so annoying to rip the blossoms apart to emasculate the flowers. The pollen of the Jovibarba types is a bit more clumpy but it does seem to pollinate just fine. I had seeds on Emerald Spring last season that I used in crosses and the seedlings definitely have the characteristics of the pollen parent.
OK on the fertility issue. If you cross two diploid plants from different species the hybrids might be fertile, depending how distantly the species are related. Surprisingly, many of these crosses show rather high fertility, even with parents of two different chromosome counts. This may be because we are really dealing with ancient allopolypoloids in our diploid species, so that there is some chromosome pairing in the hybrids. Crosses of diploids with tetraploids results in mostly sterile triploids. You do get a few seeds from these plants, but not many. Crosses between two tetraploids will give you the greatest chance for fertile hybrids between different species as all the chromosomes will have a partner with which to pair during meiosis.
The sterile plants tend to have odd, off color pollen. My Greenwich Time has a very light, almost lemon yellow pollen, much different than the more golden yellow pollen typical of the fertile semps. Other plants, like Aymon Correvon I find nearly sterile but I have been collecting open pollinated seed on these, hoping that the bees might find a compatible parent. So far only ONE seedling!
Hope that helps!
Kevin
