Besides how closely related the three species are (sanguineum, versicolor, endressii) within the genus and their overall compatibility, matching chromosome numbers is the next big hurdle to overcome. In general, more success (if any) is had by using the higher numbered chromosome plant as the pollen contributor and the lower chromosome number plant as the parent that will produce the seed.
As I would with any difficult cross, if I were to attempt it, the cross would be highly controlled, so I would know for sure that any seed produced is actually what I want:
-- protect the subject flower from unwanted pollenations, i. e. from insects, by enclosing the flower in an organza bag or similar. This begins the day before the bud opens.
-- remove stamens from the subject flower before the pollen is ripe to prevent self pollenation - this will likely be just as the bud begins to unfold. Carefully peeling back petals or removing them altogether does hurt anything.
-- one to three days after the flower opens fully (watch a same age flower on the same plant), hand pollenate with the desired pollen.
-- immediately reclose the organza bag to continue to prevent unwanted pollenations.
--leave the bag on until the seed is ripe to catch the seed. Most geranium seed pods eject their seed with force, and you will never find them otherwise.
It is likely that if any seed is produced, zero to only a few would be actually viable and have the ability to grow into a plant. And then, of course, would they be runts or a beautiful hybrid worth keeping? Only time will tell.
