Oh that's a neat flower!
I spent a few minutes looking it up. I think I have to get some.
Brent and Becky's Bulbs (bulb experts who sell online here) say it will grow in dry, average or wet soil, and has survived their Zone 7 garden.
Plant Delights Nursery (total plant nut Horticulturists) say it is from low swampy areas around Mexico City, which is a high elevation so often plants from their will survive here and in the U.K. and I would suppose Belgium.
The Pacific Bulb Society (super duper uber Plant nut geeks) mentions in one place these grow in arid areas, so I don't really know.
Anyway, there are many species of Hymenocallis, from varying habitats, some quite marshy, some periodically flooded, some from rocky slopes, so you could try to introduce other species into your program. Or just pick the best plants each generation, for your location. I saw one thread in the PBS website where someone said this genera is susceptible to Cercospora Leaf Spot, but that the species H. glauca is resistant.
Breeding plants could easily become your new hobby. I sort of got interested too, but haven't done much about it, mostly learning how to pollinate various things and saving seeds. I did get interested in iris- I know there are some really healthy really old cultivars that thrive in horrid dry shade around here, but when I bought a lot of fancy expensive new cultivars of Tall Bearded Iris they seem quite finicky, needing to be coddled, and are just not good plants for the home garden, like the old ones I see around old houses here, happily flowering for decades under trees. I bought an old out of print book from Schreiner's Iris on sale called The World of Irises, by The American Iris Society, which goes into detail about the species, genetics, and history of breeding. One could become immersed in this hobby for a long time...
What happened to your Hymenocallis that you are trying to fix with your breeding program?
