LariAnn I'd love to see more cold hardiness brought into the L. speciosa. We can't grow them up here, which is the reason I suspect we don't see much breeding work in that species.
Ashton, there doesn't seem to be much published information about hybridizing crepe myrtles, but it should be a straightforward process. The pistils and anthers are fairly separate from each other on the flower, much like daylilies. I grow a lot of crepe myrtle seedlings but have never intentionally crossed any.
