Hello all, several of you have wondered about hybridizing tropical and winter hardy hibiscus species. We are not the only ones interested in such crosses and commercial research has been conducted looking into the feasibility of such a cross. As one may expect there has been a lot of discussion on ploidy issues and pre/post fertilization barriers.
As far as pre-fertilization and post-pollination barriers there are a few that have been researched specifically in the crossing of H. syriacus and H. sinensis. Stigma/style barriers were discovered which retard and prevent the hybrid pollen from reaching the ovules efficiently (Kuligowska, Katarzyna, et al. "Breeding of Hibiscus rosa-sinensis for garden use in Denmark." II International Symposium on Woody Ornamentals of the Temperate Zone 990. 2012.).
In some instances through controlled pollination a hybrid seed will form, though at extremely low efficiency and the progeny will usually die shortly after by termination of the seed pod or shortly after germination (4-5 leaf stage).
Special pollination methods can be employed to bypass these inherent barriers of fertilization to increase the chances of getting hybrid seeds. This included cut stigma/style pollination, this is an interesting bypass as one can control the length of the style which was reported to be a factor since H.syriacus styles are far shorter than most H.sinensis and the hybrid pollen has to tunnel much farther- by using cut stigma/style pollination this barrier can be overcome, and pollen can reach the ovule.
I am tinkering with this barrier bypass myself and will report back later.
As to the post fertilization barriers there are now several methods of embryo rescue that can be performed that result in hybrid plants. I am fortunate enough to work in a genetics research lab where I can try some of these techniques for in vitro culture of hybrid ovules. I am also testing this cross method on the trees.
Anyhow, the most recent research I�ve looked at suggest these crosses should be possible with bypassing some of the barriers and that hybrids should be obtainable. In another post I have shown one such hybrid that reached maturity when a H.syriacus pollen yielded three seeds on H. sinensis- 2/3 seedlings died shortly after maturation and one has now reached 4 feet tall and is about 2 years old- this cross has not flowered yet but goes dormant in the house each fall. More to come- food for thought- if we can overcome these barriers and create the first hybrids it could open the door to more fluid crosses.