The oldest P. flaccida hybrid and the oldest Petrocosmea hybrid, for that matter, is P. 'Momo', made by Nagahide Nakayama of Japan, by crossing P. flaccida and P. nervosa. The plant, for me, grows very much like a large P. flaccida and is perhaps, a bit more floriferous, with slightly larger flowers than P. flaccida.
A couple of years ago, Mr. Nakayama did a remake of the same cross that produced P. 'Momo' and released two of the resulting seedlings. Those seedlings, named P. 'Asa Blue' and P. 'Imperial Butterflies' are improvements, in my opinion, over P. 'Momo'. I grow both of them and really like them. I have been successful in using P. 'Asa Blue' in one cross with P. begoniifolia.
P. 'Asa Blue' an improvement over P. 'Momo', was the result of a remake of the same cross that produced P. 'Momo'. The plant flowers several times a year, and the flowers are large and show. The plant also shapes very nicely. 
The photo above, shows the flowers of the parents of P. 'Asa Blue' - P. flaccida, and P. nervosa. P. 'Asa Blue' is incredibly floriferous and flowers are larger than either parent with improved form. This hybrid is also fertile for further breeding. Seedlings inherited it's early, free-flowering nature. 
Another hybrid, boasting P. flaccida as a parent is P. 'Short'nin' Bread'. This cross used P. forrestii as the seed parent. P. 'Short'nin' Bread' is a vigorous, attractive hybrid which can be grown quite large with good culture. It flowers freely at a young age, although generally only once a year, for me. 
A sibling to P. 'Short'nin' Bread', P. 'Fluffer Nutter' has the same silvery felt covered leaves, but a more compact and neat leaf habit, seeming to take this from it's P. forrestii parent. This hybrid, sadly, is not free flowering, and is the only Petrocosmea in my collection which I have never flowered, even after several years of growing. Still the foliage is attractive. 
A flower from one of the selected seedlings from my hybrid cross using P. 'Asa Blue' crossed with P. begoniifolia. These seedlings have been quite free flowering at a young age. Almost all have been attractive and selections have been difficult due to the large amount of variation I've gotten from the cross. (I wanted to keep them all!) I have so far, selected five or six to grow on for further evaluation. The seedling above is unnamed at this point. It inherited a nice deep blue color from P. 'Asa Blue' with flower shape and the white /yellow throat from P. begoniifolia. P. flaccida is a wonderful old friend among Petrocosmea admirers. If you don't grow it, you should. It's flowers are among the darkest purple of all Petrocosmea species and it is a good bloomer. These characteristics are passed along to it's progeny in hybridizing, which gives it a secure spot among our growing list of Petrocosmea species and cultivars.
