Bananas contain potassium, which is one of the three nutrients generally listed on standard fertilizers: nitrogen (N), phosphorus (P), and potassium (K).
I use compost and worm castings on my roses (although, truth to tell, I'm very behind in attending to them this year, for various reasons), and they respond well, so my roses show me that the potassium level in my compost and worm castings is to their liking. (Heaven knows, there are enough banana peels in both of them to choke a moose!
If you don't have any compost of your own yet, it would be worth your while to start one.
Sunset Zone 17, USDA Zone 9