Maybe someone has pollen from a Rex or something that might wanna share?

Congrats on your begonia blooming for the first time! Not sure why your male flowers would fall off prematurely though (inadequate light levels?) That is what I've observed too - the male flowers usually emerge before the female flowers but with enough blooms going on you should have both in abundance to do an adequate job of pollination by hand. I've read some accounts where the male flowers are collected and stored in the refrigerator for weeks at a time to use at a later date when the female flowers emerge.

Usually they will bloom if they get the right amount of light. I have a lot of rhizomatous bloom indoors under shop lights but the sad fact is rhizomatous (except rex) bloom only once a year and I wished they would bloom in late spring after I move them outdoors for summer. Some bloom very well under lights, some rather poorly so when I see photos or in person of the same begonia blooming robustly in CA or FL, it makes me jealous that we cannot achieve the same thing in a colder zone.

Here are some female peltata blooms in late Feb for example.

and Selph's Mahogany blooms in late Feb too that never truly develop to their full potential (I blame it on less than ideal light levels).

Thank you for the reply. The plant that flowered was one of about 20 plants. Some inside some outside.This plant is in an east facing window. I haven't wanted to move it for fear that it will mess up the flowering. I could however move it under a t5ho hood. That should solve the light intensity issue if that's the problem. You think that might work?

I only use T8 so it would be more than enough light - be careful not to let it burn the leaves.

My male flowers still won't open. So I found flowers on this begonia at a friends house. The leaves were long dark green and skinny any idea what kind it might be? Or if its compatible with my begonia. I really want some seeds.

Identified this as b. boliviensis