Logically, a winter or late autumn or early spring flowering crocus but the two would have to be compatible species in the first place, and then, take a fancy to each other in order to produce the seeds.
![Grin ;D](http://www.srgc.org.uk/forum/Smileys/default/grin.gif)
Depending on which you wanted to be the mother and which the father, you'd have to preserve the autumn pollen safely for a few months or the spring pollen for many months.