The normal green onions are allium fistulosum versus chives which are allium schoenoprasum. Since they are different species, I'm pretty sure that they won't cross pollinate but if you're looking to save seed and want to be 100% sure you could bag / cage one or the other. For me, chives are done flowering a couple of weeks before any of the overwintered onions start up so there's time isolation also.

This is the first year that I have had chives and green onions in the ground over winter and the first time they have flowered for me. Both have been flowering for a month or more now and neither shows any sign of quitting. I am not saving seed. But I am also not being careful to prevent the seeds from naturally falling where they may. So I am sure that some of them will germinate and was concerned that they may cross.

I was hoping that they were different species and that would prevent crossing. If it does not, I may end up with super chives or something...;)

Thank You

Steve