what phenomenon are you asking about? The baseball size or the seed heads? If you go on line and research how green mountain was bred, you'll find that both were common in the breeding.

Try to duplicate it. What zone are you in? Here in Zone 4, I would have some doubts that Green Mountain was dependably hardy. My yellow multipliers are and can be grown either spring or fall planted.

I would remove the scapes next year though, as early as possible. One of the reasons to grow Kelly's Green Mountain is their extra size. Cultural experience can only help the rest of us! Multipliers the size of cooking onions is something to crow about.

My Green Mountains all went down last week and I dug them then. Spring planted and nothing really to brag about for size.

Perhaps I wasn't clear on the most curious aspect of this. I planted them Spring of 2015, they never got dug in the late summer of 2015, overwintered, and now have grown a second season, and will be dug late summer 2016, two full growing seasons. Finally, I have very large onions, and lots of flower stalks. I am in z5, Madison, WI area.

Didn't Kelly want them to produce true seed every now & then to have a way to reproduce them easier than by division/multiplication? This way seeds can be distributed more economically.

Where did you get your starts?

I think you should collect the seed, however, you only got the very large bulbs by leaving them in for two years. I think you need to estimate whether you get more pounds pulling it every two years versus just growing more normal sized bulbs each year. But, seeds are good. I always like to encourage genetic diversity.