I'm guessing the heat and/or shortening daylight.
This is OT I think, but I wanted to comment on the multi-headed mammoth.
This volunteer I have growing has a sturdy speckled trunk that is about 2" in diameter, holding up the multi head well without any support. I'm guessing it's about 8-10 feet tall.
Except for the speckled trunk, it looked like it would be a normal single headed mammoth initially (the biggest head -- already fading and on the other side -- is about 10-12" across). Unless it was a seed dropped by birds, the seeds would have come from small flowered multiple that I grew the year before. The funny thing is that I *thought* I planted from a mammoth head of seeds I had saved.
Only explanation I have is that the original mammoth had been cross pollinated with a small-flowered multi-headed variety -- I think it was out of "sun samba mixture". I like this one a lot. I'm going to try to save the seeds from the first to bloom biggest head and see if the interesting traits this one showed will repeat in the next generation.
Learning never ends because we can share what we've learned. And in sharing our collective experiences, we gain deeper understanding of what we learned.