Quite incredible. Onions are fascinating plants I invite you to do a little experiment if you have some planted. Onions are extremely sensitive to day length. At the moment, they are sensitive to the fact that the days are getting longer, and while this is happening they put on lots of green top growth. The sooner you can get them started, the bigger your top green growth will be. Now here comes the good part.. On the very day that the days start to get shorter the hormones in the plant start to sense that it is time to store up food for the Winter, so immediately after this day the tops stop growing and the bulbs start swelling! You watch them! This can be so with many plants but it is more easily visible when you can watch the bulb swelling. Amazing!