Onions Sets.

Hi Bumbalina , I have cut and pasted from here - it does explain it well
http://www.gardeningknowhow.com/edible/vegetables/onion/onion-bolting.htm
Bolting is When an Onion Has Flower Buds
When an onion plant prematurely sends out a flower stalk, it’s referred to as onion bolting. Onion bolting is a natural process that occurs when the plant is under stress. While we gardeners may enjoy the beauty and taste of our plants, we mustn’t forget that the plants sole purpose is to reproduce. When you notice your onion has flower buds, you know Mother Nature’s whimsy has put the plant in panic mode – Help! Help! I’m going to die! The plant’s genetically coded response is to reproduce as quickly as possible and that means flowering!

 

Ahhh, OK I see. And bolting is bad because it takes energy away from the growing onion and it gets that green centre in the onion that makes it rot from the inside out?! Madness. Who thought a flower could be so undesired lol. Thanks HarryS! BTW I think we have lost silu to Wilkos online lol.