Cut off the seed heads is all I can suggest. We are looking at a very poor harvest this year. To grow and mature vegetables need sunshine - we simply are not getting this. Bulba hoed moss out from between the veggie rows today. Iy as just been so wet... I wish we had a solution as it looks like we will lose a lot of our veggies

Agree with MG 75% of my onions have gone to seed. Though they are still bulking up slightly from what i can see.

If they came from sets then that is a common problem, even in good years. Mine are grown from seed, I have one out of the 40 that survived alium leaf miner that has bolted. Biggest problem for me has been the strong winds blowing them over.

We've never succeeded from seed Scrumpyg. possibly because of where we live. Just as sowing broad beans in autumn is a waste of time, ground is too cold and wet and they rot.

Thanks to moongrowe,scrumprat and sensei89 for info on onions going to seed.I think we are all suffering through lack of sunshine. Redshed

Yes we all are, we are looking at, potentially, no onions this year whilst last year we had a superb harvest!

I'm about to start getting up the first of my onions for the under 8 oz class. Always a bit difficult to tell the weight, but a few look the size of a tennis ball which is about right.
So it's get one up and get the scales out.
I would normally expect to get them all up in about 2 weeks time, so we are not far off normal. My biggest problem with these onions as I said has been the wind blowing the leaves over, which normally means they stop growing, but I may get away with it.
The red onions are just starting to bulb up.
Sunshine would help a bit though.

I do, as I said, grow from seed sown mid January and plant out strong plants end of April/early May, so I would guess that by doing that i've avoided the problems others are getting.

This shows it in a bit more detail

http://www.growsonyou.com/scrumpygraham/blog/13335-the-onion-blog