[mdjlucan]

I planted them on the 30th of the fourth 2016 I noticed this afternoon that they started to Bolt can you tell me what is causing this

[Salmo]

This may be due to poor quality sets but is most probably due to the cold Spring.

[Trikidiki]

Any stress will cause the to bolt.

Cold spring as Salmo said, hot dry spell, sudden water after a dry spell, damaged roots from overzealous hoeing.

If the plant feels its life is threatened it will bolt to try to produce a new generation before it goes. The only 'surefire' way to stop it is to use heat treated sets that have had their flower 'embryo' destroyed. However this is not always 'surefire' as if the heat treatment is not done properly the embryo may survive, and the heat treatment itself could be a trigger for them to bolt.

Growing from seed helps as the onions are biennial so should only produce a flower in their second year.