if they are sending up a flower head, then they are bolting.

they will not get bigger bulbs since they are trying to re-seed. might as well eat them as they have been tricked into the second year and will spend all their energy sending up flower heads.

I agree with Robert. They are bolting. It happens sometimes. Sometimes it happens a lot and sometimes it only happens to a few of the onions that were the largest when planted. It often seems to be triggered by having the weather swinging back and forth from hot to very cold once your onions have achieved a certain size. One or two very cold nights won't cause it, but more than a handful of very cold nights will.

When they bolt, I immediately harvest them and use them. The flower stalk that comes up will split the bulb underground and the longer you leave it in the ground, the more likely that water gets into the split and starts rotting the onion. There is nothing to be gained by leaving them in the ground once they start bolting.

Dawn

Trees, I will have to agree with the others. I will toss in some of my own notes.

I have 143 Texas Red onions that I tried to trick into growing fast and large. Ten of the 143 have already bolted.
These onions were planted in the ground on 1-28, after they had sit in potting soil in a warm house to break dormancy. I have well over 300 other bulbing onions that I did not monkey around with and none of them have bolted at this point, but they are also smaller in size. The Texas Reds have bolted at the 7 leaf size, few of the others have reached that size. All of my onions were planted in Feb., or before.

Larry