[bayleaf]

Last year I grew some onions (S giant) from sets. Grew well and we have been eating them all winter. After winter in the shed some started to grow while in the netting they were hanging in so I put them in the plot just to see what would happen. Nearly all of them have produced twin bulbs and  are all bolting. Why is this - are they biennial? Should I just dig them out and use straight away or leave them in and do something else?

[WG.]

are they biennial? Should I just dig them out and use straight away or leave them in and do something else?

Yes.

You can eat the tops as onion greens (now or later) but you will not get a bulb.  You could allow them to seed & save that if you were so inclined (provided they are not an F1 variety).

[bayleaf]

thanks WG that expalins a lot.