Joan, I personally wouldn't want to grow onions sets in containers but each to their own. I would imagine that the difference between the ones grown in containers and the ones grown in the ground would be that the soil the containers would be free-draning and sheltered and the ones grown on the soil could be more on the heavy side and not draining so well and overwintering will produce a check in growth. If you have the right conditions then ordinary varieties like Turbo (which were always good keepers) if grown in early spring will grow on quickly without check.

They do better with good drainage. The grittier bit of my veg plot is more successful than the wetter part. So I suspect that is the reason. Dig in some grit or mulch to aid drainage.

Thanks, both of you! I suppose the answer is poor drainage and wet rot, if that is the term for onion failure. My container grown onions are always OK though some on the small size. I will try to get hold of some gritty stuff to scatter where I intend to grow the remainder of the onions.

Perhaps buy some sharp sand (doesn't have to be horticultural grade) and dig it in where you are going to plant. Onion are deep rooters. The more space between the sets will produce bigger onions.

Thanks jimmytheone: I preempted your tip and ordered some horticultural grit on Amazon! It should be here within the next few days. I will remember to dig it in deep and have been looking at how far apart to plant them. I can see why you don't favour onions in containers. They are just big enough for me but I realise I am never going to get large ones that way. The upside of containers of course is fewer weeds - of which I am not a fan - and more control over bugs! Cheers!