I leave my onions in the ground until the neck is very soft and the leaves are no longer feeding the bulb. Many times I've left them much longer with no problems.

TomNJ

Thanks Tom, that's good to know. I'll leave them in the ground awhile longer then.

As far as I know the only harm from leaving them in the ground after the tops have fallen is that they could get sun-scald.

I have had issues with wireworms entering the onion through the root end when I have left them in the ground after they stopped growing. Then the onion starts to rot.

IF you leave them in the ground too long and you get a lot of rain they may start growing again.I have had this happen a time or two.Also once the tops fall over the bulb will not get any bigger. YOPPER