Onions

Potatoes, potatoes, rhubarb, rhubarb, onions!

What’s up with my onions?

I’ve got some beautifully bulbous Radar onions growing at the plot. The problem is a few have started to split or burst. None of my gardening books mention this and I can’t find any info on the internet! Can anybody tell me what’s up with my onions?

Splitonions.