I plant onion sets every spring and, with the exception of Red Baron, they all flourish. The Red Baron variety invariably bolt in early summer, although they are planted and fed in exactly the same way as the other varieties I'm successfully growing like Sutton and Sturon. Can anyone explain why this is happening and offer advice?
I've just done a bit of a search, and quickly confirmed @TeaDrinker's observation that red onions are much more prone to bolting.
Stress is one problem (as with most plants where you want to avoid bolting) - so stable watering is going to be important.
I found one suggestion that seeds are less likely to bolt than sets. Not sure about that but it is worth a try.
Another common comment was that it is a measure of set quality. Ie. low quality sets from dollar stores, Wilko's, Walmart, etc are much more likely to bolt because they have not been treated properly.
In particular, there is meant to be a heat treatment which greatly reduces the chances of bolting (eg. this thread ).
So it is probably worth trying to find some sets from a quality nursery who has heat-treated them? If we knew what the heat treatment involved exactly, it could be something that the gardener could do.