HOW TO RECOGNISE BOLTING
The first thing you will notice is that one of the central leaves will begin to form a capsule at the top end of it. This will quickly emerge as a flower which is what you see below.
The flower head will then die off revealing the onion seeds. Even though the plant has bolted, the seeds produced will be useable next year if stored correctly.
TREATMENT OF THE ONION BOLTING
There is no chemical or biological control of onion bolting available to the amateur gardener, prevention is the only mechanism for avoiding this condition. Once an onion has bolted the best thing to do is to harvest the young onion and use it within a couple of days. Onions which have bolted will not store well at all.
The best you can do with onions that have bolted is to cut off the flower or pod as soon as you notice it and use the onions as and when you need them before any other onions which have not bolted. Bolted onions will not grow larger but they will stay in the ground in good condition for about a month. The onions will be smaller but still perfectly edible.
PREVENTING ONION BOLTING
Some gardeners believe that sowing their own onion seed reduces the risk of bolting but there simply is not enough well-researched evidence for this. There is some evidence that white and brown onions bolt less frequently than red onions but this is not guaranteed. There is however clear and proven evidence that heat treated onion sets very rarely bolt.
Heat-treated onion sets
This is the only method to prevent bolting. Heat treated sets do cost a little more compared to conventional onion sets but the difference is well worth the small extra outlay if you suffer from this problem regularly.
There is also some evidence that specific onion varieties are resistant in some degree to bolting. These are Sturon, Autumn Gold, Vulcan, Red Arrow and Stuttgarter Stanfield. For those of you who grow shallots this article applies to them in exactly the same way. Shallots which show resistance to bolting are Biztro, Yellow Moon, Springfield and Picasso.
Other common pests and diseases which affect onions include:
IDENTIFY AND PREVENT ONION BOLTING
Onions are biennial plants and this is key to understanding why they bolt. The ever changing weather in the UK fools them during some years into thinking that they are in the second year of their life and now is the time to produce flowers then seeds.
Once an onion plant has begun to bolt (produce flowers which will then produce seeds) there is no going back. There are no treatments or chemicals which will reverse the process.