[sunshineband]

Interesting as I always trim leek roots when I plant them out but never thought of doing that for onion plants.

[mumofstig]

It is just the same method used for leeks, so no doubt it would work 

[jambop]

OK so here is a you tube video of someone grow by a similar method. he uses a shallow tray to grow them on whereas I would use a 8" deep pot so that there is a decent amount of soil for the roots. He is also usinh lighting to bring them on whereas I just put them in the green house and let them grow until I was ready to transplant them. I did not trim the tops of my plants so my ones where a fair bit taller than his. The end result though was some really nice onions. 

             www.youtube.com/watch?v=xvPnkYq2QQ8

[New shoot]

The variety was an F1 called mammoth as I recall ... I have seen plenty of people on you tube videos who have grown them broadcast on seed trays and grown on until planting out. In this case the video I saw the person growing them did it the same way every year and he actually trimmed the roots at planning out time. I will try to re-find the video and post it. I have no doubt at all that the method is sound.

I've seen the YouTube videos as well, but unless it is someone I watch regularly or I've tried it myself, I tend to take things with a bit of a pinch of salt.  Onion seedlings are tough and will put up with a lot, so I'm sure you would get something.

If Mammoth grown this way ends up as a 4-5" diameter onion, it tells me they did struggle a bit.  Fine if you want smaller onions, but perhaps you need to start with seeds for a whopper to get them to this size.  It would be interesting to see how smaller varieties fared under the same treatment. 

[jambop]

To Be honest for kitchen use anything bigger than 3" diameter is excessive in my opinion so if I get onions of that size I am delighted. What was interesting that year was that I was very late getting the onions in the ground they did not go in until second week in May! The ground was not prep'd in advance just turned over with the rotavator and raked down and a bit of general fertiliser added, then we got rain and the ground was unworkable so had to just dump them in... hey presto perfect onions... makes you wonder about all this dig in plenty of well rotted manure and add a general fertiliser etc,etc