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Where do people source the seeds or bulbs - I'm after these?
I have grown a few different potato onions, but those Green Mountain Multipliers that Mr. Winterton created are by far the best. If you grow out some of his seeds they will not be the exact same. There will be some variations. Some may end up being better, but so far I have not found any better than the originals. It is worth trying the seeds if you can not source the bulbs in my opinion.
I've got some sprouting in a pot, but it's going to be a couple of years before I've got seed to share. Mine came from a source in Australia.
They get a lot bigger than the other potato onions. One of my main complaints from growing potato onions is that they are rather small. These however grow to an acceptable size. That is the primary reason I think they are the best. They taste the same to me as the more common yellow potato onion.I started with bulbs and I believe splitting into five to seven new bulbs is about average. You asked how long I have been growing them and this year is the fifth year, but I took a year off from growing them in that time span.They start out white with a little green and the outside wrap turns yellow in storage.Kelly has a real nice little web page with pictures and more details. I am on my phone or I would provide you a link.
If you are interested, here is a write-up of my 'adventures' with shallots/potato onions:http://www.growingfoodsavingseeds.co.uk/forum/breeding-and-genetics/new-channel/1902-giant-shallots I expect all UK growers of Kelly's seed will see not only diversity in what they grow from seed in the first generation, but also further favourable adaptation over the years, like I have seen with the Red Breton Giant shallots. Comparing notes over the coming years will be interesting.