the method is fine. I do not trim them. Sure they like nitrogen, who doesn't? big tops make big bulbs. In my very composted garden P, once nearly absent, is now above optimal. If you, too, have composted in the past, you would be better off giving them some urea, and that's it.

I'm not sure I'm correctly understanding your question, are you trying to grow regular bulb onions? If so, I think your techniques are very different, but I am in a very different part of the country, so take what I say with a grain of salt! If I had your tray of sprouts, I would make the holes a little less deep, and certainly not trim off the leaves. Unless it is crazy hot there, but I would mulch with straw or something rather than sacrifice the leaves. Keep them well watered for a week or so after transplant if it is dry there, or just keep an eye out for wilting if it is not.

Not so sure about the phosphorus, are you thinking of them as a root vegetable? A lot of gardening manuals do that for some reason, but onions do like nitrogen because they really are a leaf crop. That bulb we eat is the swollen base of the leaves, the roots of onions are small and comparatively unnoticable. Up the nitrogen and I think you will be happy with the results. Cheers!

Concur. the onion is a leaf vegetable with a very high requirement for nitrogen. They bulb almost on top of the ground. At three inches they may never bulb or if they do be very small. In your area, make sure you have short day onions for winter planting. Just push the plants into the surface of the soil so the top of the tiny bulb is slightly above the soil surface. Onions are easy to grow, but planting them too deep or covering the growing bulb is the major reason for failure.
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Agree that cutting off the tops is neither necessary nor desirable.

As mentioned, "make sure you have short day onions for winter planting. " That is crucial to your success. I suggest some reading about how to grow onions on the Dixondale Farm Onions website. Great information.

Dave

Thanks everybody for the feedback -- hopefully I can be more successful.

Sunnibel: Yes. Bulbs in this case.

Farmerdill and digdirt. Yep, got some Red Burgundy which I have read are a short-day.

Thanks again everybody.

Kevin