Trimming the onion leaves has no adverse affect on the future bulb size. The stem/bulb size at and just below the soil level is whats important for the transplant. Pencil thick or a little under is ideal. The existing tiny leaves will probably die but new larger leaves will replace them in a few weeks after transplant. I also trim the roots down to a couple inches prior to transplant. If you buy Dixondale onion plants they have them trimmed down to about 1".

Agree with Jack. The top trimming stimulates stem development and that is what counts with transplants. Root trimming at transplant time is standard as that triggers new root development.

Do your own experiment and plant some trimmed and some untrimmed. You'll see the difference it makes.

Dave

Thanks! They should be pencil thick or better by the time they go in the ground (Mar 10-25 depending on weather).

They're a bit more rootbound than I’d like already, so if trimming roots is OK I won’t worry as much about that.

Last year even with preserving 90% of the roots, my transplants seemed to stand still for almost a month.

The issue I’m worried about is, it took so long for my onions to start really growing, that most only had about 10 leaves by the time they started bulbing.

They did well but were smaller than I expected.

As Jack said a dormant period right after transplanting is normal for onions, just as with most vegetable transplants. But 10 leaves is excellent for most onions. Bulb size can be reduced even with that many leaves for several reasons - overly hard or compacted soil, lack of sufficient nutrients, planted too deeply, insufficient water, etc. Mulching also can reduce bulb size.

Dave

HM - if I remember correctly, you did not fertilize them much early on last year. They are heavy feeders especially on the N and need to get fed from the start. Also, I find that most years onions do great and provide big bulbs and then every few years they don't do as well with basically the same treatment by the gardener.

Yes, LoneJack, and I have a better fertilizer plan for this year.

I was looking at them today. There’s little chance to avoid root loss upon removal from the Pots as packed as they are, so I might as well plan to trim both roots and shoots.

They look just like mine...well done! My containers are opaque so I can't see the roots but I know from experience what they will look like when I get ready to transplant. Won't be long until it's time to harden them off. Hope the weather improves soon.

I find it much easier to harden off onions that it is to harden off something like tomatoes. Usually as long as it’s not gonna be any warmer than 80 or colder than 40, I can stick them on a semi shady part of the deck where they get about two hours of morning sun, leave them for three or four days, then a couple days in a place with about 4 hrs of sun, and they seem to be good to go.

So based on how mine look now, and what the likely look like in 3 to 4 weeks or so, how much should I cut off the tops and roots? Maybe leave 2 inches on both ends?

Swcondly, if stem diameter is the important factor, since I’m sure I’ll have to cull some of them (I sowed too many as I always do), would culling the thinnest stems be the best way to go?

Or eat them like green onions...

Yes they are pretty easy to harden off.

Right before transplant, I trim the tops to around 4" and roots to about 2". I would be leery of trying to cull any until you separate them just prior to transplant. If you pull some out now you are likely to damage the neighboring plants because the roots are a tangled mess. Just select the best looking seedlings for planting or you can plant them all with narrow spacing and pick every other one early on as green onions.

Looking at pictures of purchased onion plants, it looks like in many cases the roots are actually somewhat dry it out, not only just trimmed.

At some point, it seems like you’re almost just planting an onion “cutting”!

The results speak for themselves, so I’ll trim them.

So what about timing of planting? Based on the forecast and outlook in my area, we probably won’t see temperatures cold enough to out right kill the seedlings, but since the next 10 days are supposed to be no more than the 40s for highs, I feel like if I put them out now, even just to start to harden them off, it may be cold enough to trigger vernalization and then they might bolt. What do you think?