I start mine outside with planting trays in partial shade so no experience with inside planting. I transplant in December.. I would try to emulate the natural daylight hours when using lights inside. 12- 10 hours.

Thanks. Now, a definitive short-day onion is 1015, and those are normally planted on 10/15 (which is where the name comes from). Daylight hours then are 11.5 hours. It's about 12.5 now. Now, short-day bulbing is supposed to happen at 10-12 hours for short-day, so one has to believe that the daylight hours for seedlings doesn't really make a difference for 1015s, and force them to do bad stuff, since they are normally planted when the daylight hours is supposed to make them bulb. In fact, my daylight hours never get any shorter than 10 hours, so the whole issue of daylight hours for short-day seedlings gets iffy. If I had to grow short-day seedlings outside, with daylight less than what is required for bulbing, they'd never get planted!

That being said, maybe planting onion seedlings in partial shade tricks them into believing there is less daylight??

I could seed them in pots outside in partial shade, but it's kinda hot out there right now. I'm trying to jump-start my regular planting-out time by a few weeks, which would ordinarily be in mid-November, in order to get a slightly earlier harvest in the spring.

For me the only purpose is holding down soil temps. Onions hate hot feet. They have to attain some degree of maturity before they respond to trigger. They are transplanted at pencil size which is about the equivalent of puberty. If there sufficient daylight to trigger at that time, they will be small. They need to put on some growth before triggering.

Just want to say that I have never come across a reference that says to keep long day onion seedlings under lights only 10-12 hours a day. And my own experience of keeping the lights on for 14+ hours proves it has never been detrimental to them.

Rodney

I've seen it in several places.

"But, planting onion seeds indoors under grow lights is a bit more nuanced than growing other vegetables from seed. When growing the seeds of tomatoes, eggplants, and other veggies indoors under grow lights, the lights should be on for 16 to 18 hours per day. But, if you grow onion seeds indoors under grow lights and leave the lights on for that long, it will initiate an early bulb set and result in puny onions. That means that if you want to start onion seeds indoors under grow lights, start very early and only leave the lights on for 10 to 12 hours per day."

https://savvygardening.com/why-planting-onion-seeds-is-better-than-planting-sets/

"If you start onion seeds under lights, remember to turn the grow lights off to give the plants a suitable night. Onion seedlings will bulb too early if exposed to long days at any time during their development. You will not get anything bigger than sets." - Jarvis, B., Yard & Garden Line News, University of Minnesota Cooperative Extension, Volume 3 Number 2, February 1, 2001

But yes, that makes sense that the daylength bulbing triggering requires some maturity.

I think it may depend on the length of time that you carry them under lights. 10-12 weeks seems to be the sweet spot for me on my mostly intermediate day types.

When I started my own ( I just use Dixondale now for bulb onions) I would start them around New Years day and give them 16 hours/day of light. Transplant out in mid March most years with 4 or 5 leaves and about 3/16" stems at the soil line. If I kept them indoors for too long the 16 hours of light may in fact trigger premature bulbing. IDK.

I would also give them haircuts about once a week after week 5 or 6 to promote thicker stems and keep them from flopping over.

Thank you. I believe that has to be right - that hours of light doesn't make any difference until maybe the onions get older. And yes, the idea of haircuts is a good one, but I'll probably transplant out when they've got three leaves.

My nursery gets plants from Dixondale, but not usually until November. I want to plant some out in October.