Onions and day length

I’ve tried to use my Google-Fu to find such info, without a lot of solid information.

I was trying to find information on how exactly bulbing in onions is triggered. A long day needs “14 to 16 hours” but what does that really mean? Our max day length here is 14:57 on June 21, and most, but probably not all, long-day varieties will bulb up fine here.

I am guessing it has to be over the minimum length for a certain amount of time. Meaning a single “long day” over the threshold is not enough to trigger, but instead, the plant must first be physiologically mature enough, warm enough, and the day length over a minimum threshold for multiple days or weeks.

And even looking at intermediate day onions, which are said to need “12 to 14 hours”, they certainly don’t bulb the minute I put them out in the ground, even though days are already 12 hours long by then. Intermediates start bulbing in late May for me, and long days around early to mid June. I have not grown short-day onions.

Can anyone enlighten me?

I’m not sure it is what you are looking for, but there is a useful chart over at Johnny’s Seeds that lists days to maturity, day length category and most importantly the specific latitudes most suited for each variety they sell.
It is at: Full-Size Onion Varieties | Dynamic Comparison Chart | Johnny's Selected Seeds

Day length types are just general categories and we are sort of between intermediate and long day in our area. Based on the latitudes listed, some long day just require longer days than we’ll ever have at 38 north. For instance red wing looks like you need to be at 43 north (day length over 15 hours) to even consider growing them. For a red onion, I grow the intermediate day Cabernet which lists 35 as the lower end of latitude. I think this should mean it requires a slightly longer day than other intermediates that list 32 as the lower end of latitudes, giving me more days growing the tops before the energy goes into the bulbs, but that is just a guess. I know they did great for me last year.

I like Johnny’s and have seen that chart. If anything I think their latitude rankings are somewhat conservative, being aimed at commercial growers whose livelihoods depend on it. I know a few people who have grown red wing at 39° and they bulb up ok. I’ve grown Candy, Expression, and Monastrell for the intermediates. Patterson (a long day rated for 38°) does well for me, too.

I’m inquiring more about the deeper details. The science behind it.

Candy is my go to onion. Very nice for us.

Dixondale farms has good info. I have really good success using their starts.

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I just finished reading Gardening Under Lights by Leslie Halleck and I think a lot of the information you are looking for is covered in there. I would not be able to do it justice. She goes really indepth on the subject in a very easy to understand way. It has more to do with night length than day length actually and how there are phytochromes in the plants that change their chemical makeup depending on how much infrared wavelength they get without red light wavelengths being present. This daily change and chemical buildup serves as a trigger for plants depending on where they originated and their latitude. It also involves the atmosphere bending the light more blue above the 40th Parallel vs more red south of the 40th… This all makes way more sense in her book. I had been digging around the internet trying to figure out pretty much your same question when my SO reminded me that Joe Lampl’ had Leslie Halleck on one of his podcasts… I wish I’d read the book before spending over a week on Google.

I hope this is helpful.

I forgot to mention that she has some tricks about tricking plants into thinking the nights were shorter in that book as well.