Are they the same variety?

Where are you located? Did you use sets rather than plants? For most areas and varieties, April 1st is a good time to plant onion plants.

It was due to day length. Increasing day length triggers bulbing. The ones in late July were being grown when day length was decreasing, which is why they didn't bulb.

This is assuming you are in the north, where long-day onions are grown and the sets you planted were a long-day onion.

Rodney

I'm in Zone 4.5, all onion sets were the same variety. Likely it was the amount of daylight as mentioned. I definately need more study on this topic. Is it safe to assume then that if I want to try and get a second crop, I'd be able to do that with non long-day onions, or are those a southern crop only?

All onions bulb on increasing daylight. Longday onions need 16-20 hours which means the summer soltice. Short day onions can go on 10-12 hours of daylight which means the spring equinox. In both cases the plants need to have sufficient size when bulbing is triggered or you end up with pearl onions at best. Sets are limited to about 4 storage varieties. If you want two crops and your climate permit's, You might try two varieties with different DTM's. You will need plants to do this.

Sounds like I'll need to buy from a supplier that lists the long vs the short of it instead of the typical bag of 100 at Home Depot. I'm a little unclear about the short type. If shorts work during the spring equinox which I think is basically my first safe planting day May 15, it seems logical that I'd have a similar amount of light on the other side of the 6/21 max day light day (something like July 30). Is that accurate though? It can't be because you said plants would be needed to have to crops, or was that also in order to have mature enough plants due to shorter growing time?

Learn more about onions, check out Dixondale Farmshttp://www.dixondalefarms.com/learn

To get onions that bulb up, you only can plant those in the spring. If you plant onions in the fall, they will only be green onions. There are people who are experimenting with planting onions in the late fall and overwintering them. But you are looking more for July and bulb up by October and that isn't going to happen.

Anyone in the South who is more familiar with this then me, please correct me, but my understanding is that in that region, the short day onions are grown over winter to start bulbing in spring once the daylight hours start to increase.

So, I get your premise that onions planted at the height of summer, once the length of day hits ~10-12 hours in later summer or early fall, it will still trigger the same bulbing response. I am not sure, the fact that daylight hours in late June/early July already exceed the necessary 10-12 hours, they might just start to bulb as soon as you plant them. There is also the limiting factor of season length to contend with.

(And technically speaking, plants respond to length of nighttime darkness rather than length of daytime light, but we still talk about it as long/short day.)

Technically correct. Onions planted on decreasing sunlight don't do much. Short day onions spring planted will try to bulb. Unfortunately they end up as small pearl onions. Johnny's actually advertises a variety for that purpose. http://www.johnnyseeds.com/p-9910-eclipse.aspx Typically short day onions are grown in the winter. Plants are transplanted in late November - Early December for harvest of full size onions in May.

It is the changing (increasing) amount of light as well as the duration of the light that triggers bulbing. So if you plant after mid-June you are not going to get bulbs since light will be decreasing, not increasing.

You only get to "start over" in the light department after Dec. 20/21/22. Then bulbing will begin after you reach the critical day length which varies depending on your geographic latitude....assuming other conditions for growth are okay.Plants continue to amaze me with all the things they can sense and do.

Onions are typically planted in very early Spring for a summer harvest, at least for large bulbs. As others mentioned, you won't get big bulbs planting too late.

Don't plant sets IMO, they tend to bolt and not produce big bulbs. Much better to buy plants. Dixondale has them very affordable. Or grow your own.

If you want two crops, plant two varieties, and plant them at the same time (late April to midMay for me, depending on the weather) so they head up properly for you. If you choose your varieties well, they will be ready for harvest at different times. I always have one type that is a long keeper such as Copra (which will store for 5 months in my cold cellar but grows for around 115 days) and also one ready for earlier harvest after about 90 days. I rarely have to buy onions.