Onions begin bulbing in response to day length. There are short day onions, long day onions, and a group called day neutral or intermediate day onions.
The length of a day increases as you move away from the equator. The South never sees days as long as the North, therefore Gulf Coast gardeners must grow what is known as a short day onion if we are to have any expectation of bulbing. If you have ever bought one of those bags of onion sets (small immature bulbs) from a home improvement store or chain nursery and experienced failure to bulb, it wasn’t you, it was the variety. This map from Dixondale Farms makes it easy to see the zones for each type.
Short day onions begin bulbing when the day length is between 10 & 12 hours. Houston’s average day length on December 21, the Winter Solstice, is just over 10 hours. Day length reaches 12 hours about March 15. Day length never exceeds a few minutes more than 14 hours on the Summer Solstice, June 21.
Since intermediate day onions bulb between 12 & 14 hours, one would think they would bulb here, but this is not reliably the case. We hit 12 hours in mid-March, and don’t reach 14 until June. By the beginning of this window, temperatures are not favorable to onions. This is not to say that you can’t try them. I have tried a couple of varieties and may experiment with others as time passes. You might find one that just happens to bulb sufficiently to suit you as a “spring onion” – a small bulbed salad onion.
If you live in an area other than the Upper Gulf Coast, see this calculator to determine your day length, then go to the link for the map above to determine the right category for your area.
Buying the right type
If you are buying seedlings or sets, the nursery label should tell you the day length of the varieties they carry. Most of our local nurseries buy from one large wholesale supplier. They will have sent the proper varieties, but it never hurts to verify. Never buy seeds, sets, or seedlings that are just labeled “red”, “yellow” or “white”. They more than likely will not bulb.
If you are buying seed, the seed packet should tell you whether the variety is a short day, intermediate day, or long day variety. If it does not, pass the seed up unless you are already familiar with the variety or have a moment to look it up.
I have to warn you that you WILL find intermediate and long day seed on local seed racks. In their defense, the nursery may have had little choice in the matter. Seed companies often sell a seed rack as a pre-selected set of seeds. Some even come as a shrink wrapped kit, rack, seeds and all. It is buyer beware. If you end up with some intermediate or long day seeds, plant them anyway and harvest them as scallions.