Long ago (like 3 homes back), I started growing onions. My first foray into it was a 50/50% success rate.
I got lucky with the green onions (scallions), but the regular bulbing onions I planted (an onion set) produced nothing. It took me a couple of years to understand why where you live and variety mattered. That first onion set I planted? They were short day, yet were being sold in Washington State. It was my fault for buying them at a big-box store that sends the same items to every store, no matter the location. Thankfully, I only lost about $20 that time. After that hot mess, I went to growing from seed and having far more success because I had bought the right varieties for my area.
Note: Green Onions are treated as being neutral for short/long day, as you harvest them quickly. I find most of them never produce a bulb; if you ignore them, they are more likely to bolt, flower, and produce enough seed for the rest of your gardening career in summer.
Sweet onions growing in the ground, close to harvest time.
Short And Long Day Onions:
There are two distinct types of onions. Short and Long Day. Seed packages should have that noted on them, but not always. Starts will often not mention this. So always Google the type of onion in front of you before purchasing.
Short Day:
These onions begin to form bulbs when the day length is 10 to 12 hours. The Deep South has fewer hours of daylight in summer than the North, but more in winter.
Long Day:
These onions require at least 14 hours of daylight. In the north, summers can have 16 or more hours of light.
Where You Live:
In the map of the United States above, there is a line that goes across the US. It is the 36th Parallel. The “Sun Belt” is below this line (shown in red). Everything above it is in grey. Short Day onions are for those below the line. You might be in the “south” if you are below the Mason-Dixon Line, but you are not in the Deep South!
Seeds, Onion Sets & Starts:
There are multiple ways to grow onions.
Seeds:
The world is your oyster if you pick seeds. You will have a vast choice in what you can grow, only limited by the seed companies you buy from. However, you will need to start far earlier than other choices.
Onion Sets:
These come in bags as tiny bulbs. They look like pearl onions. Over the years, I have found that they have the lowest survival rate of all methods. Maybe it is me. Maybe it is that these onions are often stored in places not good for them (in warm stores, brightly lit)
For best results, if you live in a milder winter area, plant in late fall, when you plant garlic. If in an area with real winters, plant in March/April once you can work the soil. It’s OK if it is still freezing at night, just wait till you don’t freeze below 20* at night. Frosts are fine. Plant like you do garlic.
Starts:
These come in two ways.
Dormant bare roots, sold outside at plant nurseries and feed stores (often by the seed potatoes). They are usually quite sturdy and very easy to plant; you don’t have to be delicate with them. However, you need to get them in the ground before it warms up, as they need time to grow roots before it heats up, reducing the chance of onions bolting to flowers/seeds. Keep that in mind.
For good results, let your dormant starts acclimate to the outdoors for about a week. Some people soak them before planting, and I have done both ways. If they seem dry, this can be good. Before planting, trim the green tops to about 5″ tall, if not already done.
Seed starts – a 4″ pot with many sprouts, sold with other plant starts at plant nurseries. The onions are barely pencil-width. You turn out the pot and gently separate the starts, then delicately plant them in holes you make. This is highly cost-effective, yielding 20 to 50 starts per pot. Start them once the soil has warmed for the best results.
Onions are curing, using a wooden pallet.
~Sarah