Onions are a bit different than most vegetables the casual gardener will encounter. First, they are oddly finicky about daylight length. Second, they can be grown as a fresh eating vegetable or a storage vegetable. Third, there are enough onion varieties to bog down even the most decisive garden planner. Last – but certainly not least in the confusion department – you can grow them from seed, set, or from plant starts.
Let’s try to dispel some confusion and help you decide if onions are the garden crop for you and, if so, how to grow them successfully.
1. Day Length
Onions are classified as long-day, intermediate-day, or short-day.
Near Seattle where I live, at the summer solstice the sun doesn’t set until after 9 pm, and it doesn’t get hard dark until nearly 11. In the summer we have a really, really long day. Folks further north in Canada or Alaska have an even more exaggerated day length in the summer. Us Northerners pay for our summer solar bounty in the winter, when it seems like the sun only shines for a couple hours a day.
Further south, along the Gulf of Mexico or in Southern California, summer days aren’t as long (but winter days aren’t as short).
Onions respond to increasing day-length by bulbing. Different types of onions are triggered to bulb at different lengths of day. Short-day onions begin to bulb when the day length reaches 10 to 12 hours. Intermediate-day onions start bulbing at 12 to 14 hours, and long-day onions wait until day-length reaches 14 to 16 hours before they swell to size.
You have to match the daylight of your latitude at the time of year your onions should be bubing with an appropriate day-length onion variety. If you live in the south and pick a long-day variety, your latitude won’t be able to send your onion plant the message that it’s time to bulb. That means a failed onion crop for you (or very large green onions).
Conversely, if you live in the north, a short-day variety onion won’t have time to grow a mass of nice healthy leaves before it feels compelled to start bulbing. If you’re lucky, you might get very small onions.
Intermediate-day varieties are in the middle, and are ideal for gardeners in the middle latitudes of the U.S., but can typically be grown successfully by most continental U.S. gardeners not at the extreme north or south points of the country.
2. Sweet vs. Storage Onion Types
Onions are bred for varying degrees of pungency, moisture, and skin thickness. These three things in combination plus growing conditions and curing quality account for how well an onion will keep.
Sweet onions, like our famous local Walla Walla Sweets from Washington, or Georgia’s (also famous) Sweet Onions, are absolutely delicious for fresh eating. They are low in pungency, high in sugars, juicy and mild. They are not, however, great keeping onions.
Storage onions like Copra are drier and higher in the sulphurous compounds that give onions their punch. Lower moisture and higher pungency mean longer-keeping potential. A tight, small neck and close, protective skins help keeping onions dry and cure well, and limit the potential for rot in storage.
In addition, all varieties of bulb onions can also be harvested early in their growth to use as spring onions. Spring onions are tender and mild, with a small bulb and tender, scallion-like greens. I often deliberately plant my onions a bit close, and yank every-other-onion before they start to crowd each other. The thinned onions are delicious as spring onions.
3. Selecting an Onion Variety
Selecting a variety is easier once you’ve narrowed it down to varieties that suit your latitude. Within that framework, if your goal is a delicious fresh eating onion you can slice and add to summer barbecue burgers, go with a tender sweet type. If you want something you can cure and tuck away and count on for cooking all winter long, grow a dedicated storage type.
Hot tip! Sweet onions don’t actually make the best sweet, caramelized onions. Plain ol’ yellow onions are best for caramelizing onions like a pro.
The seed catalog or purveyor you are buying from should c