Some onion varieties love it here, others not so much

The courageous green tips of narcissus and daffodils, just beginning to poke up, let us know spring is on the way. After all we’ve been through this fall and winter, I suspect spring will bring some extra exuberance as beauty and warmth return to these mountains. While the soil stirs awake, I’m back to answer your gardening questions each month here in the Xpress. Please send them my way at gardening@mountainx.com.

Daylight-specific onions

I’ve heard that different kinds of onions grow better at different latitudes. Which is good to grow here?

Indeed, onions are triggered into forming bulbs (onions to harvest) by the hours of daylight they receive, also known as daylength. At first, onion plants are tiny little things and quite cute in my opinion. Their seeds are about as big as poppy seeds, and the plants emerge looking like grass that’s no thicker than a pencil lead. For their first couple of months, the plants’ attention is focused on growing green leaves, or aboveground parts, along with roots. When days get long enough, the onion plants switch gears and transform the bases of their leaves into juicy, pungent, round bulbs that we cherish to bring flavor to our food. They do this not to delight our senses but rather to store sugars for the coming year.

You see, onions are biennial plants, meaning that they complete their reproductive cycle over two growing seasons. Beets, celery, radishes and carrots are other garden vegetables that share this quality. During the first year, growth is focused on catching and storing sunlight in the form of sugars, usually cached in a starchy root or bulb. Then the plant rests over the winter, with energy on hand to fuel rapid growth the following spring. At that point, its attention will shift to flowering and making seeds.

So, back to the question of latitude. The word refers to how far north or south of the equator a particular location lies. Since we’re in the Northern Hemisphere, our latitude is measured in degrees north, usually written as ° N. The town of Asheville sits at 35.6° N, meaning it’s just over 35 degrees north of the equator. To put this in context, Miami is at 25.8° N, and Portland, Maine, at 43.7° N. What does all this have to do with onions, you ask? Well, it turns out that latitude corresponds with how long or short the days are at any given time of year. If you’re standing right on the equator, days and nights are 12 hours each, all year long. The farther you move from the equator, the longer the days will be during summer, and the shorter they’ll be during winter.

Onions are roughly grouped into three categories based on the daylength they need to trigger bulb formation. These categories are: short-day, intermediate-day (also known as day-neutral), and long-day.

Short-day varieties tend to do well from 25-35° N and begin to form bulbs when the daylength is between 10-12 hours. They are typically planted in fall and harvested the following late spring, before it gets too hot for onions to be happy in the southern regions where they’re grown. Short-day onions are more perishable than intermediate or long-day onions; they won’t keep well in a root cellar.

Intermediate-day (day-neutral) onions grow best from 32-42° N and are triggered into bulbing when days are between 12-14 hours long. In cooler climates, they are usually planted in spring and harvested in summer. In milder climates, they can be planted in fall and harvested the following spring. These onions are typically sweet in flavor and keep better than short-day types but not as well as some long-day varieties.

Long-day onions thrive from 37-47° N and bulb up when daylength is 14-16 hours. This group includes sweet and spicy varieties, with some that keep very well and others that are more delicate. The majority of the big onion-producing regions in the U.S. (Washington, Idaho, Oregon) lie in the northern region, so long-day onions are some of the most commonly grown and eaten.

Zooming back in now to your garden and this year’s onion crop: Remember that Asheville is at 35.6° N, right in the middle of the various categories. Sure, long-day varieties usually thrive just a smidge to the north, but remember that these are rough categories. Daylength adaptation is on a continuum, and specific varieties in each category may form bulbs in a wider daylength range than others.

Since my own onion-growing goals have been storage onions to keep through fall and winter, I’ve mostly grown long-day varieties, and with good success. My two favorite yellow varieties are Dakota Tears and Clear Dawn. For a red, I like Rosa di Milano. All of these have performed for me under organic growing conditions and moderate fertility; they’re tough, work-horse onions that are also tasty.

Intermediate-day onions will likely do great here, too, and are a good choice if you like big sweet slices of fresh onions on burgers, in salads, or otherwise eaten fresh, rather than stored for later. Short-day onions would probably not work well here because they are typically sown in the fall and harvested in the following spring. This is a great way to get early onions if the winter is mild, but our winters (especially this most recent one) are hardly that. If you happen to have a hoop house, you could try short-day onions overwintered there, where they will be protected.

The problem with planting the wrong type of onion for your location is that they either will begin to bulb too soon, resulting in small bulbs, or they won’t bulb at all. Other factors that contribute to onion-growing success are fertility, water and sunlight. Onions have pretty shallow roots and will suffer from underwatering. They are also heavy feeders, requiring fairly rich soil to grow large. Since they have slender, upright leaves with less surface area for photosynthesis than, say, a bean or squash plant, it’s especially important that onions get plenty of direct sunlight. In fact, if you grow onions in a shady spot, or your garden is in a deep valley, this can affect how they perceive daylength and can affect bulbing. If onions are under stress from poor conditions, they can begin to bulb earlier than they’re “supposed to.” This makes a lot of sense, since bulbing is the first step toward making seeds and ensuring the next generation of onions. Heavy stress tells the onions, “Hey, you might not live until next season, might as well make seeds now so there can be a future.” Keeping onions well weeded is another important step to reduce stress and improve growth. I like to weed them once or twice after transplanting, then mulch heavily once they’re 6-8 inches tall with straw or leaves so that weeds are inhibited and soil moisture is retained.

Hopefully, this helps your onion endeavors. Remember, too, that you can get a similar tangy taste from scallions (green onions, no bulbing involved), and there are a great variety of smaller-bulbed perennial onions to explore, too. I recommend checking those out at Southern Exposure Seed Exchange.