Onions are one of the most frequently grown garden vegetables, yet most gardeners know little more about them than how to push a handful of sets into the ground and wait for the tops to fall over. That’s a shame, because there are a lot of different kinds of onions that you can’t grow from sets: ribbon-winning state fair bulbs the size of softballs, special “sweet” onions, flat- or bottle-shaped Italian heirloom strains—even storage onions that last much longer than those grown from sets—well into the following summer!
Growing onions is easy if you keep a few things in mind. Most people think of onions as a root vegetable, but don’t you believe it! Onion bulbs are actually swollen leaf bases attached to a small, disk-shaped stem. If you want good yields you need much higher fertility than you’d normally provide for root crops. Onions are one of a group of vegetables needing abundant nitrogen early on, so they can grow a big plant, but less once they start to mature and prepare to go dormant. So, although we include them in the third part of our rotation, we make sure that section of the bed gets a bushel or so of compost or well-rotted manure for every hundred square feet to bring nutrient levels up early in the season. If you can’t get the plants up to size before the average daily temperature and the number of daylight hours trigger the bulbing reflex, you simply won’t get a big onion.
This day length (actually night length, but it means the same thing to us as gardeners) distinction is very important in deciding what kind of onions to grow, as bulb onions can be generally grouped in three categories: short-day onions, long-day onions, and day-neutral onions. While every cultivar has specific adaptations, and there is a fairly large gray area in the middle of the country, the standard rule of thumb is that south of a line running from between North and South Carolina out to San Francisco, you should stay away from long-day onions; north of that line you should avoid planting short-day onions. Southern gardeners who ignore this advice will likely end up with huge scallions, as the long-day types will keep growing without the bulbing reflex ever being triggered; Northern gardeners who plant short-day types will end up with tiny onions, as the plants will bulb while still seedling size. If your seed source doesn’t know the adaptation of the cultivars they list, buy elsewhere.
This bulbing reflex is also affected by a combination of other factors as well. High nitrogen levels at what should be the normal bulbing time will delay it or cause the bulbs to form two centers, which ruins their storage ability. Phosphorus deficiencies in the soil will also delay maturity. In addition, the temperature must also be above a certain level—about 60˚ F (16˚ C) for most cultivars—or your onions simply won’t start to bulb. To complicate matters further, once bulbing has begun the speed at which it proceeds is also temperature-related. Hot weather right after bulbing has started will reduce yields, and cool weather will increase yields, because the onions will have longer to grow before going dormant. Though high nitrogen and/or rainy weather at harvest time can reduce the storage life of onions, all other factors being equal, there seems to be no connection between the size of the bulb and its keeping quality and flavor.
The keeping quality and flavor of onions are related, though. Storage onions keep well for the same reason they make you cry when you slice them: sulfur. Sulfur is a potent fungicide, bactericide, and eye irritant. Well-grown, thoroughly cured storage onions have a high sulfur content. Many onion cultivars are also quite sweet, containing up to 8 percent sugar by weight, but if they are high in sulfur, it’s hard for their natural sweetness to shine through the tears. The sweetest onions are grown on soils naturally low in sulfur. So it’s unlikely you’ll ever grow an onion as sweet as they do in places like Vidalia, Georgia, or Walla Walla, Washington, where the low sulfur soil, climate, and cultivar all work together to produce an onion you can eat almost like an apple. But you can come close if you go about it right.
We start our bulb onions—sweet and storage—indoors in flats as much as three months before the frost-free date, so that by the time the garden is ready to plant we already have vigorously growing plants. That way we can be sure that the plants will reach a decent size before they bulb. Using a twenty-row tray, we sow the seed 1/4 inch deep, water, and then cover with a humidity dome until the seed germinates; at 72˚ F (22˚ C) it should take no more than three or four days. The first leaf to break ground will be doubled over like a hairpin. This is called the “flag” leaf, and once it unfolds, you can thin the seedlings to 1/4 inch apart in the rows. If you want to grow really big onions—blue-ribbon bulbs for the state fair—and you’ve chosen a cultivar that is appropriate for that, transplant the thinnings to individual cell packs. Plants kept in the twenty-row trays should be given an occasional haircut, if necessary, to limit their height to only three to four inches tall, so they stay upright. Just clip the tops with scissors, being careful not to cut off the tip of the young leaf in the center of the plant, which is the growing point.
These transplants can be set out as soon as they have adjusted to the outside weather and the soil is dry enough to prepare thoroughly. We might put out the first of our transplants as early as April 15, only six weeks after seeding; but the main crop doesn’t go in until almost a month later. For really big onions, set them out six inches apart on center or in rows. If you’re more interested in continuous yields than in bulb size, start extra and set the seedlings only a couple of inches apart, and pull every other one of scallions once they have grown up a bit.
Some of the classic sweet onions like the Vidalias are grown on an overwintering schedule. Southern gardeners should sow the seed of a Granex hybrid (that’s what a Vidalia actually is) 1/4 inch deep in a nursery bed or flats in late September, early October, then transplant to the garden sixty days later. Early plantings are more likely to run to seed without bulbing, according to the experts at the University of Georgia. Trim the plants to three or four inches tall and set them 1/4 inch or so deeper than they were in the flat, to help protect the bulbs from cold. Six-inch spacing is the normal recommendation for large bulbs, which, in an average season, will be ready for harvest in May.
Northern gardeners who want to try overwintering should direct-seed one of the sweet varieties like Walla Walla in a nursery bed or (if in a very cold climate) in a cold frame. Sow the seed sparsely, say four seeds to the inch 1/4 inch deep, around mid-to late August , and cover them with a floating row cover. Don’t fertilize; you want them to grow just to about 1/8 inch in stem diameter and two to three inches tall before winter. Smaller plants are more likely to winterkill; larger ones are likely to run to seed without bulbing. As soon as the ground can be worked in spring, gently lift the plants, trim the roots and tops, and set them in the growing bed on a six-inch spacing. Care from that point on is the same as for spring-sown plants. NOTE: we did extensive testing in our Vermont garden (minus 35˚ F. annual low) and found that spring transplants, if started very early and grown rapidly, work just as well.
Most onion diseases are best prevented by crop rotation. The two major pests are root maggots in the North and thrips in the South. The onion root maggot is very closely related to the cabbage root maggot and controlled the same way. If you have a history of infestation, place a floating row cover over the bed immediately after seeding or transplanting. My grandfather used wood ash as a maggot control on onions as well as cabbage family plants. Thrips can be controlled in a number of ways. They can simply be washed from the plants by a strong stream of water, or sprayed with an insecticidal soap; since they migrate into the garden from drying field grasses, keeping the area around the garden mowed will help some. Vigorous plants can outgrow thrip damage, so be sure to keep your onions well fed. Here is a good digest of onion pests and diseases from UC Davis.
To produce the best bulbs, make sure your onions don’t lack for water. During their first few months in the garden they need an inch or more of water a week. Irrigation should be cut back at bulbing time, though, so they can cure properly.
The biggest problem with growing onions is weeds. Because of their limited foliage, onions don’t compete well for space with most of the common garden weed species. To make sure the weeds never get established, we go over the bed every few weeks, using a narrower hoe each time. Once the onions begin to bulb, stop cultivating and just keep an eye out for any weeds that grow up and threaten to set seed; it’s rarely more than a few, and they are easy to pull up and discard.
With overwintered plantings that will not be moved, a new technique that works well is to cultivate the bed in the early spring and apply granules of corn gluten meal. This is a by product of the production of high fructose corn syrup, and has two qualities that make it very useful in onion culture. First it has an analysis of 10-0-0, which means if applied in spring, it gives the plants a shot of nitrogen just when they need it, that wears off b y the time they start to bulb up. Second, as it dissolves, it releases a natural substance that inhibits the germination of annual plants — in this case, annual weeds!
Once half of the tops have fallen over, bulb onions are ready to harvest. If you can, wait for the beginning of a dry spell, then bend over the remaining plants, and a few days later pull the plants and lay them beside the row to cure for two or three days. If wet weather threatens they can be removed to a dry, airy shed to finish curing. Once the outer layers of onion skin become dry and opaque, and the necks are shriveled and dry, either braid them or put them in mesh bags and store in a cool, dry place. Discard or use right away those bulbs with thick necks, double hearts, or any other imperfection; they won’t store well.
This curing process is very important. The storage life of an onion is closely related to curing conditions, because the resistance of the bulbs to rot organisms depends partly on how well the scales, or outer skin layers, of the onion have dried. Also, the internal color of red onions will not be consistent unless they are allowed to mature fully, and then cured properly.
Onion cultivars are classified by shape and color. There are flat, top-shaped, globular, and bottle-shaped onions. The range of colors is red, yellow to golden-brown, and white, though not every combination of shape and color is represented by a cultivar currently available as seed.
Most onion sets are yellow, though you’ll sometimes see red and white sets, too. While the sets themselves are usually globular, the bulbs at maturity are likely to be flattish, as it is the nature of onions to be flatter at maturity than as sets. When buying sets, choose those that are 1/2 to 3/4 inch in diameter; smaller than that they may not be sufficiently vigorous, and larger sets are more likely to run to seed without forming a bulb. If you have to buy them pre-bagged, simply plant the off-sized sets in a different spot where they can be pulled for green onions, and fill your main planting with properly sized ones.
In the North, the standard for long-day sweet onions is Walla Walla, an open-pollinated heirloom cultivar first brought to this country by a Corsican immigrant almost a hundred years ago. It takes its name from the town in Washington State that has become famous for growing it. For many years the seed stocks were jealously guarded, but it is now available. It produces a very large, mild bulb anywhere it can be overwintered, and is reliably hardy to about 0˚ F (-18˚ C) unprotected, if there is decent snow cover. We have had good luck with early-sown, transplanted spring crops as well. There are other sweet onions of this type, including one called the Willamette Sweet, which many consider a Walla Walls strain particularly well adapted to the Willamette Valley of Oregon. A newer high-yielding, hybrid, overwintering sweet onion that is even hardier is Sweet Winter.
In the South, the standard short-day sweet onion is the Granex hybrid. There are a number of strains, each adapted to different conditions or growing schedules. The actual cultivar used to grow Vidalia onions is Granex 33. This hybrid is a cross between two venerable Southern cultivars, Bermuda and Texas Grano 502. If you are a seed saver you might like to try these parents instead of the hybrid.
A red sweet very popular where it can be grown—it is from an area on the line between long-day and short-day—is Stockton Red; some sources list it as Fresno Sweet Red.
I especially like bottle onions. They don’t store well, but they are quite beautiful, not too pungent, and their shape makes them easy to slice. They are also relatively daylength-insensitive, and, due to their shape, tolerate closer planting than other varieties. At maturity they are about the size of a standard pickling cucumber: 1 to 1 1/2 inches in diameter and four to six inches long. The only two cultivars I know that are currently available are Owa, a golden yellow cultivar from Denmark, and Red Florence, which is also known as Rouge de Florence or Red Lucca, a nonhybrid heirloom from the region around Florence, Italy. This is the cultivar that many mass market seed sources list as Italian Red Torpedo, or even just Red Torpedo.
To grow exhibition-size onions, try the cultivars Lancastrian or Kelsae Sweet Giant; these two can get as big as a soccer ball if pampered, and average five pounds or more each. In the extreme North Ailsa Craig may work better. All three of these are English cultivars and are adapted to very long days due to England’s latitude. For the best results, make sure to start them early, and provide plenty of fertility and water early on, with at least eight-inch spacing. None of these varieties is a good keeper.
Most of the good storage onions are yellow, globe-shaped, long-day cultivars. In fact, the traditional onion of this type is named Yellow Globe. There are, however, white and red strains as well, and a whole plethora of names, some of which, like Southport Red Globe, include a place name to indicate their region of adaptation. One short-day onion that is a good keeper is Creole or Red Creole, a flat, pungent onion resistant to thrips but prone to bolt if planted too soon in the fall. Any of these is likely to be a good bet for the kitchen gardener, but keep two things in mind: first, the red and white cultivars rarely keep as well as the yellow; and second, many of the traditional cultivars are not as well maintained, now that most seed companies have shifted their attention to hybrids, so you should be certain to buy only from reliable, well known companies, not just from a seed rack at the hardware store.
For storage, try the newer hybrid types. They change fairly frequently in the seed catalog trade, so if the cultivars I note aren’t listed, look for a reference to them in the catalog description to find one with similar characteristics. For an early storage onion, try Buffalo; it is milder than many others, hardy enough to overwinter, and relatively insensitive to day length, so it can be grown a bit farther south than many others. It stores moderately well, lasting until around the first of the year. For a longer storing onion, try Copra. It matures a full month later than Buffalo, but will keep until the following spring with little loss. It is also high in sugar, so once cooked—which reduces pungency—it is quite sweet. For a storage onion that gets sweet on its own during storage, try Sweet Sandwich. This excellent-keeping, super-high-yielding variety matures at the same time as Copra; yet after three months of storage becomes sweet enough for fresh use in salads. It does not hold up well when cooked, though, so it should only be used fresh. For a red storage onion you can try the hybrid Mambo, or the open-pollinated Bennie’s Red, which will store at least until Christmas.