Growing Onions: Some Tips and Techniques

Growing Onions: Some Tips and Techniques

The onion: it’s in almost every kitchen, but we’d never thought about growing onions until we moved to farm country (Minnesota), where it seems everyone has a huge kitchen garden. Here is where we first learned about short-day and long-day onions, seedlings and sets, and storing these pungent bulbs that add so much to our simple living cuisine.

Onions are another member of the Allium genus, and like garlic, they contain high levels of beneficial nutrients. World’s Healthiest Foods, the authoritative nutrition website, says of onions, “With their unique combination of flavonoids and sulfur-containing nutrients, the allium vegetables such as onions belong in your diet on a regular basis.”

But we would use onions even if they weren’t powerhouses of healthful polyphenols because they taste so good! We add them to beans while they’re cooking, to vegetable/bean filling for burritos, and too roasted root veggies and pasta sauce.

Want to Grow Your Own?

Growing onions is easy if you are patient and know which kind to grow in your part of the world. Here are some points we think are important if you want to grow your own:

Onions can be grown in three different ways: from seeds, from sets, and from transplants. We’ve done all three; each has its advantages and disadvantages.

Onion sets come from seeds germinated last year. They are grown to marble-sized bulbs and then stored over the winter, sold for spring planting, and harvested in the late summer or fall. They are convenient, but can be expensive and don’t keep as well as onions are grown directly from seed. This method is our least favorite way of growing onions.





You can direct-seed onions in the garden in the early spring. This method was frustrating for us because onion seeds take a long time to germinate – it seems like they will never come up! With patience and protection when they’re tiny, this method produces onions that keep well and is the method of choice if you want to grow a lot of onions.

The best method for us is transplanting. Seeds are germinated indoors in flats of potting soil (be patient!), nurtured in the cold frame during late winter, and planted out into the garden when cold weather is past. This method makes the most of every seed, and the onions are grown this way keep well if they’re stored properly.

Another consideration when you’re growing onions is day length. Onions respond to the length of the day to know when it’s time to form a bulb. Different varieties are adapted to different latitudes. In the northern U.S. summer days can be 16 hours long, whereas here in Arizona the longest days are only about 14 hours long. Gardeners in Minnesota grow long-day varieties while growers in Florida grow short-day types. Your seed catalog or garden center will tell you which types are adapted for your latitude.

So you’ve chosen a variety, a growing method, and your onions are lookin’ good. How do you tell when they’re “done”? When the day length is right your onions will form bulbs of golf-ball to softball size, and the leaves will begin to turn yellow and die back. The necks of the onions will soften and the tops will fall over. This falling over is the signal your onions are ready to harvest.

Gently lift them out of the soil with a spading fork and let them dry for a few days in the garden if the weather will allow. If it’s rainy, bring them indoors to a warm dry place. Let the tops dry and the necks thin down. Don’t cut the tops off until they’re completely dry! Store your onions in a cool dry place indoors where there’s plenty of air circulation.

Foundation of a great dish

Onions are an essential, tasty part of our cuisine. They’re members of the large Allium genus, which also includes garlic, leeks, and chives, as well as the many ornamental alliums in perennial gardens.

There are three edible groups. The familiar bulb onion (A. cepa) is in the Cepa Group. Picked young, when it has a small, white bulb and green tops, it’s known as a scallion, or a green or spring onion. Left to mature, it’s used in cooking or salads. Shallots produce multiple bulbs from a single planted bulb and belong to the Aggregatum Group. Welsh, or bunching, onions belong to a different species altogether—A. fistulosum.

Bulb onions can be grown from seed or sets, which are small bulbs harvested the previous summer and stored over the winter. Although sets are easy to plant and give a quick start, the resulting onions are more prone to disease and bolting, and don’t store as well as those grown from seed. Also, there’s a wider selection of varieties available from seed.

In Canada, Spanish-type onions should be started indoors in February; any later and the bulbs will be small and too strong-tasting. Onions you plan to store over the winter can be seeded directly outside at the same time as peas, but in most parts of Canada, a better crop will result from starting seeds indoors at the beginning of April.

Plant them in either flats or cell packs filled with the seed-starting mixture. In flats, sow five millimeters deep, five millimeters apart in rows five centimeters apart. For cell packs, plant three seeds per cell. Maintain a temperature of 18 to 21°C. Once they germinate (seven to 10 days), place seedlings under a grow light or in a sunny, south-facing window and keep at a cool temperature (around 15°C). Keep plants well-watered (water-stressed plants produce small, strong-tasting bulbs) and feed with a half-strength organic fertilizer every two weeks. When the seedlings are 2.5 centimeters high, thin to one centimeter apart in flats or one seedling per cell. Trim tops to 10 centimeters. When the danger of heavy frost has passed, harden them off by gradually exposing them to cooler temperatures and direct sunlight.

Plant seedlings in fertile, well-drained soil in full sun. Prepare the area by removing weeds and digging in lots of compost or composted manure, adding extra compost and coarse sand to heavy clay soil. If the area has poor drainage, grow the onions in raised beds.

Dig individual holes or a long trench about five centimeters deep. For seedlings grown in flats, use a knife to remove them, then gently separate individual plants. Spread out the roots and firm the soil around its base. For seedlings grown in cell packs, keep the soil ball intact. Plant 10 to 15 centimeters apart, depending on the mature size of the variety.

Buy firm, disease-free sets that have no sprouts. Avoid larger bulbs, as they’ll go to seed. Plant with the tips just below the surface, with the flat ends down. Once a month, side-dress with compost or organic fertilizer.

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