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 seed-starting mixture. In flats, sow five millimetres deep, five millimetres apart in rows five centimetres 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 centimetres high, thin to one centimetre apart in flats or one seedling per cell. Trim tops to 10 centimetres. When 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 centimetres 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 centimetres 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.