Onions form bulbs in response to the number of daylight hours. In Canada, grow the long-day varieties because short-day onions produce smaller bulbs.
Unfortunately, Canadians can't grow very mild, sweet Spanish onions, such as the popular ‘Vidalia', but ‘Walla Walla', ‘Super Star', ‘Ailsa Craig', ‘Riverside Sweet Spanish' and ‘Candy' are mild, sweet varieties that grow well here. Red-skinned ‘Red Burgermaster' has red-and-white flesh. None of the Spanish onions are suitable for long-term storage. (They'll keep for about one month; cooking onions will keep for four to seven months under ideal conditions.)
Not all regular cooking onions store well over the winter either. ‘Copra', with its firm flesh, stores longer than any other variety. ‘Frontier' is an earlier onion that stores nearly as long. ‘Redwing' is the longest storing of the reds.
‘Crystal Wax' and ‘Barletta' have silvery white skin, small, round bulbs and mild, crisp flesh that make them ideal for pickling. ‘Pacific' is a scallion that can be left in the ground to form small bulbs that are also suitable for pickling.
Harvesting and Storage
Toward the end of summer, when about half the tops have fallen over and bulbs are forming a papery skin, gently knock over the tops by hand or with a hoe, bending but not breaking them. Pull out any that have flower stalks and eat them first, as they won't store well. Leave the rest in the ground, without watering, until the tops begin to wither. Then dig them up, gently brushing off the soil (don't wash).
For successful storage, it's essential to cure the onions: Spread them out to dry for a couple of weeks in a warm, airy place, out of rain and sun. Once they're cured, braid the tops or cut them off, leaving a 2.5-centimetre stub. Store at 60 to 70 per cent humidity, as close to 0°C as possible without letting them freeze, in mesh bags or on wire mesh shelves to allow for good air circulation. Check regularly and remove any that are sprouting or rotting.
Diseases and Pests
Onion root maggots Adult flies lay eggs at the base of young plants. The white maggots burrow into the underground stem, causing plants to wilt and turn yellow. Later generations burrow into the bulbs of larger onions and may cause them to rot. Mulch immediately after planting or, in damp areas, sprinkle the soil with diatomaceous earth. Destroy infested plants and do a thorough fall cleanup.
Onion thrips Thrips suck out plant juices, causing silver streaks that develop into silver patches. Serious infestations cause stunted, bleached leaves and plants that ripen prematurely, produce small bulbs and may die. Spray with insecticidal soap. Don't plant onions near fields of alfalfa or grain (alternative hosts for the thrips).
Fungal diseases Onions are susceptible to a number of fungal diseases that can damage leaves and bulbs. To minimize problems, plant in well-drained soil, practise a four-year crop rotation and don't water late in the day.