How to Grow Onions in Canada: Sets, Transplants & Direct Seeding

Why Grow Onions in Canada?

Homegrown onions are incomparably flavourful -- sharper, more complex, and far more satisfying than anything from a grocery bin. Better yet, a well-grown and properly cured onion will store for 6 to 12 months in a cool, dry location, turning a late-summer harvest into a year-round pantry staple. Onions are a long-season crop that requires some planning, but the effort is straightforward and the rewards are excellent.

Canada's latitude is actually an advantage for onion growing. Our long summer days trigger bulb formation in long-day varieties earlier and more reliably than in warmer, southerly climates. The key is understanding the long-day vs short-day distinction and choosing the right varieties for Canadian conditions.

Long-Day vs Short-Day Onions: Choose Correctly

This is the most critical decision for Canadian onion growers.

Long-day onions begin forming bulbs when day length reaches 14 to 16 hours. This is what happens across Canada in June and July. Long-day varieties are the correct choice for all Canadian provinces.

Short-day onions bulb up when day length is around 10 to 12 hours -- suitable for southern US states and sub-tropical climates. Planted in Canada, short-day onions form bulbs prematurely when still small, producing walnut-sized bulbs. They are the wrong choice for Canadian gardens.

Day-neutral (intermediate) onions bulb between 12 and 14 hours and can work reasonably well in southern Canadian zones (5-6), though dedicated long-day varieties typically outperform them.

Always read catalogue descriptions before purchasing. Any seed packet or listing intended for Canadian gardens should specify "long-day" or list a suitable zone.

Sets vs Transplants vs Direct Seeding

There are three ways to start an onion crop. Each has trade-offs worth understanding.

Onion Sets

Sets are small, dry onion bulbs grown from seed the previous year, then dried and sold in bags. They are the most widely available option at garden centres and are the easiest to plant.

Pros: Extremely easy -- just push them into the soil. No indoor seed starting required. Available everywhere. Germinates reliably even in cold soil.

Cons: Limited variety selection (mostly yellow cooking onions and red varieties). Sets have a higher tendency to bolt (go to seed) than transplants, especially if planted too early in cold conditions or if the sets are large. Bolted onions are edible but won't store well. Choose sets no larger than 1.5 cm (0.6 inch) in diameter to reduce bolting risk.

Transplants (Seedlings Started Indoors)

Starting onions from seed indoors gives access to the full range of varieties and produces high-quality bulbs with low bolt rates.

How to start: Sow seeds 10 to 12 weeks before transplant date, roughly 1 cm (0.5 inch) deep. Onion seeds germinate in 7 to 10 days. Grow under lights at 15 to 18 C (60 to 65 F) -- onion seedlings actually prefer slightly cooler temperatures than most other transplants. Trim the tops with scissors to 10 to 12 cm (4 to 5 inches) when they flop over, which encourages stronger growth. Transplant to the garden 2 to 4 weeks before the last frost date -- onions can handle light frost.

Pros: Wide variety selection. Lower bolt rate than sets. Excellent bulb quality.

Cons: Requires 10 to 12 weeks of indoor seed starting. Seedlings are thin and grass-like -- they look fragile but are tougher than they appear.

Direct Seeding

Sowing onion seeds directly in the garden is possible but requires a long season and is not recommended for zones 3 and 4. In zones 5 and warmer, direct-seeded onions sown as early as the soil can be worked can produce reasonable bulbs by late summer, but they will be smaller than transplant-grown onions.

For most Canadian gardeners, transplants (started indoors) or sets offer better results than direct seeding.

When to Plant by Province

Onions tolerate light frost and can go into the ground earlier than most crops.

• British Columbia (coastal, Zone 8): Plant sets or transplants in April; start seeds indoors in January
• Southern BC Interior (Zones 5-6): Plant sets mid-April; transplants late April to early May; start seeds indoors late January to early February
• Southern Ontario (Zones 5-6): Plant sets late April to early May; transplants early May; start seeds indoors late January to early February
• Quebec (Zones 4-5): Plant sets and transplants early to mid-May; start seeds indoors mid to late February
• Maritimes (Zones 5-6): Plant sets and transplants early to mid-May; start seeds indoors early February
• Prairies -- Manitoba/Saskatchewan (Zones 3-4): Plant sets late April to early May as soon as soil is workable; transplants mid-May; start seeds indoors mid-February
• Alberta (Zones 3-4): Plant sets and transplants mid to late May; start seeds indoors early March

Use our free garden calculator to fine-tune timing based on your specific city's last frost data.

Spacing in 30-Inch Beds

Onions can be planted densely in biointensive beds, making them very productive per square metre.

Four rows with 10 cm (4-inch) in-row spacing fills a 75 cm (30-inch) bed efficiently. A 3.6 m (12-foot) bed holds approximately 144 onion plants. For larger storage onions, increase spacing to 15 cm (6 inches) in-row. Tighter spacing produces smaller but still flavourful bulbs suitable for fresh use and pickling.

Growing Tips

Soil. Onions grow best in loose, fertile, well-drained soil with a pH of 6.0 to 7.0. They develop bulbs just below the soil surface and need loose soil to expand without restriction. Raised beds and deeply worked ground are ideal. Incorporate compost before planting but avoid fresh manure, which can promote diseases.

Watering. Onions have shallow root systems and need consistent moisture, especially during bulb development in midsummer. About 2.5 cm (1 inch) per week is adequate. Reduce watering significantly about two weeks before harvest to help bulb skins cure while still in the ground.

Weeding. This is the most labour-intensive aspect of onion growing. Onion tops are narrow and don't shade out competing weeds. Hand-weed carefully -- onion roots are shallow and easily disturbed by cultivation tools. A straw mulch applied after planting reduces weed pressure significantly.

Fertilizing. Side-dress with a nitrogen-rich fertilizer or liquid fish emulsion once in early summer to support leaf growth. Stop feeding when the tops begin to flop naturally in late summer, signalling bulb maturation.

Companion planting. Onions and carrots are classic companions. Onions repel carrot fly while carrot foliage deters onion fly. Alternate rows or plant adjacent beds for best effect. See our companion planting guide for more combinations.

Curing and Storing Onions

Proper curing is essential for long storage. Without it, onions develop soft spots, mould, and rot within weeks.

Harvest when half to two-thirds of the tops have naturally fallen over. Gently loosen bulbs with a fork and leave them on the soil surface for a day or two if the weather is dry and warm.

Cure by spreading bulbs in a single layer in a warm (25 to 30 C / 77 to 86 F), dry, well-ventilated location for 2 to 4 weeks. A porch, greenhouse, or garage with good airflow works well. The outer skins will paper over, the necks will dry out and tighten, and the bulbs will firm up. Curing is complete when the neck feels bone-dry and the outer skin is papery and crisp.

Store in a cool (4 to 10 C / 40 to 50 F), dry, dark place with good airflow -- mesh bags, old pantyhose, or slatted crates all work. Properly cured storage varieties keep 6 to 9 months. Check regularly and remove any that show softness or sprouting.

Recommended Varieties for Canadian Gardens

• Cortland -- Long-day storage onion; large, globe-shaped, excellent flavour; one of the best for Canadian conditions
• Patterson -- Long-day storage variety; very high storage quality; good disease resistance
• Copra -- Long-day yellow storage onion; widely grown commercially in Canada; stores 6 to 9 months without quality loss
• Stuttgarter -- The classic set variety; yellow, globe-shaped; widely available at garden centres; reliable producer
• Walla Walla -- Sweet, mild onion; best used fresh; does not store long but outstanding flavour fresh from the garden
• Red Burgundy -- Long-day red onion; excellent for fresh use and mild storage; good flavour
• Ailsa Craig -- Long-day yellow variety; produces very large exhibition-type bulbs; milder flavour; a favourite with gardeners who want impressive bulbs

Seed and sets available from William Dam Seeds, Stokes Seeds, OSC Seeds, and most Canadian garden centres.

Plan Your Onion Garden

Our free garden calculator determines your indoor seed start date, transplant date, and expected harvest window based on your location. For information on companion planting and crop rotation with alliums, see our companion planting guide.