When to Plant Onions in Ontario — City Guide
Indoor seed dates, set planting dates, long-day variety selection, pest and disease control, and curing and storage for Toronto, Ottawa, Hamilton, Windsor, London, Kingston, Kitchener, Sudbury, and Thunder Bay.
When to plant onions in Ontario starts indoors in late January — earlier than almost any other vegetable on the Ontario calendar. Onions need the longest indoor start of any common Ontario crop, and getting that timing right is what determines whether you pull large, firm storage bulbs in August or thin, undersized ones that rot before December.
This guide covers indoor seed start dates and set planting dates for every major Ontario city, why long-day variety selection is non-negotiable in Ontario, the difference between growing from seed versus sets, best varieties by zone, how to grow and care for onions through Ontario's season, the pests and diseases Ontario growers encounter most, and how to cure and store a full-season harvest.
Ontario onions at a glance: Long-day varieties only. From seed: start indoors late Jan–late Feb depending on city. Sets: plant outdoors when soil reaches 10°C, 2–4 weeks before last frost. Harvest when half the tops fall over. Cure 2–3 weeks. Row cover from planting through June for onion maggot.
Outside Ontario? See the Canada-wide onion planting guide for dates in BC, Quebec, the Prairies, and the Maritimes — plus why Canada needs long-day varieties.
Ontario Onion Planting Dates by City — 2026
Onions started from seed need 10–12 weeks indoors before transplanting. Sets go directly into the ground as soon as soil reaches 10°C — 2–4 weeks before last frost in most Ontario cities.
| City | Start Indoors | Transplant Out | Sets Direct | Harvest |
|---|---|---|---|---|
| Windsor | Jan 15–Feb 1 | Apr 1–15 | Mar 25–Apr 10 | Aug |
| Toronto | Jan 20–Feb 5 | Apr 10–25 | Apr 1–15 | Aug–Sept |
| Hamilton | Jan 20–Feb 5 | Apr 10–25 | Apr 5–20 | Aug–Sept |
| London | Feb 1–15 | Apr 15–30 | Apr 10–25 | Aug–Sept |
| Kitchener | Feb 1–15 | Apr 15–30 | Apr 10–25 | Aug–Sept |
| Kingston | Feb 5–20 | Apr 20–May 5 | Apr 15–May 1 | Aug–Sept |
| Ottawa | Feb 10–25 | Apr 25–May 10 | Apr 20–May 5 | Aug–Sept |
| Sudbury | Feb 15–Mar 1 | May 5–20 | Apr 25–May 10 | Sept |
| Thunder Bay | Feb 20–Mar 5 | May 15–30 | May 5–20 | Sept |
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🌿 Free Seed Starting CalculatorDay Length — The Rule That Determines Everything
Onions form bulbs in response to day length, not temperature. Getting this wrong — planting the wrong type — means no bulb at all, regardless of how well the plants are tended.
Long-day onions — the only type for Ontario
Long-day onion varieties bulb when days reach 14–16 hours of light. Across all of Ontario, this threshold is crossed in June — southern Ontario (Windsor, Toronto) in early June, northern Ontario and Ottawa by mid-June. The plant builds green tops through spring, then triggers bulb formation once the day length is reached. This is correct behaviour for Ontario's latitude. All onion seed and sets sold at Ontario garden centres should be long-day — confirm on the packet or label before buying. This applies whether you are in Windsor or Ottawa.
Short-day onions — will not work in Ontario
Short-day varieties (bred for Georgia, Texas, and other southern US states) bulb at only 10–12 hours of daylight. By the time Ontario days are short enough to trigger bulbing in these varieties — October or November — it is far too late in the season. The plant simply continues producing tops and never forms a bulb. This is one of the most frustrating and entirely avoidable Ontario onion failures. Recognise short-day varieties by name: Vidalia, Texas 1015, Bermuda, Granex. If you see these names, do not plant them in Ontario. The label "sweet onion" alone does not indicate day length — check explicitly.
Intermediate day-length onions — for Ottawa and Kingston
Intermediate or day-neutral varieties bulb at 12–14 hours and work across a wider latitude range than strict long-day types. For Ottawa and Kingston where the season is shorter and the goal is the earliest possible bulbing, Candy (85 days) is the top intermediate variety — it matures 2–3 weeks faster than standard long-day types like Copra or Patterson and is a safe first-season choice for Ontario gardeners new to growing from seed. All standard long-day varieties work across all Ontario latitudes — the day-length threshold is met comfortably even in Ottawa.
Seed, Sets, or Transplants — Which to Use in Ontario
From seed indoors — best results, most effort
Starting from seed gives access to the best storage varieties (Copra, Patterson) not sold as sets, produces the largest bulbs, and results in plants less prone to bolting than set-grown onions. Sow into 72-cell trays using sterile seed-starting mix, 2–3 seeds per cell at 6 mm depth. Thin to one seedling per cell after germination. Onion seedlings grow slowly and look like thin grass for several weeks. Grow lights running 14–16 hours are needed for January and early February starts in Ontario's short winter daylight. Pot up into 8 cm pots once seedlings reach pencil thickness. Harden off for 10–14 days before transplanting — onions tolerate light frost once hardened. Ontario gardeners who start onions from seed once rarely go back to sets.
From sets — easiest, most widely available
Sets are small dormant onion bulbs — typically Stuttgarter — sold in bags at Ontario garden centres from March onward. Plant them 2–3 cm deep, 10 cm apart, as soon as soil reaches 10°C. They establish quickly with no indoor start. The tradeoffs: limited variety range, smaller average bulbs than seed-grown onions, and more prone to bolting — especially if planted into cold soil or if large sets are used. Choose firm, small sets (coin-sized or smaller) over large ones; large sets are significantly more likely to bolt. Discard any that feel soft or show signs of mold at purchase. For Ottawa and Kingston gardeners who find the indoor start timing challenging, sets planted promptly in late April are a reliable fallback.
From transplants — a practical middle path
Some Ontario garden centres sell onion transplants (bundles of bare-root seedlings) in April — typically Walla Walla Sweet or a similar variety. These give the quality advantages of seed-grown onions without the January indoor start. Transplants establish well when planted at pencil thickness, 10 cm apart, roots tucked in with the white base just at soil level. Look for bundles where seedlings are uniform and not yellowed or tangled. Avoid transplants thicker than a finger — they have likely been held too long and are more prone to bolting. If purchasing from a garden centre, ask when the transplants arrived and plant them within a week of purchase.
Best Onion Varieties for Ontario
All varieties below are long-day types suited to Ontario. Intermediate-day varieties are noted where relevant for Ottawa and Kingston or for first-time seed growers who want the fastest results.
| Variety | Days | Type | Best Use | Notes |
|---|---|---|---|---|
| Candy | 85 days | Intermediate | All zones | Fastest-maturing reliable option. Best for Ottawa and Kingston. Mild flavour, moderate storage (2–3 months). |
| Stuttgarter | 100 days | Long-day / sets | All zones | Standard Ontario set variety. Widely available. Flat globe shape. Good storage. Most practical choice for sets growers. |
| Copra | 105 days | Long-day | Storage | Best-keeping Ontario onion — cures hard, stores 6–8 months. The top choice when long storage is the goal. Grow from seed. |
| Patterson | 105 days | Long-day | Storage | High-yielding storage onion with excellent disease resistance. 6–8 months storage life. Widely grown commercially across Ontario. |
| Ailsa Craig | 110 days | Long-day | Fresh use / Toronto south | Grows very large globe bulbs. Mild flavour. Poor storage (2–3 months) — eat first. Best for Windsor and Toronto's longer season. |
| Red Baron | 105 days | Long-day | All zones | Best reliable red onion for Ontario. Deep burgundy colour throughout. Good storage (3–4 months). Widely available as seed. |
| Walla Walla Sweet | 110 days | Long-day | Fresh / Windsor, Toronto | Very mild, sweet, large bulbs. Short storage (1–2 months). Best in Windsor and Toronto where 110-day season fits comfortably. |
Ontario Onion Planting Dates by Region
Onion timing shifts 3–4 weeks across Ontario's zones. Long-day varieties (the standard for all of Ontario) need 14–16 hours of daylight to start bulbing — meaning every region bulbs around the same week in late June regardless of when you planted. Below: planting windows by Ontario region. Earlier-planted onions produce LARGER bulbs because they have more time to build leaf mass before bulb-up triggers.
| Ontario region | Zone | Start seed indoors | Transplant outdoors | Cities |
|---|---|---|---|---|
| Niagara / Carolinian | 6b/7a | Jan 15–25 | April 5–15 | St. Catharines, Niagara Falls, Welland, Leamington, Windsor |
| Southwestern ON | 6a/6b | Jan 20–Feb 1 | April 10–20 | London, Sarnia, Chatham-Kent, Stratford |
| GTA / Golden Horseshoe | 6a/6b | Jan 25–Feb 5 | April 15–25 | Toronto, Mississauga, Brampton, Hamilton, Oshawa |
| Central ON / Lake Simcoe | 5a/5b | Feb 5–15 | May 1–10 | Barrie, Orillia, Peterborough, Lindsay, Cobourg |
| Eastern ON / Ottawa Valley | 5a/5b | Feb 5–15 | April 25–May 5 | Ottawa, Kingston, Belleville, Cornwall, Pembroke |
| Northern ON / Canadian Shield | 3b/4b | Feb 20–March 5 | May 15–25 | Sudbury, North Bay, Sault Ste. Marie, Thunder Bay, Timmins |
Onions need 10–14 weeks indoors before transplant. They tolerate light frost in spring and can go out 2–3 weeks before last frost (zone 5+) for biggest bulbs. Onion sets (small dormant bulbs) skip the indoor-start step and can be planted directly outdoors 2 weeks before last frost.
Starting Onion Seeds Indoors in Ontario
Onions have the longest indoor start of any common Ontario vegetable — 10–12 weeks before transplanting. Getting the setup right from the beginning pays off in stronger transplants and larger bulbs at harvest.
Containers, mix, and sowing
Sow into 72-cell trays or shallow flats using sterile seed-starting mix — not garden soil or potting mix, which are too heavy and harbour disease. Sow 2–3 seeds per cell at 6 mm depth. Germination occurs in 7–10 days at 18–21°C. Thin to one seedling per cell by snipping the weaker ones — never pull, which disturbs the surviving root. Onion seedlings look like thin grass blades for the first several weeks — this is normal. Once seedlings reach pencil thickness (approximately 8–10 weeks), they are ready to pot up into 8 cm pots for 2 more weeks before transplanting outdoors.
Light and grow lights
January and February in Ontario deliver weak, short daylight — insufficient for compact, strong onion seedlings without supplemental lighting. Without a grow light, seedlings started in late January will be tall, floppy, and weak by transplant time. A T5 or LED grow light running 14–16 hours per day produces markedly better transplants — keep lights 5–8 cm above seedling tops and raise as plants grow. For March starts, a very bright south-facing window may be adequate as daylight improves; for January and February starts it is not. This is the single most important setup decision for Ontario onion growers starting from seed.
Bulb size is determined by leaf count at bulbing
The number of leaves a plant has at the point the day-length trigger is reached in June directly determines bulb size — each healthy leaf corresponds roughly to one ring inside the bulb. A plant with 13 leaves can form a 13-ring bulb; a plant with 8 leaves will produce an 8-ring bulb. This is why the January start date matters and why grow lights matter: every week of additional indoor growth before the transplant window builds more leaf capacity, which translates directly into a larger bulb at harvest. Starting too late or growing weak seedlings under insufficient light produces small onions regardless of how well the summer season goes.
How to Grow Onions in Ontario
Soil and spacing
Onions prefer loose, well-drained soil with a pH of 6.2–6.8. They perform poorly in compacted or waterlogged ground — bulb development is stunted and rot pressure increases. Ontario's heavy clay soils (common in Hamilton, London, Ottawa) need amendment: work in 5–7 cm of aged compost before planting to improve drainage and loosen structure. Avoid fresh manure, which promotes lush tops at the expense of bulb development. Space onions 10 cm apart in rows 25–30 cm apart. Plant sets 2–3 cm deep with the pointed tip just above the soil surface. As bulbs develop through summer they push themselves upward slightly — do not hill soil over developing bulbs.
Weeding — the most important summer task
Onions have shallow, fibrous roots and almost no competitive ability against weeds — even moderate weed pressure through June and July significantly reduces final bulb size. This is one crop where consistent hand weeding pays large dividends. Weed thoroughly every 1–2 weeks through June when weeds grow fastest. Use a shallow hoe or hand tool carefully — onion roots are close to the surface and are easily damaged by deep cultivation. Mulching with 3–4 cm of straw after transplanting or set establishment suppresses weeds and retains moisture through Ontario's dry July and August.
Watering and fertilising
Water consistently through spring and early summer — 2–3 cm per week. Ontario's April and May rainfall often covers early needs, but the drier July–August period requires irrigation through the bulbing stage. Stop watering entirely once tops begin to yellow and fall — at this point the plant is curing the bulb and excess moisture promotes neck rot. Fertilise with a balanced feed (10-10-10) at transplanting or when sets show 3–4 cm of green growth. Apply a second nitrogen-rich feed when leaves are actively growing and the plant is a few weeks from full size. Stop all nitrogen fertilising by early July — late nitrogen pushes thick, soft necks that cure poorly and rot in storage.
Hardening off transplants
Onion transplants moved directly from a warm indoor growing environment to Ontario's outdoor conditions in April suffer transplant shock — and cold shock in particular triggers the vernalisation response that causes bolting. Harden off for 10–14 days before transplanting: start with 1–2 hours outside in a sheltered spot during the warmest part of the day, increasing exposure gradually over two weeks. Once hardened, onion transplants tolerate light frost well. Do not skip this step or rush it — a cold snap immediately following transplanting of un-hardened seedlings is one of the most reliable bolt triggers in Ontario's spring garden. Transplant on a calm, overcast day if possible to reduce moisture stress during establishment.
Green onions from thinnings
If you sow seeds closer than the final 10 cm spacing — which many Ontario gardeners do to hedge against germination losses — the thinnings pulled to reach final spacing make excellent green onions (scallions). Pull thinnings when they reach pencil thickness, before they crowd their neighbours. This converts what would otherwise be discarded seedlings into a bonus early harvest from early June onward, weeks before the main bulb crop. For sets planted at 5 cm apart with the intention of thinning to 10 cm, pull every other plant as early green onions once tops reach 20–25 cm. The remaining plants have the full spacing they need to develop into full-size storage bulbs.
Pests and Diseases in Ontario
Ontario's primary onion threats are onion maggot in the field and neck rot in storage. Both are largely preventable with row cover and proper curing.
Onion maggot — Ontario's primary onion pest
The onion maggot (Delia antiqua) lays eggs at the base of onion stems in spring. Larvae tunnel into bulbs, causing plants to collapse and rot from the inside. In Ontario the first and most damaging emergence is May through June, with a second generation in July. Row cover (floating fabric) installed at planting and sealed at the edges is the most effective control — keep in place through late June. Remove row cover for occasional weeding and replace it carefully. There is no effective spray once larvae are inside the bulb. Individual wilting plants surrounded by healthy ones, especially in May and June, are the first sign of maggot damage. Crop rotation to a new bed each year reduces pressure from overwintering pupae in the soil.
Thrips
Onion thrips (Thrips tabaci) are tiny insects that rasp leaf surfaces, causing silver-white streaking and leaf tip distortion. Populations build rapidly during Ontario's hot dry July periods and can cause significant yield reduction in severe infestations. Inspect leaves closely — thrips are visible as tiny dark specks on leaf surfaces. Insecticidal soap or spinosad spray applied every 5–7 days controls established populations. Consistent irrigation during hot dry spells reduces stress that makes plants more vulnerable. Blue sticky traps near the bed monitor for early population buildup and help time sprays before populations peak.
White rot and neck rot
White rot (Sclerotium cepivorum) causes white fluffy mycelium at the bulb base, yellowing tops, and plant death — and like clubroot, persists in soil for 20+ years once established. Prevention: strict 4-year rotation of all alliums; never compost affected plants; maintain soil pH above 6.5. Neck rot (Botrytis allii) is a storage disease that enters through poorly cured necks — appearing as gray mold inside stored bulbs weeks after harvest. It is entirely preventable: ensure necks are fully dry and papery before storage and maintain storage at 0–5°C with good airflow. Neck rot is the most common cause of Ontario home gardeners' stored onions rotting within 2–3 months — it is a curing failure, not a field disease.
Common Onion Problems in Ontario
No bulb forming — wrong variety or late start
The most common cause is wrong variety — short-day onions planted in Ontario will produce abundant tops but never bulb. Check variety descriptions before purchasing, especially from online seed suppliers where US and international varieties are common. A second cause is starting seeds too late indoors: onions need maximum top growth before the June day-length trigger arrives — a small plant with few leaves at bulbing time will produce a small bulb regardless of growing conditions through July. The January start date for Ontario is not conservative; it is the minimum needed to build sufficient leaf mass before the trigger.
Bolting — seed stalks instead of bulbs
Bolting is triggered when seedlings or sets experience temperatures below 10°C after reaching pencil thickness — the plant interprets this as winter and flowers instead of bulbing when days lengthen. In Ontario this most commonly happens when: sets are planted into soil still below 10°C in April; transplants are moved outside before adequate hardening off; or a cold snap follows transplanting. A bolted onion forms a hard, woody central stalk and will not produce a usable storage bulb. Harvest bolted plants immediately and use as green onions — they will not recover. Small coin-sized sets bolt less readily than large ones; this is the most important set selection rule for Ontario growers.
Small bulbs at harvest
Small bulbs are almost always caused by one or more of: starting seeds too late (insufficient leaf growth before the June bulbing trigger); planting too closely (competition reduces bulb size); weed competition through June and July; or nitrogen fertiliser applied too late, pushing leafy growth when the plant should be channelling energy into the bulb. The most impactful fix for consistently small onions is to start seeds 2–3 weeks earlier the following season and commit to consistent weeding through June. The leaf-to-ring relationship means every additional leaf the plant has at bulbing time directly adds one more ring — and one more layer of size — to the final bulb.
Doubles — two bulbs from one plant
A double is an onion that has split at the base and formed two separate bulbs side by side from a single set or transplant. Doubles store poorly — the shared neck dries unevenly — and should be used or given away first. In Ontario, doubles are most commonly caused by over-large sets (large sets often have two distinct growing points inside), by transplanting seedlings that were potted into the same cell and not properly thinned, or by stress events during early bulbing that interrupt the formation of a single central bulb. Prevention: use coin-sized sets, thin rigorously to one seedling per cell indoors, and avoid significant irrigation interruptions during the bulbing trigger period in June.
Thick necks and soft bulbs
Bulbs with a thick, spongy, or pithy neck — rather than a thin, tight, papery one — are caused by nitrogen applied too late in the season or by watering that continued too long after tops began to die back. Late nitrogen keeps the plant in vegetative growth mode when it should be hardening and curing the neck tissue; extra moisture after die-back softens bulb tissue before it cures. Both make storage life very short regardless of how the bulb looks externally. Prevention is entirely in the growing and harvest decisions: stop all nitrogen by early July and stop all watering as soon as tops begin to fall. Thick-necked onions are still edible and should be used immediately — they will not improve in storage.
Yellowing leaves in-season
Some yellowing of the lowest leaves is entirely normal as the plant ages — the plant draws nutrients back from old leaves as it focuses on bulb development from July onward. Widespread yellowing of younger or middle leaves signals nitrogen deficiency; side-dress with a balanced fertiliser, though after early July this is not advisable as it pushes soft necks. Yellowing with streaking or distortion on new growth in June suggests thrips damage — inspect leaf surfaces closely for small dark specks. Yellowing accompanied by wilting despite adequate moisture warrants root inspection: pull one plant and check the base for the white fluffy mycelium of white rot, or for the soft, collapsed tissue of onion maggot damage inside the bulb.
Curing and Storing Ontario Onions
Proper curing after harvest determines how long your onions keep. Skipping or rushing this step is the primary reason home-grown Ontario onions rot in storage while supermarket varieties last months.
In-ground and post-harvest curing
After tops fall naturally, leave bulbs in the ground for 10–14 days in dry weather for in-ground curing — the skin sets and toughens substantially during this period. Ontario's August is usually warm and dry enough for this step to work well in the field. If heavy rain is forecast, pull bulbs and move to a covered drying location immediately. After lifting, lay bulbs on slatted racks, screens, or hang in loose bundles of 8–10 in a warm (25–30°C), dry, well-ventilated space for 2–3 weeks. A garage, shed with good airflow, or covered porch works well in Ontario's August. Do not wash before curing; brush off loose soil only.
How to know when curing is complete
Curing is complete when the outer wrapper is fully papery and rustles when handled, the neck feels dry and tight with no give when squeezed firmly, and the roots are completely shrivelled. This takes 2–3 weeks under good Ontario August conditions. Moving onions to cold storage before curing is complete is the single most common cause of premature rot. When in doubt, leave them curing an additional week. After curing, trim roots to 1 cm and tops to 2–3 cm before moving to storage.
Storage conditions and shelf life by variety
Store cured onions in mesh bags, open crates, or braided ropes (softneck types only) at 0–5°C in darkness with good airflow. A cold cellar, unheated basement, or insulated garage through Ontario's autumn and winter are ideal. Never store near apples (ethylene gas accelerates sprouting) or potatoes (moisture promotes rot in both). Shelf life by variety: Candy 2–3 months; Red Baron 3–4 months; Stuttgarter 4–5 months; Patterson and Copra 6–8 months. Sweet varieties (Walla Walla, Ailsa Craig) store only 1–2 months and should always be used or given away first. Set aside your largest, best-formed bulbs as seed sets for spring replanting if growing from sets each year.
Frequently Asked Questions
When should I plant onions in Ontario?
From seed indoors: Windsor January 15–February 1, Toronto/Hamilton January 20–February 5, London February 1–15, Kingston February 5–20, Ottawa February 10–25. Transplant out 10–12 weeks later. Sets direct in ground: Windsor/Toronto late March to mid-April, London/Hamilton mid-April, Kingston/Ottawa late April to early May. Harvest when half the tops fall over, typically August–September.
Why did my onions grow lots of tops but no bulbs?
Almost certainly a wrong variety — short-day onions will not bulb in Ontario. Check that your variety or sets are labelled long-day. Names like Vidalia, Texas 1015, and Bermuda are short-day and unsuitable for Ontario. The second cause is starting seeds too late, giving the plant insufficient leaf development before the June day-length bulbing trigger. A large leafy plant produces a large bulb; a small plant produces a small one or none at all.
Are sets or seeds better for Ontario onions?
Sets are easier and need no indoor space — the practical choice for most home gardeners. Seeds give access to better storage varieties (Copra, Patterson), produce larger bulbs, and are less prone to bolting. If you have the setup for a late-January indoor start with grow lights, seed-grown onions consistently outperform sets. For Ottawa and Kingston, sets planted promptly in late April are a reliable fallback when the indoor start timing is difficult to manage.
How do I prevent onion maggot damage in Ontario?
Row cover (floating fabric) installed at planting and sealed at the edges, kept in place through late June, is the only reliable prevention for the first generation. There is no effective spray once larvae are inside the bulb. Remove row cover briefly for weeding, replace it immediately, and ensure edges stay sealed. Crop rotation each season reduces overwintering pupae in the soil and is the most effective long-term pressure reduction strategy.
Can I start onions indoors without a grow light in Ontario?
For February starts, a very bright south-facing window may produce acceptable seedlings as daylight improves. For January starts required by Windsor and Toronto gardeners, a grow light running 14–16 hours per day is needed — Ontario's January daylight is insufficient for compact, well-developed onion seedlings. Leggy, weak seedlings from inadequate light translate directly to smaller bulbs at harvest because the leaf count at the June bulbing trigger is what determines final bulb size.
How long do Ontario-grown onions store?
Storage life depends heavily on variety and curing quality. Copra and Patterson store 6–8 months at 0–5°C when properly cured. Red Baron 3–4 months. Stuttgarter 4–5 months. Sweet varieties (Walla Walla, Ailsa Craig) 1–2 months only — use first. Thorough curing (2–3 weeks until necks are fully dry and papery) is the most important factor. Improperly cured onions begin rotting within weeks regardless of variety or storage conditions.