When to Plant Tomatoes in Ontario 2026 (Zone 5 & 6 Guide)

Start your tomatoes indoors 6–8 weeks before your last frost date, then transplant outdoors after the last frost has passed and nighttime temperatures are consistently above 10°C. In most of Ontario, that means starting seeds indoors between late March and mid-April, and transplanting between mid-May and early June. Your exact dates depend on which zone you're in — read on for a city-by-city breakdown.

Ontario Tomato Planting Dates by City (2026)

These dates are based on 30-year Canadian climate normals from Environment Canada and are consistent year over year. The 2026 calendar aligns with historical averages — no unusual late-frost patterns are forecast for southern Ontario.

Not sure which zone you're in? Use the Ontario planting dates guide to look up your city, or enter your postal code in the MyGardenPlanner frost date calculator for a personalized result.

How to Start Tomatoes Indoors in Ontario

Step 1: Choose the Right Seeds

Before you fill a single cell tray, think about your season length. Ontario's growing season ranges from roughly 90 days in Thunder Bay to 160+ days in the Windsor-Essex area. That gap matters enormously for tomatoes.

• Determinate varieties (bush tomatoes) ripen all at once and suit shorter seasons. Good for zone 5 and northern growers who need to beat the frost.
• Indeterminate varieties keep producing until frost kills them. They reward zone 6 gardeners in Toronto and Hamilton who have the full season to work with.

Look for the "days to maturity" number on the seed packet. For Ottawa and Sudbury (zone 5), choose varieties under 70 days. For Toronto and Hamilton (zone 6), you can go up to 80–85 days with room to spare.

Step 2: Sow at the Right Depth and Temperature

Fill cell trays or small pots with a quality seed-starting mix — not garden soil, which compacts and can carry pathogens. Sow seeds about 6 mm (¼ inch) deep, two seeds per cell.

Tomato seeds germinate best at 21–27°C soil temperature. Most Ontario homes in March run cooler than that, especially overnight. A seedling heat mat under your trays makes a real difference — germination typically happens in 5–7 days with bottom heat, versus 10–14 days without.

Once seedlings emerge, move them to your brightest south-facing window or set up grow lights 5–7 cm above the tops of the plants for 14–16 hours per day. Leggy, pale seedlings are the number-one symptom of insufficient light.

Step 3: Harden Off Before Transplanting

This is the step most Ontario gardeners skip — and it's the step that determines whether your transplants thrive or sulk for two weeks after going in the ground.

Starting 7–10 days before your transplant date, begin setting your seedlings outside for short periods each day:

• Days 1–2: 1–2 hours in a sheltered, shaded spot
• Days 3–4: 3–4 hours, some indirect sun
• Days 5–6: half a day, including some direct sun
• Days 7–10: full day outside, including wind exposure

Bring plants in if overnight temperatures drop below 10°C. After a proper hardening-off period, your tomatoes will be stocky, dark green, and ready to hit the ground running.

Best Tomato Varieties for Ontario by Zone

Zone 5 (Ottawa, Sudbury, and Surrounding Areas)

With a last frost in late May and first fall frost arriving as early as mid-September, zone 5 Ontario gardeners are working with a compressed window. Prioritize short-season, cold-tolerant varieties:

• Early Girl (57 days) — reliable, classic, widely available at Ontario garden centres
• Sub-Arctic Plenty (45 days) — bred specifically for short Canadian seasons; sets fruit even in cool weather
• Glacier (55 days) — sweet, prolific, handles cool nights better than most

Zone 6 (Toronto, Hamilton, London, and the Golden Horseshoe)

Zone 6 opens up the full tomato catalogue. You still want to transplant after May long weekend, but you have the heat units to ripen even large, slow-maturing heirlooms:

• San Marzano (78 days) — the Italian paste tomato; thrives in Toronto's summer heat
• Cherokee Purple (80 days) — rich, complex flavour; a favourite at Ontario farmers' markets
• Brandywine (85 days) — the benchmark heirloom; needs a full zone 6 season to deliver

For market gardeners growing at scale, pair variety selection with a succession planting plan to spread harvest across the season rather than getting slammed all at once.

Common Ontario Tomato Planting Mistakes

Planting Too Early

Cold soil — below 15°C — stuns root development and opens the door to disease. A tomato planted into warm soil on May 20 will almost always outperform one planted into cold soil on May 5, even though the earlier plant had two extra weeks in the ground. Use a soil thermometer, not the calendar, as your final check before transplanting.

Skipping the Hardening-Off Step

Seedlings grown indoors under artificial light have never experienced wind, UV radiation, or temperature swings. Transplanting them directly outside causes transplant shock — wilting, leaf scorch, and stunted growth for 2–3 weeks. Ten days of gradual outdoor exposure costs you nothing and saves you weeks of recovery time.

Not Staking Before You Plant

Drive your stake or set up your cage at transplant time, before the roots establish. Doing it later risks slicing through roots and destabilizing a plant that has already settled in. Indeterminate varieties in zone 6 can reach 1.5–2 metres — plan your support system on day one.

Get Your Exact Ontario Tomato Dates

The table above covers major Ontario cities, but frost dates vary at the neighbourhood level — elevation, proximity to the Great Lakes, and urban heat islands all shift your local last frost by days or even weeks.

Enter your Ontario postal code in the MyGardenPlanner frost date calculator to get your personalized tomato start date, transplant date, and a full planting schedule for every crop in your garden. It's free and takes about 30 seconds.

For a complete Ontario planting calendar across all vegetables, see the Ontario planting dates guide. And once your tomatoes are in the ground, check the seed starting schedule guide to plan out the rest of your season.

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Dates based on 30-year Canadian climate normals from Environment Canada. Consistent year over year — these figures apply to the 2026 growing season.