Ontario spans four distinct plant hardiness zones. Windsor and Leamington sit in Zone 7a, while Ottawa, Kingston, and the Kawartha Lakes fall into Zone 5b. That gap translates to roughly three to four weeks of difference in last-frost timing between the province's warmest and coldest growing areas. Planning around those differences is the starting point for every spring garden.
Understanding Ontario's Hardiness Zones
Agriculture and Agri-Food Canada publishes the official plant hardiness zone map, updated most recently using 1981–2010 climate data. The zones reflect average annual minimum temperatures and factor in variables like snowfall, wind, and humidity. For spring planning, the most relevant number is the average last frost date for your area.
The following zones cover the major settled areas of Ontario:
- Zone 7a (Windsor, Leamington, Kingsville): Last frost typically falls between April 1 and April 15. The mildest winters in the province allow for earlier transplanting and longer warm-season windows.
- Zone 6b (Hamilton, Niagara, St. Catharines): Average last frost around April 20. The Niagara Escarpment provides some cold-air drainage, which gardeners on the escarpment face more frequently than those on the lakeplain below.
- Zone 6a (Toronto, Mississauga, Oshawa): Last frost commonly falls April 20–28. Lake Ontario moderates temperature significantly — properties within 5 km of the lakeshore often gain one to two weeks compared to inland areas.
- Zone 5b (Ottawa, Kingston, Peterborough): Last frost May 1–10. Gardeners here rely on indoor starts and cold frame protection to make the most of the shorter frost-free period.
- Zone 5a (Haliburton, Parry Sound, Bracebridge): Last frost May 10–18. The Muskoka region sits at the transition into the Canadian Shield, where cold air settles into low-lying areas and can catch gardeners off guard.
Environment and Climate Change Canada's historical climate data portal allows you to look up frost probability dates for the weather station nearest your property — more precise than zone averages alone.
Indoor Start Windows by Crop
Most tender vegetables and many herbs require indoor starts in Ontario, where the frost-free period is too short to direct-sow warm-season crops from seed. The timing below is calculated from an assumed last frost date of May 10 (Zone 5b/Ottawa baseline). Adjust by the number of weeks your zone differs from this reference point.
10–12 Weeks Before Last Frost (Late February – Early March)
- Celery and celeriac (slow-growing; benefits from longest indoor window)
- Onions from seed
- Leeks
- Peppers (especially thick-walled varieties like bell peppers)
- Eggplant
8–10 Weeks Before Last Frost (Early to Mid March)
- Tomatoes (cherry, paste, and beefsteak alike)
- Ground cherries
- Parsley (slow germinator; benefits from early start)
- Broccoli, cabbage, and Brussels sprouts (for early outdoor transplant)
4–6 Weeks Before Last Frost (Late March – Mid April)
- Cucumbers
- Summer and winter squash
- Zucchini
- Pumpkins
- Melons (for Zone 6 and warmer only)
- Basil
Direct-Sow Dates for Cold-Hardy Crops
A number of vegetables tolerate light frost and can go into the ground while temperatures are still unpredictable. These crops are typically sown directly, without indoor starts, as soon as the soil is workable — usually when it reaches 7–10°C at a 5 cm depth.
In Zone 5b, the soil becomes workable around April 10–25 in most years. In Zone 6a (Toronto), that window typically opens in late March to early April.
- Peas: Sow 6–8 weeks before last frost. Peas prefer cool soil and will stall in heat. In Zone 5b, that means late March to early April.
- Spinach: Can be sown as soon as soil is workable. Tolerates light frost once established. Succession-sow every 10 days through mid-May.
- Lettuce: Direct sow or transplant out 3–4 weeks before last frost. Start a second sowing in early August for fall harvest.
- Kale and Swiss chard: Transplant out 2–4 weeks before last frost. Cold-tolerant and productive through fall.
- Carrots and parsnips: Sow directly as soon as soil temperature reaches 7°C. Parsnips are particularly slow — a late April sow in Zone 5b gives a reliable fall harvest.
- Beets: Sow from late April through late June for a continuous harvest. Thin to 8–10 cm spacing for full root development.
- Radishes: Fast crop (25–35 days). Sow every two weeks starting from the first workable soil date through mid-May.
After-Frost Transplant Schedule (Warm-Season Crops)
Warm-season crops cannot go outdoors until nighttime temperatures stay consistently above 10°C and all frost risk has passed. Soil temperature at transplant time matters as much as air temperature — tomatoes transplanted into cold soil (below 15°C) will stall and show little growth for several weeks.
A soil thermometer is inexpensive and takes the guesswork out of transplant timing. For Zone 5b, the reliable transplant window for tomatoes, peppers, and cucumbers typically falls May 18–25. For Zone 6a (Toronto), that window shifts to May 10–18.
Using black plastic mulch or dark-coloured rock mulch in early May can raise soil temperature by 3–5°C and allow earlier transplanting of heat-demanding crops like melons and peppers.
Cold Frame and Row Cover Extensions
Unheated cold frames and floating row cover (e.g., Reemay or equivalent non-woven fabric) extend usable growing time by two to four weeks on either end of the season. A cold frame built over a raised bed and covered with a clear polycarbonate lid can hold temperatures 5–8°C above ambient on clear nights, allowing brassica transplants and lettuce to go out in early April across most of Zone 5b.
Row cover fabric rated at 1.5 oz/yd² provides approximately 4°C of frost protection and is permeable to rain, making it a low-maintenance option for protecting direct-sown crops from late spring frosts in April and May.
The Ontario Ministry of Agriculture, Food and Agribusiness publishes updated seasonal growing guides, including vegetable trial data from research stations across the province.