frost
When to Plant Vegetables in Ontario
Planting dates for the most common vegetables in Zone 5b and 6a — what goes in the ground early, what waits for warm soil, and what has to be started indoors in March if you want a harvest at all.
Zone 5b/6a frost dates · direct-sow vs start indoors · Easy — a calendar job · Plan once, plant all season
THE DATES The two frost dates that run the garden
Everything in this guide hangs off these averages for Durham Region and the east GTA (Canadian hardiness Zone 5b to 6a). Averages are not promises — a low-lying garden can frost a week after the neighbourhood average — so keep an eye on the overnight forecast through May.
| Date | What it means |
|---|---|
| Average last spring frost | Early May (about May 1–10) |
| Safe transplant window for tender crops | Mid-to-late May; the May long weekend is the traditional marker |
| Warm-soil crops (beans, squash, cucumbers) | Late May to early June, once soil feels warm, not just frost-free |
| Average first fall frost | Mid-October (about Oct 10–20) |
| Garlic planting window | October, a few weeks before the ground freezes |
THE CHART Crop by crop: sow, start, or transplant
Three ways a plant gets into your garden: direct-sown seed, seed started indoors and transplanted, or transplants bought at the garden centre. This chart sorts the common crops for Zone 5b/6a.
| Vegetable | How | When (Zone 5b/6a) |
|---|---|---|
| Peas | Direct sow | Mid-April to early May, as soon as soil can be worked |
| Lettuce & spinach | Direct sow | Mid-April; sow again every 2 weeks until June, and again in late August |
| Carrots & beets | Direct sow | Late April to May; carrots again in July for a fall crop |
| Radishes | Direct sow | Mid-April onward, small sowings every week or two |
| Potatoes | Plant seed potatoes | Late April to mid-May |
| Onions | Sets direct, or start indoors | Sets in late April; seed indoors in February for transplants |
| Tomatoes | Start indoors | Sow indoors mid-March to early April; transplant out late May |
| Peppers | Start indoors | Sow indoors early-to-mid March (they are slow); transplant out late May |
| Cucumbers & squash | Direct sow or start indoors | Direct sow late May to early June; or start indoors early May for a head start |
| Beans (bush & pole) | Direct sow | Late May to early June — seed rots in cold soil and seedlings die in frost |
| Corn | Direct sow | Mid-to-late May, in blocks for pollination |
| Broccoli & cabbage | Start indoors or buy transplants | Transplant out late April to May; they tolerate light frost |
| Pumpkin & winter squash | Direct sow | Late May; needs the longest warm runway you can give it |
| Garlic | Plant cloves | October — the one crop planted in fall |
THE METHOD How to use the chart without getting burned
Timing mistakes in Ontario are almost always the same one: planting tender crops on the first warm weekend in April. Work through these checks instead.
- Sort your seeds into three pilesFrost-hardy (peas, lettuce, spinach, carrots, onions, brassicas), frost-tender (tomatoes, peppers, cucumbers, squash), and warm-soil-only (beans, corn). The piles get planted weeks apart.
- Start the slow indoors crops in MarchPeppers first (early-to-mid March), then tomatoes (mid-March to early April). Count back 6–8 weeks from your late-May transplant date — earlier than that and seedlings just get leggy on the windowsill.
- Direct-sow the hardy crops as soil allowsFrom mid-April, if a handful of soil crumbles instead of clumping into mud, peas, lettuce, spinach, and onion sets can go in — even if nights still frost.
- Hold tender transplants until nights settleTender crops go out when overnight lows are reliably above 10°C and the 10-day forecast shows no frost — in most years, the second half of May. Harden seedlings off over 7–10 days first.
- Keep sowing the fast cropsLettuce, radish, beans, and carrots reward small repeated sowings every 2 weeks. One big April sowing gives you a glut, then nothing.
WATCH OUT Common mistakes
- Planting tomatoes on the first warm April weekendA single night at 0°C kills them. Garden centres sell tomatoes early because people buy them early — keep yours indoors until late May.
- Sowing beans into cold soilBean seed sits in cold, wet soil and rots. Wait until the soil is genuinely warm — late May at the earliest.
- Starting seeds too early indoorsSeedlings started in February for a late-May transplant become tall, weak, and rootbound. Later and compact beats early and leggy.
- Treating the frost date as a guaranteeIt is an average. Roughly half of years see a frost after it. Watch the forecast, and keep row cover or old bedsheets handy in May.
Good to know
Soil temperature beats air temperature for seeds. A cheap soil thermometer — or the old rule of sitting the back of your hand on the soil — tells you more than the calendar: most seeds germinate poorly below 10°C soil, and beans and squash want closer to 16°C. In Durham’s clay, soil also warms a few days later than sandy soil a block away.