Last Frost Date Markham — April 28 (Zone 6b)

Markham's last spring frost date is April 28 for the central core — Zone 6b. This is the historical 50th-percentile average from Environment and Climate Change Canada climate normals (1991–2020). South Markham (Thornhill, Milliken, German Mills) runs 2–4 days earlier (April 24–27) thanks to closer Lake Ontario moderation and urban heat island. North Markham (Berczy Village, Cathedraltown, Wismer Commons, Greensborough) runs 3–5 days later (May 1–5) because they sit further north and at slightly higher elevation. Unionville and central Markham track the city average. The full historical range spans April 12 (earliest recorded) to May 12 (latest).

The historical average last frost in Markham is April 28, and that average is what 2026 planning should be based on — frost dates are climate normals that do not change year to year. Individual years vary by 2–3 weeks earlier or later. For 2026 specifically, monitor Environment Canada forecasts through the second week of May. After May 12, frost risk in central Markham is very low in any year. North Markham gardeners and rural York Region should add a full week of buffer through mid-May. Victoria Day weekend (third Monday of May) is the traditional safe-transplant deadline for tomatoes, peppers, and basil across the GTA.

If today's date is past April 28, the average last frost has passed in central Markham. In any given year there is roughly a 15–20% chance of a frost after April 28 — Markham is further from Lake Ontario than Toronto or Mississauga, so its spring climate is slightly more variable. For frost-sensitive plants like tomatoes, peppers, and basil, wait until May 5–15 across Markham. After Victoria Day weekend, frost risk is very low. North Markham and rural York Region (Stouffville, Aurora boundary) should hold tender transplants until mid-to-late May.

Markham's neighbourhoods vary by latitude and elevation. Thornhill (south, bordering Toronto): April 24–27. Milliken, German Mills: April 24–27. Markville, Bullock, Aileen-Willowbrook: April 26–30. Unionville (central): April 26–30. Markham Village, Box Grove: April 27–May 1. Cornell, Cathedraltown: April 30–May 4. Berczy Village (north-central): May 1–5. Wismer Commons, Greensborough (north): May 1–5. Northeast corridor near Stouffville border: May 3–7. Rural Markham approaching Whitchurch-Stouffville: May 5–10.

Markham is officially Zone 6b for the central and southern parts under the Canadian Plant Hardiness Zone system — same as Toronto and Mississauga thanks to general Lake Ontario regional moderation. South Markham (Thornhill, Milliken) sits in the warmer end of Zone 6b approaching Zone 7a in lakeshore-proximate areas. North Markham (Cathedraltown, Wismer, Greensborough) drops to the cooler end of Zone 6b. Rural Whitchurch-Stouffville to the immediate north is Zone 6a. A Zone 6 plant will reliably overwinter anywhere in Markham; Zone 7 plants survive most years in south Markham/Thornhill but die in severe polar-vortex winters.

Frost-tolerant cool-season crops can go in 4–6 weeks before the April 28 last frost — meaning mid-to-late March in Markham. Direct sow as soon as soil is workable (typically late March): peas, spinach, radishes, lettuce, arugula, kale. Direct sow early April: carrots, beets, Swiss chard, turnips. Transplant mid-April: broccoli, cauliflower, cabbage, kohlrabi, onions (set out 3–4 weeks before last frost). Wait until after May 5–15 for tomatoes, peppers, basil, beans, and squash. Markham's growing season is approximately 180 days — long enough for full-season vegetables including indeterminate tomatoes, peppers, and winter squash.

Late frost in Markham can drop to -3°C in worst-case scenarios. Protection options: floating row cover (Reemay, Agribon) over transplants protects to about -3°C and is the GTA workhorse. Cloches or inverted pots over individual seedlings for predicted overnight frosts. Heavy mulch around transplants to insulate roots. Plant tender crops in the warmest spot — south-facing fence, against a heat-radiating brick wall. Watch the forecast for clear, still nights after warm April days — these are the radiation-frost conditions that catch gardeners off guard. Don't transplant tender crops if nights below 3°C are forecast through May 10. Available at Plant Paradise Country Gardens, Sheridan Nurseries Unionville, Loblaws Garden Centre, and Home Depot Markham.

Markham's April 28 last frost matches Brampton and Newmarket but trails Toronto and Mississauga by 8 days. Compared to other GTA cities: 8 days later than Toronto and Mississauga (April 20), 5 days later than Hamilton (April 23), same as Brampton (April 28), 4 days earlier than Newmarket (May 2), 11 days earlier than Barrie (May 9). Markham's 180-day growing season is roughly 10 days shorter than Toronto's but still supports reliable cultivation of full-season tomatoes, peppers, sweet corn, and winter squash. The largest single climate driver is distance from Lake Ontario — Toronto and Mississauga benefit from the lake's heat capacity; Markham loses most of that moderation by being 25–30 km inland.