Last Frost Date London Ontario — April 22 (Zone 6a)
Last frost date London Ontario: April 22 for the central core (Zone 6a). The downtown Thames valley runs a couple of days earlier; rural Middlesex County runs 6–14 days later. Great Lakes microclimate, historical range, frost protection.
Last frost date London Ontario 2026: April 22 for the central core (downtown, Old North, Old South) — hardiness Zone 6a. Thames valley core: April 20–24. North & west London: April 23–28. Rural Middlesex County: April 28–May 6. Wait until May 5–10 to transplant tomatoes, peppers, and basil (May 10–18 in rural Middlesex). Historical range: April 5 (earliest) to May 14 (latest). Source: Environment and Climate Change Canada climate normals (1991–2020).
❄️ London Ontario Frost Dates at a Glance
Historical Average and Range
The last frost date for London Ontario — April 22 for the central core — is the 50th-percentile historical average drawn from Environment and Climate Change Canada climate normals for the 1991–2020 reference period. In plain terms: roughly half of recent years have seen London's last spring frost before April 22, and half after. It is a planning anchor, not a guarantee.
The full historical range tells the supporting story. The earliest recorded last spring frost in London's central core in modern records lies around April 5; the latest sits around May 14. That's a 39-day window. London sits in a sheltered inland position in southwestern Ontario, moderated by Lake Erie about 35 km to the south and Lake Huron about 60 km to the west — the two lakes keep both spring lows and fall frosts gentler than London's latitude alone would suggest. The downtown Thames River valley adds an urban heat island effect, keeping the city core a degree or two warmer overnight than the surrounding farmland.
The 1991–2020 climate normals replaced the older 1981–2010 normals in 2021. Compared to the older reference period, London's average last frost has shifted about 3–4 days earlier due to gradual warming, consistent with broader Great Lakes trends. ECCC updates its 30-year normals every decade. The April 22 figure is current and will remain the official average until the next update around 2031.
Last Frost by London Neighbourhood and Surrounding Community
London's last frost varies by urban density and distance from the city core. The downtown Thames River valley and the older central neighbourhoods hold the most heat. North, west, and outer suburbs run a few days later. Rural Middlesex County — open farmland where cold air drains and pools on clear nights — runs a week or more behind the city. The Lake Erie shore at Port Stanley is the warmest pocket in the wider region.
| Neighbourhood / Community | Avg. Last Frost | Zone | Notes |
|---|---|---|---|
| Downtown, Thames valley core | April 20–24 | 6a/6b | Urban heat island + river valley — warmest in the city |
| Old North, Old South, Wortley Village | April 21–25 | 6a | Mature central neighbourhoods; tree canopy moderates |
| Byron, Lambeth, south London | April 22–26 | 6a | South-west suburbs; close to the city average |
| Masonville, north London | April 23–28 | 6a | Northern suburbs; slightly higher, more open |
| Hyde Park, west London | April 23–28 | 6a | West-end suburbs; newer development, open exposure |
| Rural Middlesex County | April 28–May 6 | 5b/6a | Open farmland; cold-air drainage, no urban heat |
| St. Thomas | April 22–27 | 6a | South of London, toward Lake Erie |
| Strathroy, Woodstock | April 26–May 3 | 5b/6a | Surrounding towns; inland, slightly cooler than London |
| Port Stanley (Lake Erie shore) | April 18–23 | 6b | Lake Erie waterfront; warmest pocket in the region |
Dates derived from ECCC climate normals (1991–2020) and station-level observations from London International Airport (YXU), London CS, and surrounding southwestern Ontario stations. Treat as historical averages; actual frost dates vary year to year by up to 2 weeks.
How to Protect Plants from a Late London Frost
Frost after April 22 happens in roughly 1 in 5 years in London's central core, more often in rural Middlesex County. London's late frosts are moderate — rarely below −3°C — but cold air pools on open farmland, so rural gardeners need to be more careful. Standard frost protection covers any scenario.
Floating row cover (the workhorse)
Spun-bonded fabric (Reemay, Agribon) draped loosely over transplants traps ground heat overnight and protects to about −3°C — more than enough for typical London late frosts. Drape in late afternoon before temperatures drop, weight the edges with stones or soil, and remove in the morning once temperatures rise above 5°C. A single 1.5 m × 10 m roll covers a typical vegetable bed for a full season. Available at garden centres across London and Middlesex County for $15–25.
Urban heat island advantage (Thames valley core)
If you garden in downtown London or the older central neighbourhoods, the urban heat island and Thames River valley keep overnight lows 1–2°C warmer than rural Middlesex. You can transplant tender crops a few days earlier than the April 22 average suggests, and a south-facing city garden is the warmest growing situation in the area. Rural Middlesex gardeners do not get this buffer and should hold to the May 10 rule.
Cloches and inverted containers
For individual transplants when a frost is forecast, an inverted plastic milk jug (bottom cut off), large yogurt container, or commercial cloche provides emergency protection to about −2°C. Set in place before sunset, remove first thing in the morning so plants don't cook. A 4-pack of garden cloches runs $15–20 and lasts years — useful in rural Middlesex where late frosts are more frequent.
Victoria Day rule (rural Middlesex)
Traditional Ontario gardening wisdom: wait until the Victoria Day long weekend (third Monday of May) to transplant tomatoes, peppers, and basil. For London's central core, this rule is conservative — you can safely transplant May 5–10 with row cover ready. For rural Middlesex County, Strathroy, and Woodstock, the Victoria Day rule is exactly right and shouldn't be broken without protection.
A lightweight floating row cover you drape over seedlings and beds when a late frost threatens — it buys several degrees of protection on cold nights and extends your growing season at both ends.
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What to Plant Before vs. After London's Last Frost
The April 22 last frost date is the pivot point of the London vegetable garden calendar. Cool-season crops can go in 4–6 weeks before; warm-season crops have to wait at least 1–2 weeks after. London's ~178-day growing season provides ample margin for heat-loving crops.
❄️ Plant before April 22 (frost-tolerant)
- Direct sow mid-to-late March: peas, spinach, radishes, lettuce, arugula, kale
- Direct sow late March/early April: carrots, beets, Swiss chard, turnips
- Transplant early April: broccoli, cauliflower, cabbage, kohlrabi
- Transplant mid-April: onions, leeks, parsley, hardy herbs
- Plant fall (mid-Oct): garlic (hardneck Music, Russian Red)
⚠️ Wait until after May 5 (frost-sensitive)
- Tomatoes: transplant May 5–10 (rural Middlesex: May 10–18)
- Peppers: transplant May 10–18 (need 15°C soil)
- Basil: May 12 minimum — cold damage stunts permanently
- Beans, cucumbers, squash: direct sow May 5–15
- Eggplant, melons, sweet potato: May 18–25 (London's season handles them)
How London's Frost Date Compares to Other Canadian Cities
London Ontario has among the earliest last frosts of any Canadian city — close behind Toronto and Windsor, behind only coastal BC. The ~178-day growing season is a solid mid-range Ontario season, and the southwestern Ontario position between Lake Erie and Lake Huron gives London a warm, well-moderated climate.
| City | Last Frost | Zone | Season | vs. London |
|---|---|---|---|---|
| Vancouver | March 15 | 8b | ~260 days | 38 days earlier |
| Windsor | April 20 | 7a | ~190 days | 2 days earlier |
| Toronto | April 20 | 6b | ~197 days | 2 days earlier |
| London Ontario | April 22 | 6a | ~178 days | — |
| Hamilton | April 25 | 6b/7a | ~186 days | 3 days later |
| Montreal / Ottawa | May 9 | 5a/5b | ~145–150 | 17 days later |
| Calgary | May 23 | 3b | ~120 days | 31 days later |
Common Questions about London's Last Frost
When can I safely transplant tomatoes outdoors in London Ontario?
May 5–10 in the central core, May 10–18 in rural Middlesex County. The traditional Victoria Day rule (third Monday of May) works for any London gardener without protection. Tomatoes need both frost-free conditions and warm soil (above 12°C at 5 cm depth). London's ~178-day season allows most varieties, including 80–85 day heirlooms. Always harden off seedlings for 7–10 days before transplanting.
Why does rural Middlesex County frost later than the city of London?
The urban heat island. Downtown London and the older central neighbourhoods are full of buildings, pavement, and the Thames River valley, which all store and radiate heat — keeping the city core 1–2°C warmer overnight in spring. Rural Middlesex County is open farmland, where there is nothing to hold heat and cold air drains and pools in low ground on clear, still nights. The net result is a last-frost gap of 6–14 days between the city centre and the surrounding countryside.
Is London Ontario Zone 6a or Zone 6b?
London is officially Zone 6a for the central core under the Canadian Plant Hardiness Zone system. The downtown Thames River valley sits in a slightly warmer Zone 6a/6b pocket thanks to the urban heat island. Rural Middlesex County drops to Zone 5b/6a. A Zone 6 plant overwinters reliably across the London area. London is one zone cooler than Hamilton and Windsor (Zone 6b/7a) but its sheltered southwestern Ontario position between Lake Erie and Lake Huron keeps it milder than continental Ontario cities at the same latitude.
When is the first fall frost in London Ontario?
Around October 18 for the central core, giving London a roughly 178-day growing season. Rural Middlesex County can frost a week or more earlier. Fall frost arrives faster and harder than spring frost: a clear, cold mid-October night can drop temperatures to −2°C and end the tomato season overnight. Covering tomatoes and peppers with row cover during the first one or two light frosts of October can buy 7–10 extra days of ripening.
Where does this frost date data come from?
Environment and Climate Change Canada (ECCC) climate normals for the 1991–2020 reference period, supplemented by station-level observations from London International Airport (YXU), London CS, and surrounding southwestern Ontario stations. The April 22 average reflects central-core conditions. Rural Middlesex dates are adjusted for distance from the city, cold-air drainage, and the loss of the urban heat island.
📍 Related London Garden Resources
Build Your London Planting Calendar
The London planting guide turns April 22 into a full month-by-month schedule for 25+ vegetables — indoor start dates, transplant dates, succession sowing windows, and harvest timing for London's 178-day Zone 6a growing season.