Ontario Hardiness Zones — Zone by City & Region

Ontario spans a wide range of plant hardiness zones — from roughly Zone 2 in the far north (Hudson Bay lowlands, Moosonee) to Zone 7a in the extreme southwest (Windsor, Essex County, and Pelee Island, which is among the mildest inland sites in Canada). The great majority of Ontarians garden in Zone 5a to Zone 6b: Toronto, Mississauga, Brampton, Hamilton, Markham, Niagara Falls, and St. Catharines are Zone 7a; London, Kitchener, and Oshawa are Zone 6a; Guelph and Kingston are Zone 5b; Ottawa and Barrie are Zone 5b; Sudbury is Zone 4b and Thunder Bay is Zone 3b. Zones are assigned by Natural Resources Canada based on winter minimum temperature plus summer heat, rainfall, snow cover, wind, and frost-free days — so a Canadian zone is not identical to the U.S. (USDA) zone of the same number. Check your exact location on the NRCan Plant Hardiness Zone Map at planthardiness.gc.ca.

Toronto's urban core is Zone 6b (average annual winter minimum of about -20.6°C to -23.3°C). Its lakeshore neighbourhoods — the Beaches, Leslieville, harbourfront, and the Toronto Islands — are effectively Zone 7a thanks to Lake Ontario's moderating effect, while higher inland districts north of the city (Vaughan, Richmond Hill, north Markham) run cooler at Zone 6a. Toronto's average last spring frost is around May 10 and first fall frost around October 15, giving a frost-free growing season of roughly 155–160 days. For perennial and shrub selection use the zone (6b); for vegetable transplant timing use the frost dates.

The warmest hardiness zone in Ontario is Zone 7a, found in the extreme southwest — Windsor, Leamington, Kingsville, and the rest of Essex County, plus Pelee Island in Lake Erie (Canada's southernmost point, which some sources rate 7b). This Carolinian corner has the province's earliest last frost (Windsor around April 25), its longest growing season, and enough summer heat to reliably grow crops that struggle elsewhere in Ontario — sweet potatoes, okra, long-season melons, and tender figs in protected sites. It is one of only a handful of inland Zone 7 pockets anywhere in Canada; almost everywhere else that mild is coastal British Columbia.

Northern Ontario is considerably colder than the populous south. Thunder Bay sits in Zone 3b and Sudbury in Zone 4b (both confirmed in our city dataset). Further north and inland, the zone drops steadily: towns such as Timmins, Kapuskasing, Cochrane, and Chapleau fall into roughly Zone 2b–3a, and the far northern Hudson Bay lowlands (Moosonee, Fort Severn) reach Zone 1 or lower. Because these smaller communities aren't in our city dataset, don't take a single number as gospel — look up your exact town on the Natural Resources Canada Plant Hardiness Zone Map at planthardiness.gc.ca, which maps every location in the country. Northern Ontario gardeners lean on short-season varieties, season-extension (row covers, cold frames, Wall-O-Water), and cold-hardy perennials rated to Zone 3 or below.

Three ways, most precise first: (1) Use the official Natural Resources Canada Plant Hardiness Zone Map at planthardiness.gc.ca — enter your address or postal code for a location-specific zone that accounts for local terrain. (2) Use our interactive Zone Finder (growersguide.ca/zone-finder) — click Ontario to see zones and frost dates for cities across the province. (3) Match to the nearest city in the table on this page. Remember that zones can shift by one or two sub-zones within a single municipality: lakeshore and downtown-urban-heat-island sites run warmer, while inland, higher-elevation, and low-lying frost-pocket sites run colder. When a plant tag says 'hardy to Zone 6,' choosing one rated a zone colder than yours is the safe hedge.

No — and this trips up Ontario gardeners who buy plants or read growing advice from U.S. sources. The USDA system is based on winter minimum temperature alone. Natural Resources Canada's system factors in seven variables — winter minimum, summer maximum, frost-free period, rainfall, snow cover, and wind — so a Canadian 'Zone 6' is defined differently from a USDA 'Zone 6,' even though the numbers look the same. NRCan does also publish a separate USDA-method map for cross-border comparison. Practical rule for Ontario: use the Canadian (NRCan) zone from planthardiness.gc.ca for Canadian-sourced plants and seed catalogues, and treat a U.S. plant tag's zone as a rough guide only — when in doubt, pick the hardier (lower-numbered) option.

Your hardiness zone tells you which perennials, shrubs, and trees will survive a typical Ontario winter; your frost dates tell you when to plant tender vegetables. In Zone 6b (Toronto, Hamilton, London, Niagara) you can grow most perennials sold in Ontario garden centres, plus borderline Zone 7 plants in sheltered spots. In Zone 5b (Ottawa, Kingston, Barrie, Guelph) stick to Zone 5-or-hardier perennials and expect a last frost in mid-May. In Zone 4a Sudbury and Zone 3b Thunder Bay, choose Zone 3–4 perennials (Northern Lights azaleas, hardy roses, most native shrubs) and lean on season-extension for warm-season vegetables. For crop timing, see our Ontario city planting guides linked below and our frost calculator.

Yes — microclimates routinely move a site one to two sub-zones off the city's headline zone. The three big drivers in Ontario are the Great Lakes (shorelines on Ontario, Erie, and Huron run milder — Toronto's Beaches approach 7a, Hamilton's waterfront is warmer than the mountain), elevation and the Niagara Escarpment (higher ground is colder — Hamilton mountain is 6a while the lower city is 6b), and the urban heat island (dense downtown cores hold winter warmth, while low-lying rural frost pockets and river valleys just outside the city can be a full zone colder). This is why a location-specific NRCan map lookup beats relying on your city's single published number.

Hamilton is Zone 6b. Because the city straddles the Niagara Escarpment, it splits in two for gardeners: the lake-moderated lower city and waterfront hold Zone 6b (and sheltered bayfront pockets edge toward 7a), while Hamilton 'mountain' and the higher ground above the escarpment run a colder Zone 6a. Hamilton's average last spring frost is early-to-mid May. Use 6b for perennial and shrub selection in the lower city; if you garden on the mountain, hedge to Zone 6a-hardy plants.

Sudbury is Zone 4b — the coldest of Ontario's larger cities in our dataset, well north of the Zone 5b–6b belt where most Ontarians garden. Expect a short season and hard winters: a last spring frost around late May and a first fall frost by mid-to-late September. Choose perennials, shrubs, and trees rated to Zone 4 or (safer) Zone 3, and lean on season-extension — row covers, cold frames, Wall-O-Water — for warm-season vegetables like tomatoes and peppers.

Kitchener–Waterloo is Zone 6a. Because Waterloo Region sits inland, away from the moderating influence of Lake Ontario, it runs about a sub-zone colder than the lakeshore Golden Horseshoe cities (Toronto, Mississauga, Hamilton lower city) that are Zone 6b. The average last spring frost is around mid-May. Pick Zone 6-or-hardier perennials, and note that low-lying rural pockets around the region can drop to 5b.

Barrie is Zone 5b. It sits in central Ontario between Lake Simcoe and the Georgian Bay snowbelt, which makes it about a full zone colder than the Golden Horseshoe cities an hour south — Toronto and Mississauga are Zone 6b. Barrie's average last spring frost is around mid-May and the first fall frost arrives in early October, so the frost-free season runs roughly 140 days. For gardening this means choosing perennials, shrubs, and trees rated to Zone 5 or hardier, and holding tender vegetables — tomatoes, peppers, basil, squash — until the last frost has passed in mid-May. The heavy lake-effect snow Barrie receives is an advantage for perennials: consistent snow cover insulates crowns through the coldest weeks.

Windsor is Zone 7a — the warmest hardiness zone anywhere in Ontario and one of the very few inland Zone 7 pockets in Canada. Windsor and the rest of Essex County sit in the Carolinian southwest, with the province's earliest last spring frost (around April 25), its latest first fall frost, and its longest growing season. For planting, that means you can grow essentially any perennial sold in Ontario, plus heat-loving crops that struggle elsewhere in the province: sweet potatoes, okra, long-season melons, and figs in a protected spot. Nearby Pelee Island, Canada's southernmost point, is rated even milder by some sources.

Brampton is Zone 6b. It sits inland in Peel Region, north of the Lake Ontario shoreline, so it runs marginally cooler than lakefront Mississauga even though both carry the same 6b rating — Brampton's first fall frost arrives roughly a week earlier and spring frost risk lingers a few days longer. For planting, Zone 6b means most perennials, shrubs, and trees sold in southern Ontario garden centres will overwinter reliably. Hold tender vegetables until after the average last frost in early May, and note that low-lying parts of the city act as frost pockets that can catch a late frost after surrounding neighbourhoods are clear.

Mississauga is Zone 6b. Sitting directly on the Lake Ontario shoreline in Peel Region, it benefits from strong lake moderation — Port Credit, Lakeview, and the waterfront strip edge toward Zone 7a, while northern Mississauga away from the lake behaves closer to 6a. The average last spring frost is around April 20 and the first fall frost near October 28, giving one of the longer growing seasons in the Greater Toronto Area. For planting, Zone 6b covers nearly every perennial, shrub, and tree stocked by southern Ontario nurseries, and the long season suits full-size tomatoes, peppers, melons, and winter squash.

St. Catharines is Zone 7a. It sits in the Niagara fruit belt between Lake Ontario and the Niagara Escarpment — a combination that produces one of the mildest microclimates in the province, which is exactly why Niagara is Ontario's tender-fruit and wine region. Sheltered lakeshore sites in and around St. Catharines edge toward Zone 7a. For growing, this means peaches, sweet cherries, grapes, and other tender fruit that fail across most of Ontario succeed here, and Zone 6-rated perennials and shrubs overwinter without protection.

Sault Ste Marie is Zone 4b — the mildest of the major Northern Ontario cities, and milder in practice than Sudbury, which shares the 4b rating but frosts nearly two weeks later, and than Zone 3b Thunder Bay. The Sault's position on the St Marys River between Lake Superior and Lake Huron gives it real thermal moderation: its average last spring frost is May 18, roughly two weeks earlier than Sudbury's. Waterfront neighbourhoods approach Zone 5a, while rural Algoma District at elevation drops back to Zone 4a or 3b. Zone 4 plants overwinter reliably across the city — Saskatoon berries, hardy apples, prairie roses, Korean lilacs, and panicle hydrangeas are dependable choices.