Last Frost Date New Brunswick — By City and Region

New Brunswick's last frost dates vary dramatically across the province thanks to the strong contrast between Bay of Fundy coastal moderation and interior continental climate. Saint John (Bay of Fundy coast): May 3 (Zone 6a, mildest in NB). St. Stephen, St. Andrews (south coast): May 8 (Zone 6a). Moncton (Northumberland Strait coast): May 12 (Zone 5b). Sussex: May 18 (Zone 5b, inland). Fredericton (interior, Saint John River valley): May 24 (Zone 5b, classic frost pocket). Miramichi: May 22 (Zone 5a). Bathurst, Acadian Peninsula: May 24 (Zone 5a, cool north coast). Campbellton: May 26 (Zone 4b). Edmundston (Madawaska region, far northwest): May 26 (Zone 4b). The 1991–2020 climate normals from Environment and Climate Change Canada are the source for all dates.

Saint John sits directly on the Bay of Fundy, which has Canada's strongest tidal moderation effect — the massive twice-daily tides constantly cycle cold ocean water through coastal embayments, producing extremely stable spring temperatures. Saint John's last frost (May 3) reflects this exceptional moderation. Fredericton sits 100 km inland up the Saint John River valley, losing all Bay of Fundy moderation and gaining classic continental frost-pocket behaviour — cold air drains down from surrounding hills into the river valley on still spring nights. Fredericton's May 24 average last frost is 21 days later than Saint John despite being only 100 km apart. The Saint John River valley creates a frost-pocket corridor that extends from Fredericton up through Woodstock and beyond.

Tomato transplant dates for New Brunswick: Saint John, south coast: May 15–25 (mildest area). St. Stephen, St. Andrews: May 18–28. Moncton: May 22–June 1. Sussex: May 28–June 5. Fredericton: June 3–10 (inland frost pocket; wait longer). Miramichi: June 1–8. Bathurst, Acadian Peninsula: June 3–10. Campbellton, Edmundston: June 5–12. The general New Brunswick rule: wait 10–12 days past your local average last frost for tomatoes, peppers, basil, beans, and squash. Inland frost pockets (Fredericton valley, Sussex valley) need longer buffers than the calendar suggests — cold air drainage on still June nights still drops temperatures unexpectedly.

New Brunswick spans hardiness zones from Zone 6a along the Bay of Fundy (Saint John, St. Stephen) to Zone 4b in the far northwest (Edmundston, Campbellton). Most of New Brunswick is Zone 5a–5b. Moncton is Zone 5b, Fredericton Zone 5b, Bathurst Zone 5a, Edmundston Zone 4b. The dramatic zone variation reflects the province's combination of Bay of Fundy moderation, interior continental cold pockets, and northern Acadian/Madawaska elevation. Plants rated Zone 5 thrive across most of populated New Brunswick except the far northwest interior. The Bay of Fundy coastal strip can grow Zone 6 plants (similar to Halifax) including hardy figs in protected spots.

Fredericton's last spring frost date is May 24 for the city centre — Zone 5b. This is significantly later than the Bay of Fundy coast despite being a southern New Brunswick city, because Fredericton sits inland in the Saint John River valley where cold air pools on still spring nights. The historical range spans early May (earliest recorded) to mid-June (latest). Riverfront and downtown Fredericton run a few days earlier than higher-elevation neighbourhoods (Skyline Acres, north Fredericton) which can see frost into early June. Plant tomatoes, peppers, and basil after June 3–10 for reliable success. The University of New Brunswick maintains the local frost data.

The Bay of Fundy has the world's highest tidal range — up to 16 metres twice daily. This massive tidal cycling constantly mixes deep cold ocean water with surface water and brings stable temperatures to surrounding shorelines. Saint John's effective spring climate is more like coastal British Columbia than mainland New Brunswick: stable temperatures, slow seasonal transitions, rare frost after early May. The trade-off is summer fog and cool ocean temperatures that make summers cooler than inland New Brunswick. Saint John gardens grow plants impossible elsewhere in NB — hardy figs, certain camellias, lavender that overwinters reliably — thanks to the Bay's exceptional moderation.

Late frost in New Brunswick inland frost pockets (Fredericton, Sussex valley) can drop to -3 to -5°C even in late May. Protection options: floating row cover (Reemay, Agribon) over transplants protects to about -3°C. Heavier woven frost blankets protect to -5°C for severe cold snaps. Cloches or inverted pots over individual seedlings. Heavy mulch around transplants to insulate roots. Plant tender crops in the warmest spot — south-facing fence, sheltered from cold-air drainage. Watch the forecast for clear, still nights after warm May days — classic radiation-frost conditions in valley locations. Bay of Fundy coast rarely needs serious frost protection. Available at Halifax Seed (also ships to NB), Corn Hill Nursery, Cedar Rim Nursery (Fredericton), Atlantic Superstore garden centres.

New Brunswick has the widest range of last-frost dates of any Atlantic province thanks to its Bay of Fundy / interior climate split. Saint John (May 3) is the earliest of any Atlantic city — earlier than Halifax (May 10) and PEI (May 13). Fredericton (May 24) is among the latest in Atlantic Canada — later than Halifax, PEI, or coastal NS. Within Atlantic Canada: Saint John NB May 3, Yarmouth NS May 1, Halifax May 10, Moncton NB May 12, PEI Charlottetown May 13, Truro NS May 18, Fredericton NB May 24, Sydney NS May 22, Campbellton/Edmundston NB May 26, St. John's NL May 30. The Saint John vs Fredericton split (21 days apart) is the largest within-province variation in Atlantic Canada.