When Is the Last Frost in Boston? Planning Around Zone 6b Reality

Quick Answer

The average last frost in Boston lands April 25–May 5 depending on neighborhood, and the safe planting date for tender vegetables is May 15. The 30-year NOAA record shows Boston has had frost as late as May 19 (logan Airport, 1947) and as early as April 7 (warm year). Plan around the safe date, not the average — a single late frost wipes out a season's tomato transplants. Coastal Boston (Charlestown, East Boston, Dorchester waterfront) frees up earlier than interior Boston (Roslindale, West Roxbury, parts of Brighton).

Why "Last Frost" Is a Range, Not a Date

The last-frost question gets asked every March in Boston because the answer depends on three things: the neighborhood, the year, and how much risk you accept.

• The neighborhood matters because Boston Harbor and the salt marshes moderate overnight lows. Coastal neighborhoods stay 3–5°F warmer than interior on a clear, calm spring night — the conditions when frost actually forms.
• The year matters because variability is high. Boston's standard deviation on last-frost date is about 10 days. Some years it's April 18; some years it's May 12.
• Risk acceptance matters because the average and the safe date are different numbers. The average is the 50/50 line. The safe date is the 90% line.

Q: What's the official last frost date for Boston?

A: April 27 average, with a 90% safe date of May 12. That's pulled from the NOAA NWS Boston climate normals at Logan Airport, 1991–2020 averaging period.

Translation: - April 27: historically, half of Boston spring seasons have ended frost by this date. - May 12: historically, 90% of Boston spring seasons have ended frost by this date. - May 19: the latest recorded last-frost date at Logan in the modern record.

Q: What's the difference between "frost" and "freeze"?

A: Frost forms at 36°F air temp; freeze damage starts at 32°F. Plant damage thresholds:

• Light frost (36–32°F): Cosmetic damage on tender vegetables; cool-season crops fine.
• Moderate frost (32–28°F): Tender vegetables die back. Tomato transplants gone.
• Hard freeze (28°F or below): Most cool-season vegetables damaged. Spring blossoms on fruit trees lost.

The "frost" date in NOAA records is the last date with a low at or below 32°F. Use that as your anchor.

Q: Which Boston neighborhoods free up earliest?

A: Coastal first; interior last. The order, in a typical year:

1. East Boston, Charlestown, downtown waterfront — last frost around April 18–22.
2. South Boston, Dorchester waterfront, Castle Island — April 20–25.
3. Most of Cambridge, Somerville, Allston — April 25–May 1.
4. Roslindale, West Roxbury, Mattapan, JFK area — May 1–8.
5. Hyde Park, Mission Hill, parts of Brighton — May 3–10.
6. Roxbury, Jamaica Plain (low-lying) — May 1–10 (the cold air pools).

For Roslindale-specific zone analysis, see how to read the USDA Hardiness Zone Map for Roslindale and West Boston neighborhoods.

Q: When can I plant cool-season vegetables?

A: Any time after soil hits 50°F — typically March 28–April 5 in coastal Boston, April 5–12 inland. Cool-season crops (peas, lettuce, spinach, kale, radishes) tolerate frost down to 28°F — they're not the limiting variable. Soil temperature is.

For specific crops to plant late March, see 5 crops to plant in a Plymouth raised bed at the end of March — Plymouth runs about 5 days ahead of Boston, but the same crops fit Boston by April 5.

Q: When can I plant warm-season vegetables?

A: After May 15 for safe planting; May 10 if you're willing to cover for frost events.

• Tomatoes, peppers, eggplant: May 15 transplant.
• Cucumbers, squash, beans: May 15–20 direct sow (germinate at 60°F+).
• Basil and tender herbs: May 20+.
• Sweet corn: May 25–June 5.

Planting earlier means accepting risk. A May 8 transplant of tomatoes will probably be fine, but a single overnight at 30°F kills the plants and you start over from the garden center.

Q: What if I plant before the safe date?

A: Cover for frost events. Floating row cover (lightweight) keeps tender crops 4–6°F warmer than ambient. Old bedsheets work in a pinch. Heavier frost cover (Reemay 1.5 oz) keeps crops 8°F warmer.

Watch the NWS Boston frost-and-freeze forecasts closely between May 1 and May 15. A clear, calm overnight with predicted low under 35°F is when frost actually forms.

Q: Do greenhouses or cold frames change the math?

A: Yes — by 2–3 weeks earlier. A simple PVC-and-plastic cold frame maintains 8–12°F above ambient, extending the planting window from late March (vs. April 25) for warm-season transplants. Worth the investment if you're serious about extending the season.

Q: How is climate change shifting the dates?

A: Boston's last-frost date has shifted ~6 days earlier since 1990. The 1961–1990 normal at Logan was May 3; the 1991–2020 normal is April 27. The trend is real, the year-to-year variability is still real, and the safe planting date hasn't moved much because that's the 90% line, not the average.

In 2026, plan around historical norms. Don't bet on warming-trend earliness — the cold years still happen.

Q: What about the Zone 6b vs Zone 7a question?

A: Boston is borderline. The USDA Plant Hardiness Zone Map places coastal Boston in Zone 7a (0 to 5°F average annual extreme low) and interior Boston in Zone 6b (-5 to 0°F). The zone map is about winter cold tolerance for perennials, not last-frost dates for vegetables — but the same coastal-vs-interior pattern applies to both.

For perennials and shrubs, a Zone 7a-rated plant performs in coastal Boston but might struggle in West Roxbury. For vegetables, the last-frost timing follows the same coastal/interior gradient.

The Boston Planting Calendar (2026)

Working backward from a May 15 warm-season planting target:

• March 28: Cool-season direct-sow (peas, lettuce, spinach, radishes) in coastal Boston. See 5 crops to plant in a Plymouth raised bed at the end of March.
• April 5: Same in interior Boston.
• April 15: Pre-emergent on lawns (Plymouth County pre-emergent guide covers timing).
• April 25–May 5: Average last frost. Tender plants still risky.
• May 15: Safe warm-season planting. Tomatoes, peppers, eggplant, cucumbers, squash, beans go in.
• May 25: Sweet corn, basil, last warm-season crops.

For the broader regional calendar context, the 2026 Plymouth County outlook covers material timing and seasonal dynamics across eastern MA.

The short version: average last frost April 27, safe date May 15. Coastal Boston frees up first, interior Boston last. Plant cool-season now, warm-season May 15. Don't bet on the average.