What does USDA hardiness zone 6b mean in Maine?

Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 3 locations in Maine fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Maine's zone 6b is the growing season longest?

Thomaston runs the longest season on this page at about 178 days; Rockland is shortest at about 175 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in Maine's zone 6b?

Using Southwest Harbor's frost dates (this page's median-season location): start seeds indoors about Mar 2 – Mar 16, then transplant outside about May 4 – May 11. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in Maine's zone 6b have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 3 locations listed (see the table above for each location's own dates).

USDA zone 6b in Maine

−5 to 0 °F avg. annual extreme minimum · 3 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6b covers 3 locations across Maine (average annual extreme minimum −5 to 0 °F): Rockland, ME, Southwest Harbor, ME, Thomaston, ME. Average last spring frost across this group ranges April 27 (Rockland) to April 27 (Thomaston), and growing seasons run 175–178 days (Rockland to Thomaston) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Maine location's own frost dates; the printable planting calendar uses Southwest Harbor, this group's median-season location.

USDA zone: 6b−5 to 0 °F
Last frost range: Apr 27–Apr 27avg, 32°F
First frost range: Oct 19–Oct 25avg, 32°F
Growing season range: 175–178days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Rockland	Southwest Harbor	W Rockport 1 Nnw	Apr 27	Oct 19	175
Southwest Harbor *	Northeast Harbor	Acadia Np	Apr 27	Oct 22	177
Thomaston	Boothbay Harbor	Tenants Harbor	Apr 27	Oct 25	178

* Southwest Harbor is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Southwest Harbor, representative)

Computed from Southwest Harbor's average frost dates — the median-season location in this Mainezone 6b group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Southwest Harbor (Maine, zone 6b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 2 – Mar 16	May 4 – May 11	Jul 3 – Jul 23	—	matures comfortably
Pepper	Very tender	Feb 16 – Mar 2	May 11 – May 18	Jul 10 – Aug 9	—	matures comfortably
Cucumber	Tender	Mar 30 – Apr 6	May 4 – May 11	Jun 23 – Jul 13	—	matures comfortably
Summer squash / zucchini	Tender	—	May 4 – May 11	Jun 18 – Jul 3	—	matures comfortably
Bush bean	Tender	—	May 4 – May 11	Jun 23 – Jul 3	Aug 23 – Sep 2	matures comfortably
Sweet corn	Tender	—	Apr 27 – May 11	Jun 26 – Jul 26	—	matures comfortably
Basil	Very tender	Mar 16 – Mar 30	May 4 – May 11	Jun 3 – Jun 18	—	matures comfortably
Lettuce	Half-hardy	Mar 16 – Mar 30	Mar 30 – Apr 13	May 14 – May 29	Aug 9 – Aug 24	matures comfortably
Pea	Hardy	—	Mar 16 – Mar 30	May 10 – May 25	Jul 30 – Aug 14	matures comfortably
Spinach	Hardy	—	Mar 16 – Mar 30	Apr 25 – May 5	Aug 19 – Aug 29	matures comfortably
Carrot	Half-hardy	—	Apr 6 – Apr 13	Jun 5 – Jun 25	Jul 20 – Aug 9	matures comfortably
Broccoli	Half-hardy	Mar 2 – Mar 16	Mar 30 – Apr 13	May 24 – Jun 13	Jul 25 – Aug 14	matures comfortably

Which crops fit this growing season

Using Southwest Harbor's own 177-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Southwest Harbor)

From NOAA's 1991–2020 Climate Normals at station USC00170100. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Southwest Harbor.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 8	May 23	Oct 8	Oct 25	151
32°F (freeze)	Apr 27	May 8	Oct 22	Nov 5	177
28°F	Apr 15	Apr 27	Nov 2	Nov 16	201
24°F	Apr 4	Apr 15	Nov 14	Nov 27	222

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 3 locations is 2,043; Southwest Harbor's own reading is shown below.

Annual growing degree days for Southwest Harbor (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,291	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,296	cool-season crops (brassicas, greens)

Hardiness zone 6b in Maine

Zone 6b means an average annual extreme minimum winter temperature of about −5 to 0 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. Maine spans 8 hardiness zones in this dataset — see all Maine locations for the full range, including zones3b, 4a, 4b, 5a, 5b, 6a, 7a.

Explore zone 6b in other states at zone 6b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6b mean in Maine?: Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 3 locations in Maine fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Maine's zone 6b is the growing season longest?: Thomaston runs the longest season on this page at about 178 days; Rockland is shortest at about 175 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in Maine's zone 6b?: Using Southwest Harbor's frost dates (this page's median-season location): start seeds indoors about Mar 2 – Mar 16, then transplant outside about May 4 – May 11. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in Maine's zone 6b have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 3 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.