What does USDA hardiness zone 4a mean in Maine?

Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 9 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 4a is the growing season longest?

Greenville runs the longest season on this page at about 153 days; Eagle Lake is shortest at about 102 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 4a?

Using Limestone's frost dates (this page's median-season location): start seeds indoors about Mar 21 – Apr 4, then transplant outside about May 23 – May 30. 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 4a 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 9 locations listed (see the table above for each location's own dates).

USDA zone 4a in Maine

−30 to −25 °F avg. annual extreme minimum · 9 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4a covers 9 locations across Maine (average annual extreme minimum −30 to −25 °F): Blaine, ME, Caribou, ME, Eagle Lake, ME, Fort Kent, ME, Greenville, ME, Limestone, ME, and 3 more. Average last spring frost across this group ranges May 6 (Madawaska) to June 2 (Eagle Lake), and growing seasons run 102–153 days (Eagle Lake to Greenville) — 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 Limestone, this group's median-season location.

USDA zone: 4a−30 to −25 °F
Last frost range: May 6–Jun 2avg, 32°F
First frost range: Sep 15–Oct 9avg, 32°F
Growing season range: 102–153days

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)
Blaine	Mars Hill	Bridgewater	May 25	Sep 18	115
Caribou	Washburn, Van Buren, Fort Fairfield	Caribou Muni Ap	May 7	Oct 1	143
Eagle Lake	—	Allagash	Jun 2	Sep 15	102
Fort Kent	Eagle Lake	Ft Kent	May 23	Sep 24	121
Greenville	—	Greenville Maine Forestry Svc	May 7	Oct 9	153
Limestone *	—	Limestone 4 Nnw	May 16	Sep 24	129
Madawaska	Eagle Lake, Grand Isle	Frenchville Aroostook Ap	May 6	Oct 2	147
Presque Isle	Ashland, Mapleton, Mars Hill	Presque Isle	May 14	Sep 26	131
Van Buren	—	Van Buren 2	May 23	Sep 26	125

* Limestone 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 (Limestone, representative)

Computed from Limestone's average frost dates — the median-season location in this Mainezone 4a 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 Limestone (Maine, zone 4a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 21 – Apr 4	May 23 – May 30	Jul 22 – Aug 11	—	matures comfortably
Pepper	Very tender	Mar 7 – Mar 21	May 30 – Jun 6	Jul 29 – Aug 28	—	matures comfortably
Cucumber	Tender	Apr 18 – Apr 25	May 23 – May 30	Jul 12 – Aug 1	—	matures comfortably
Summer squash / zucchini	Tender	—	May 23 – May 30	Jul 7 – Jul 22	—	matures comfortably
Bush bean	Tender	—	May 23 – May 30	Jul 12 – Jul 22	Jul 26 – Aug 5	matures comfortably
Sweet corn	Tender	—	May 16 – May 30	Jul 15 – Aug 14	—	matures comfortably
Basil	Very tender	Apr 4 – Apr 18	May 23 – May 30	Jun 22 – Jul 7	—	matures comfortably
Lettuce	Half-hardy	Apr 4 – Apr 18	Apr 18 – May 2	Jun 2 – Jun 17	Jul 12 – Jul 27	matures comfortably
Pea	Hardy	—	Apr 4 – Apr 18	May 29 – Jun 13	Jul 2 – Jul 17	matures comfortably
Spinach	Hardy	—	Apr 4 – Apr 18	May 14 – May 24	Jul 22 – Aug 1	matures comfortably
Carrot	Half-hardy	—	Apr 25 – May 2	Jun 24 – Jul 14	Jun 22 – Jul 12	matures comfortably
Broccoli	Half-hardy	Mar 21 – Apr 4	Apr 18 – May 2	Jun 12 – Jul 2	Jun 27 – Jul 17	matures comfortably

Which crops fit this growing season

Using Limestone's own 129-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 (Limestone)

From NOAA's 1991–2020 Climate Normals at station USW00094645. 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), Limestone.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 31	Jun 13	Sep 14	Sep 27	104
32°F (freeze)	May 16	Jun 1	Sep 24	Oct 5	129
28°F	May 2	May 16	Oct 4	Oct 19	153
24°F	Apr 24	May 3	Oct 18	Nov 4	178

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 9 locations is 1,663; Limestone's own reading is shown below.

Annual growing degree days for Limestone (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,435	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,020	cool-season crops (brassicas, greens)

Hardiness zone 4a in Maine

Zone 4a means an average annual extreme minimum winter temperature of about −30 to −25 °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, 4b, 5a, 5b, 6a, 6b, 7a.

Explore zone 4a in other states at zone 4a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 4a mean in Maine?: Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 9 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 4a is the growing season longest?: Greenville runs the longest season on this page at about 153 days; Eagle Lake is shortest at about 102 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 4a?: Using Limestone's frost dates (this page's median-season location): start seeds indoors about Mar 21 – Apr 4, then transplant outside about May 23 – May 30. 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 4a 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 9 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.