What does USDA hardiness zone 5a mean in Maine?

Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 26 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 5a is the growing season longest?

Norridgewock runs the longest season on this page at about 159 days; Bethel is shortest at about 112 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 5a?

Using Mattawamkeag's frost dates (this page's median-season location): start seeds indoors about Mar 13 – Mar 27, then transplant outside about May 15 – May 22. 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 5a 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 26 locations listed (see the table above for each location's own dates).

USDA zone 5a in Maine

−20 to −15 °F avg. annual extreme minimum · 26 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5a covers 26 locations across Maine (average annual extreme minimum −20 to −15 °F): Bethel, ME, Bethel, ME, Bethel, ME, Bridgton, ME, Brownville Junction, ME, Dover-Foxcroft, ME, and 20 more. Average last spring frost across this group ranges May 2 (Norridgewock) to May 30 (Bethel), and growing seasons run 112–159 days (Bethel to Norridgewock) — 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 Mattawamkeag, this group's median-season location.

USDA zone: 5a−20 to −15 °F
Last frost range: May 2–May 30avg, 32°F
First frost range: Sep 19–Oct 10avg, 32°F
Growing season range: 112–159days

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)
Bethel	—	Rumford 6sw	May 12	Sep 29	139
Bethel	—	Bethel 6 Sse	May 6	Oct 2	147
Bethel	—	Berlin Muni Ap	May 30	Sep 19	112
Bridgton	Norway, Naples, Bethel	Bridgton 3 Nw	May 5	Oct 5	151
Brownville Junction	—	Barnard	May 12	Oct 1	141
Dover-Foxcroft	Milo, Howland, Dexter, Guilford	Dover-foxcroft Wwtp	May 15	Sep 28	134
Greenville	Guilford	Greenville Maine Forestry Svc	May 7	Oct 9	153
Hartland	—	Harmony	May 11	Oct 1	141
Howland	Lincoln, Mattawamkeag	Lincoln Sanitary District/wtp	May 4	Oct 6	153
Kingfield	Bingham	Long Falls Dam	May 13	Sep 30	138
Mattawamkeag *	Patten, Island Falls, East Millinocket	Millinocket Muni Ap	May 8	Oct 1	143
Mattawamkeag	Danforth, Woodland, Vanceboro	Topsfield 2	May 6	Oct 8	155
Norridgewock	Clinton, Winslow, Oakland, Fairfield, Unity	Waterville Trtmt Plt	May 2	Oct 10	159
Old	Bangor, Brewer, Bucksport, Hampden, Orono, Winterport, +1 more	Bangor Intl Ap	May 4	Oct 4	152
Old	Orono, Milford, Bradley	Old Town 2 W	May 13	Sep 28	137
Patten	—	Patten 2	May 13	Sep 30	138
Pittsfield	Dexter, Newport, Hartland, Corinna, Unity	Corinna	May 8	Oct 3	148
Rumford	Mexico	Rumford 1 Sse	May 4	Oct 6	155
Rumford	Mexico	Middle Dam	May 19	Oct 1	133
Rumford	Norway, South Paris, Chisholm, Dixfield	Hartford	May 6	Oct 3	148
Skowhegan	Norridgewock, Madison, Hartland, North Anson, Anson	Madison	May 8	Oct 5	149
Steep Falls	Lake Arrowhead, Cornish	Hollis	May 14	Sep 30	136
Steep Falls	Cornish, Naples, Kezar Falls	East Hiram	May 12	Oct 1	140
Vanceboro	—	Vanceboro No 2	May 13	Oct 1	139
Wilton	Farmington, Kingfield, Chisholm, Mexico	Farmington	May 16	Sep 26	130
Winterport	Unity	Belfast	May 7	Oct 5	151

* Mattawamkeag 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 (Mattawamkeag, representative)

Computed from Mattawamkeag's average frost dates — the median-season location in this Mainezone 5a 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 Mattawamkeag (Maine, zone 5a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 13 – Mar 27	May 15 – May 22	Jul 14 – Aug 3	—	matures comfortably
Pepper	Very tender	Feb 27 – Mar 13	May 22 – May 29	Jul 21 – Aug 20	—	matures comfortably
Cucumber	Tender	Apr 10 – Apr 17	May 15 – May 22	Jul 4 – Jul 24	—	matures comfortably
Summer squash / zucchini	Tender	—	May 15 – May 22	Jun 29 – Jul 14	—	matures comfortably
Bush bean	Tender	—	May 15 – May 22	Jul 4 – Jul 14	Aug 2 – Aug 12	matures comfortably
Sweet corn	Tender	—	May 8 – May 22	Jul 7 – Aug 6	—	matures comfortably
Basil	Very tender	Mar 27 – Apr 10	May 15 – May 22	Jun 14 – Jun 29	—	matures comfortably
Lettuce	Half-hardy	Mar 27 – Apr 10	Apr 10 – Apr 24	May 25 – Jun 9	Jul 19 – Aug 3	matures comfortably
Pea	Hardy	—	Mar 27 – Apr 10	May 21 – Jun 5	Jul 9 – Jul 24	matures comfortably
Spinach	Hardy	—	Mar 27 – Apr 10	May 6 – May 16	Jul 29 – Aug 8	matures comfortably
Carrot	Half-hardy	—	Apr 17 – Apr 24	Jun 16 – Jul 6	Jun 29 – Jul 19	matures comfortably
Broccoli	Half-hardy	Mar 13 – Mar 27	Apr 10 – Apr 24	Jun 4 – Jun 24	Jul 4 – Jul 24	matures comfortably

Which crops fit this growing season

Using Mattawamkeag's own 143-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 (Mattawamkeag)

From NOAA's 1991–2020 Climate Normals at station USW00014610. 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), Mattawamkeag.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 22	Jun 4	Sep 22	Oct 3	120
32°F (freeze)	May 8	May 24	Oct 1	Oct 14	143
28°F	Apr 28	May 9	Oct 13	Oct 30	168
24°F	Apr 18	Apr 29	Oct 28	Nov 11	192

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 26 locations is 1,885; Mattawamkeag's own reading is shown below.

Annual growing degree days for Mattawamkeag (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,929	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,697	cool-season crops (brassicas, greens)

Hardiness zone 5a in Maine

Zone 5a means an average annual extreme minimum winter temperature of about −20 to −15 °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, 5b, 6a, 6b, 7a.

Explore zone 5a in other states at zone 5a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5a mean in Maine?: Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 26 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 5a is the growing season longest?: Norridgewock runs the longest season on this page at about 159 days; Bethel is shortest at about 112 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 5a?: Using Mattawamkeag's frost dates (this page's median-season location): start seeds indoors about Mar 13 – Mar 27, then transplant outside about May 15 – May 22. 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 5a 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 26 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.