What does USDA hardiness zone 6a mean in Connecticut?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 7 locations in Connecticut fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Connecticut's zone 6a is the growing season longest?

Bethlehem runs the longest season on this page at about 181 days; Lakeville is shortest at about 146 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 Connecticut's zone 6a?

Using Winsted's frost dates (this page's median-season location): start seeds indoors about Mar 8 – Mar 22, then transplant outside about May 10 – May 17. 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 Connecticut's zone 6a 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 7 locations listed (see the table above for each location's own dates).

USDA zone 6a in Connecticut

−10 to −5 °F avg. annual extreme minimum · 7 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 7 locations across Connecticut (average annual extreme minimum −10 to −5 °F): Bethlehem, CT, Bristol, CT, Lakeville, CT, North Granby, CT, Quinebaug, CT, Torrington, CT, and 1 more. Average last spring frost across this group ranges April 20 (Bethlehem) to May 11 (Lakeville), and growing seasons run 146–181 days (Lakeville to Bethlehem) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Connecticut location's own frost dates; the printable planting calendar uses Winsted, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: Apr 20–May 11avg, 32°F
First frost range: Oct 4–Oct 20avg, 32°F
Growing season range: 146–181days

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)
Bethlehem	New Preston	Danbury	Apr 20	Oct 20	181
Bristol	West Simsbury, Terryville, Canton Valley, Thomaston	Burlington	Apr 25	Oct 18	172
Lakeville	Sharon, Millerton	Falls Village	May 11	Oct 4	146
North Granby	Salmon Brook	Hartford Bradley Intl Ap	Apr 21	Oct 16	177
Quinebaug	North Grosvenor Dale, Quasset Lake	W Thompson Lake	May 1	Oct 9	161
Torrington	Northwest Harwinton, North Granby, Bethlehem, New Preston, New Hartford Center, Litchfield, +4 more	Bakersville	May 8	Oct 5	151
Winsted *	Cornwall Bridge, Canaan, Falls, West Cornwall, Norfolk, Cornwall, +2 more	Norfolk 2 Sw	May 3	Oct 11	161

* Winsted 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 (Winsted, representative)

Computed from Winsted's average frost dates — the median-season location in this Connecticutzone 6a 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 Winsted (Connecticut, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 8 – Mar 22	May 10 – May 17	Jul 9 – Jul 29	—	matures comfortably
Pepper	Very tender	Feb 22 – Mar 8	May 17 – May 24	Jul 16 – Aug 15	—	matures comfortably
Cucumber	Tender	Apr 5 – Apr 12	May 10 – May 17	Jun 29 – Jul 19	—	matures comfortably
Summer squash / zucchini	Tender	—	May 10 – May 17	Jun 24 – Jul 9	—	matures comfortably
Bush bean	Tender	—	May 10 – May 17	Jun 29 – Jul 9	Aug 12 – Aug 22	matures comfortably
Sweet corn	Tender	—	May 3 – May 17	Jul 2 – Aug 1	—	matures comfortably
Basil	Very tender	Mar 22 – Apr 5	May 10 – May 17	Jun 9 – Jun 24	—	matures comfortably
Lettuce	Half-hardy	Mar 22 – Apr 5	Apr 5 – Apr 19	May 20 – Jun 4	Jul 29 – Aug 13	matures comfortably
Pea	Hardy	—	Mar 22 – Apr 5	May 16 – May 31	Jul 19 – Aug 3	matures comfortably
Spinach	Hardy	—	Mar 22 – Apr 5	May 1 – May 11	Aug 8 – Aug 18	matures comfortably
Carrot	Half-hardy	—	Apr 12 – Apr 19	Jun 11 – Jul 1	Jul 9 – Jul 29	matures comfortably
Broccoli	Half-hardy	Mar 8 – Mar 22	Apr 5 – Apr 19	May 30 – Jun 19	Jul 14 – Aug 3	matures comfortably

Which crops fit this growing season

Using Winsted's own 161-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 (Winsted)

From NOAA's 1991–2020 Climate Normals at station USC00065445. 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), Winsted.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 15	May 28	Oct 2	Oct 14	139
32°F (freeze)	May 3	May 17	Oct 11	Oct 28	161
28°F	Apr 20	May 6	Oct 25	Nov 8	187
24°F	Apr 7	Apr 20	Nov 5	Nov 19	210

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 7 locations is 2,576; Winsted's own reading is shown below.

Annual growing degree days for Winsted (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,192	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,119	cool-season crops (brassicas, greens)

Hardiness zone 6a in Connecticut

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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. Connecticut spans 5 hardiness zones in this dataset — see all Connecticut locations for the full range, including zones5b, 6b, 7a, 7b.

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

Frequently asked questions

What does USDA hardiness zone 6a mean in Connecticut?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 7 locations in Connecticut fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Connecticut's zone 6a is the growing season longest?: Bethlehem runs the longest season on this page at about 181 days; Lakeville is shortest at about 146 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 Connecticut's zone 6a?: Using Winsted's frost dates (this page's median-season location): start seeds indoors about Mar 8 – Mar 22, then transplant outside about May 10 – May 17. 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 Connecticut's zone 6a 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 7 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.