Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated

Planting zone for 06791 — Harwinton, CT

ZIP code 06791 (Harwinton, Northwest Hills County) is in USDA zone 6a: the coldest winter night averages −10 to −5 °F. The median last spring frost is May 8 and the first fall frost October 5.

Where ZIP 06791 is

Outlined: the ZIP code area · tap the map for the zone at any spot · scroll or pinch to zoom

Not the official USDA Plant Hardiness Zone Map. Zones come from the 2023 USDA hardiness-zone data by the PRISM Group, Oregon State University; we re-mapped and recolored them. The official map is at planthardiness.ars.usda.gov.

Zone6a−10 to −5 °F
Coldest night−23.3 to −20.6 °C
Last frostMay 8
First frostOct 5
2012 zone5b

Was 5b on the 2012 map — half a zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 6a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Harwinton, CT…

Forecast data: National Weather Service (api.weather.gov), public domain. This is not a warning service — for official frost and freeze warnings see weather.gov.

Frost dates in Harwinton, CT

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 8median — 50% chance of a later frost
Safe for tender plantsMay 22only 10% chance of frost after
First fall frostOctober 5median — 10% chance by Sep 25
Growing season151 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 26 and returns around October 18.

From the NOAA station Bakersville, CT — 6 miles away, elevation 686 ft. NOAA NCEI U.S. Climate Normals 1991–2020, 32 °F threshold. Low spots and valleys can freeze later in spring than the station.

When to plant vegetables in 06791

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 27 – Apr 10
SpinachSow outdoors · Mar 27 – Apr 17
Broccoli, cabbageStart indoors · Feb 27 – Mar 13
Broccoli, cabbageTransplant · Apr 17 – May 1
Onion setsPlant outdoors · Apr 10 – Apr 24
PotatoesPlant outdoors · Apr 17 – May 1
LettuceSow outdoors · Apr 10 – Apr 24
Carrots, beetsSow outdoors · Apr 17 – May 1
TomatoesStart indoors · Mar 13 – Mar 27
TomatoesTransplant · May 15 – May 29
Peppers, eggplantStart indoors · Feb 27 – Mar 13
Peppers, eggplantTransplant · May 22 – Jun 5
BeansSow outdoors · May 15 – May 29
Sweet cornSow outdoors · May 15 – May 29
Cucumbers, squashSow outdoors · May 15 – May 29
Fall lettuce, spinachSow outdoors · Aug 10 – Aug 24
GarlicPlant cloves · Sep 21 – Oct 26
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 8) and first fall frost (Oct 5) with the rules of thumb U.S. extension services publish. Cool-season crops take a light frost; warm-season crops do best once the soil is above about 60 °F.

When spring arrived in 2026

In 2026, spring leaf-out near 06791 came on Apr 3 — 9 days earlier than the 1991–2020 average (Apr 12).

JanFebMarAprMayJun

The Spring Index marks the day when accumulated warmth is enough for lilac and honeysuckle to leaf out — a widely used signal that the growing season has started. Source: USA National Phenology Network, www.usanpn.org — Spring Indices, first leaf, 2026 (data as of 2026-09-08) and 30-year average 1991–2020 (CC BY 4.0).

Native plants for zone 6a in Connecticut

528 plants native to the lower 48 states, recorded in Connecticut, and rated by the USDA to survive −10 °F or colder — the bottom of zone 6a. Each links to its USDA PLANTS profile.

Trees 64

American basswood100 ft · blooms late spring · low shade tolerance · to −46 °FAmerican beech80 ft · blooms mid spring · low shade tolerance · to −44 °FAmerican chestnut115 ft · blooms early summer · high shade tolerance · to −33 °FAmerican elm120 ft · blooms early spring · medium shade tolerance · to −46 °FAmerican sycamore100 ft · blooms spring · medium shade tolerance · to −34 °Farborvitae50 ft · blooms mid spring · medium shade tolerance · to −33 °F

Shrubs 105

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °FAllegheny serviceberry35 ft · blooms early spring · low shade tolerance · to −38 °Falternateleaf dogwood25 ft · blooms mid spring · low shade tolerance · to −47 °FAmerican black currant3 ft · blooms early spring · to −46 °FAmerican hazelnut10 ft · blooms spring · medium shade tolerance · to −33 °FAmerican holly40 ft · blooms mid spring · low shade tolerance · to −20 °F

Wildflowers & perennials 185

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °FAmerican bur-reed3 ft · blooms indeterminate · medium shade tolerance · to −33 °FAmerican eelgrass1 ft · blooms mid summer · high shade tolerance · to −33 °FAmerican licorice4 ft · blooms late spring · medium shade tolerance · to −38 °FAmerican lotus3 ft · blooms mid summer · high shade tolerance · to −36 °FAmerican pokeweed7 ft · blooms mid summer · low shade tolerance · to −33 °F

Grasses 149

American beachgrass2.5 ft · blooms spring · high shade tolerance · to −28 °Farctic brome3 ft · blooms early summer · low shade tolerance · to −33 °Fawlfruit sedge4 ft · blooms mid spring · low shade tolerance · to −38 °FBailey's sedge2.1 ft · blooms late spring · low shade tolerance · to −33 °Fbeaked spikerush3 ft · blooms late spring · high shade tolerance · to −28 °FBebb's sedge3 ft · blooms late spring · low shade tolerance · to −38 °F

Vines 8

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °Fcatbird grape30 ft · blooms late spring · medium shade tolerance · to −13 °Fdevil's darning needles15 ft · blooms mid summer · medium shade tolerance · to −38 °Friverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Fsummer grape30 ft · blooms late spring · low shade tolerance · to −23 °Ftrumpet creeper2 ft · blooms summer · high shade tolerance · to −13 °F

Source: USDA, NRCS. 2026. The PLANTS Database (plants.usda.gov), National Plant Data Team — state distribution, native status and the “Temperature, Minimum” characteristic. Check light, moisture and soil needs before buying; local nurseries and your state native plant society can confirm what thrives in Harwinton, CT.

Soil in Harwinton, CT

The soil mapped at the center of ZIP 06791 is Canton — slightly decomposed plant material, well drained.

TextureSlightly decomposed plant materialtop layer
DrainageWell drainednatural drainage class
Organic matter95%high
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Canton and Charlton fine sandy loams, 3 to 15 percent slopes, extremely stony.

Soil Survey Staff, Natural Resources Conservation Service, USDA. Soil Survey Geographic (SSURGO) Database, via Soil Data Access, accessed 2026-10-29. The soil at the center point of ZIP 06791; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Harwinton, CT

Summer high82 °Faverage high, July
Winter low15 °Faverage low, January
Hot days7days a year at 90 °F or more
Freezing nights158nights a year at 32 °F or less
Growing degree days2,345base 50 °F — the season’s usable heat
Longest day15 h 13 mindaylight on June 21
Shortest day9 h 08 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation52 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°32 °FJanuary: average high 33 °F, low 15 °F 33°15° JanFebruary: average high 37 °F, low 16 °F 37°16° FebMarch: average high 45 °F, low 23 °F 45°23° MarApril: average high 59 °F, low 33 °F 59°33° AprMay: average high 69 °F, low 44 °F 69°44° MayJune: average high 77 °F, low 53 °F 77°53° JunJuly: average high 82 °F, low 58 °F 82°58° JulAugust: average high 80 °F, low 56 °F 80°56° AugSeptember: average high 72 °F, low 49 °F 72°49° SepOctober: average high 60 °F, low 38 °F 60°38° OctNovember: average high 49 °F, low 29 °F 49°29° NovDecember: average high 38 °F, low 21 °F 38°21° Dec

Precipitation by month inches

0″1″2″3″4″5″6″January: 3.43 in precipitation 3.4 JanFebruary: 3.21 in precipitation 3.2 FebMarch: 4.20 in precipitation 4.2 MarApril: 4.51 in precipitation 4.5 AprMay: 4.29 in precipitation 4.3 MayJune: 4.47 in precipitation 4.5 JunJuly: 4.72 in precipitation 4.7 JulAugust: 4.39 in precipitation 4.4 AugSeptember: 4.60 in precipitation 4.6 SepOctober: 5.38 in precipitation 5.4 OctNovember: 4.02 in precipitation 4.0 NovDecember: 4.74 in precipitation 4.7 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Bakersville, CT (6 mi away); precipitation: Thomaston Dam, CT (5 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

2 of the 8 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

067926a
Torrington, CT · <1 mi
067786a
Northfield, CT · 4 mi
060136a
Burlington, CT · 6 mi
067906a
Torrington, CT · 6 mi
067876a
Thomaston, CT · 6 mi
060576a
New Hartford, CT · 6 mi
067866b
Terryville, CT · 7 mi
067816b
Pequabuck, CT · 7 mi

Connecticut planting zones →

Questions about 06791

What planting zone is ZIP code 06791?

ZIP code 06791 (Harwinton, CT) is in USDA hardiness zone 6a on the 2023 map: the coldest night of an average winter reaches −10 to −5 °F (−23.3 to −20.6 °C).

When is the last frost in Harwinton, CT?

The median last spring frost (32 °F) near 06791 is May 8; after May 22 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Bakersville station.

When is the first frost in Harwinton, CT?

The median first fall frost near 06791 is October 5, giving a median frost-free growing season of 151 days.

When should I plant tomatoes in 06791?

Start tomato seeds indoors around Mar 13 – Mar 27 and set transplants outside around May 15 – May 29, one to three weeks after the median last frost.

How much rain and snow does 06791 get?

About 52 inches of precipitation a year (NOAA 1991–2020 normals, Thomaston Dam station).

How hot do summers get in Harwinton, CT?

The average high peaks at 82 °F in July, with about 7 days a year at 90 °F or more.

What kind of soil does 06791 have?

The USDA soil survey maps Canton at the center of ZIP 06791: slightly decomposed plant material, well drained.

Did spring come early in Harwinton, CT in 2026?

Spring leaf-out near 06791 came on Apr 3 in 2026, 9 days earlier than the 1991–2020 average (Apr 12), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Harwinton, CT?

Long-day onions: the longest day near 06791 is 15 h 13 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 06791 change?

Yes. The 2012 USDA map put 06791 in zone 5b; the 2023 map puts it in zone 6a. The newer map uses 1991–2020 data and more weather stations.