Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 29, 2026

Planting zone for 50255 — Thornburg, IA

ZIP code 50255 (Thornburg, Keokuk County) is in USDA zone 5b: the coldest winter night averages −15 to −10 °F. The median last spring frost is April 24 and the first fall frost October 15.

Where ZIP 50255 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.

Zone5b−15 to −10 °F

Coldest night−26.1 to −23.3 °C

Last frostApr 24

First frostOct 15

2012 zone5a

Was 5a 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 5b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Thornburg, IA…

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 Thornburg, IA

JanFebMarAprMayJunJulAugSepOctNovDec

Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free

Last spring frostApril 24median — 50% chance of a later frost

Safe for tender plantsMay 9only 10% chance of frost after

First fall frostOctober 15median — 10% chance by Sep 30

Growing season172 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 12 and returns around October 24.

From the NOAA station Sigourney, IA — 10 miles away, elevation 732 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 50255

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Mar 13 – Mar 27

SpinachSow outdoors · Mar 13 – Apr 3

Broccoli, cabbageStart indoors · Feb 13 – Feb 27

Broccoli, cabbageTransplant · Apr 3 – Apr 17

Onion setsPlant outdoors · Mar 27 – Apr 10

PotatoesPlant outdoors · Apr 3 – Apr 17

LettuceSow outdoors · Mar 27 – Apr 10

Carrots, beetsSow outdoors · Apr 3 – Apr 17

TomatoesStart indoors · Feb 27 – Mar 13

TomatoesTransplant · May 1 – May 15

Peppers, eggplantStart indoors · Feb 13 – Feb 27

Peppers, eggplantTransplant · May 8 – May 22

BeansSow outdoors · May 1 – May 15

Sweet cornSow outdoors · May 1 – May 15

Cucumbers, squashSow outdoors · May 1 – May 15

Fall lettuce, spinachSow outdoors · Aug 20 – Sep 3

GarlicPlant cloves · Oct 1 – Nov 5

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 24) and first fall frost (Oct 15) 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 50255 came on Mar 25 — 10 days earlier than the 1991–2020 average (Apr 4).

average
Apr 42026
Mar 25

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 5b in Iowa

473 plants native to the lower 48 states, recorded in Iowa, and rated by the USDA to survive −15 °F or colder — the bottom of zone 5b. Each links to its USDA PLANTS profile.

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 Thornburg, IA.

Soil in Thornburg, IA

The soil mapped at the center of ZIP 50255 is Mahaska — silty clay loam, somewhat poorly drained.

TextureSilty clay loamholds water and nutrients; slow to drain and warm up

pH6.1slightly acidic; most vegetables like 6.0–7.0

DrainageSomewhat poorly drainedconsider raised beds

Organic matter4%moderate

Hydrologic groupC/Dslow when drained; wet otherwise

Farmland classprime farmlandUSDA classification

Map unit: Mahaska silty clay loam, 0 to 2 percent slopes.

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 50255; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Thornburg, IA

Summer high85 °Faverage high, July

Winter low12 °Faverage low, January

Hot days16days a year at 90 °F or more

Freezing nights142nights a year at 32 °F or less

Growing degree days3,046base 50 °F — the season’s usable heat

Longest day15 h 11 mindaylight on June 21

Shortest day9 h 10 mindaylight on December 21

Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)

Precipitation37.4 inper year, rain and melted snow

Average high and low, by month

Precipitation by month inches

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Sigourney, IA (10 mi away); precipitation: Sigourney, IA (10 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 5b.

501365b

Keswick, IA · 2 mi

502685b

What Cheer, IA · 4 mi

Barnes City, IA · 7 mi

522225b

Deep River, IA · 8 mi

525505b

Delta, IA · 10 mi

525865b

Rose Hill, IA · 10 mi

Iowa planting zones →

Questions about 50255

What planting zone is ZIP code 50255?

ZIP code 50255 (Thornburg, IA) is in USDA hardiness zone 5b on the 2023 map: the coldest night of an average winter reaches −15 to −10 °F (−26.1 to −23.3 °C).

When is the last frost in Thornburg, IA?

The median last spring frost (32 °F) near 50255 is April 24; after May 9 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Sigourney station.

When is the first frost in Thornburg, IA?

The median first fall frost near 50255 is October 15, giving a median frost-free growing season of 172 days.

When should I plant tomatoes in 50255?

Start tomato seeds indoors around Feb 27 – Mar 13 and set transplants outside around May 1 – May 15, one to three weeks after the median last frost.

How much rain and snow does 50255 get?

About 37.4 inches of precipitation a year (NOAA 1991–2020 normals, Sigourney station).

How hot do summers get in Thornburg, IA?

The average high peaks at 85 °F in July, with about 16 days a year at 90 °F or more.

What kind of soil does 50255 have?

The USDA soil survey maps Mahaska at the center of ZIP 50255: silty clay loam, somewhat poorly drained, pH about 6.1 (slightly acidic).

Did spring come early in Thornburg, IA in 2026?

Spring leaf-out near 50255 came on Mar 25 in 2026, 10 days earlier than the 1991–2020 average (Apr 4), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Thornburg, IA?

Long-day onions: the longest day near 50255 is 15 h 11 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 50255 change?

Yes. The 2012 USDA map put 50255 in zone 5a; the 2023 map puts it in zone 5b. The newer map uses 1991–2020 data and more weather stations.