Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 30, 2026

Planting zone for 50112 — Grinnell, IA

ZIP code 50112 (Grinnell, Poweshiek County) is in USDA zone 5b: the coldest winter night averages −15 to −10 °F. The median last spring frost is May 3 and the first fall frost October 6.

Where ZIP 50112 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 frostMay 3

First frostOct 6

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 Grinnell, 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 Grinnell, IA

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 3median — 50% chance of a later frost

Safe for tender plantsMay 17only 10% chance of frost after

First fall frostOctober 6median — 10% chance by Sep 22

Growing season155 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 23 and returns around October 17.

From the NOAA station Grinnell 3 Sw, IA — 2 miles away, elevation 906 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 50112

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Mar 22 – Apr 5

SpinachSow outdoors · Mar 22 – Apr 12

Broccoli, cabbageStart indoors · Feb 22 – Mar 8

Broccoli, cabbageTransplant · Apr 12 – Apr 26

Onion setsPlant outdoors · Apr 5 – Apr 19

PotatoesPlant outdoors · Apr 12 – Apr 26

LettuceSow outdoors · Apr 5 – Apr 19

Carrots, beetsSow outdoors · Apr 12 – Apr 26

TomatoesStart indoors · Mar 8 – Mar 22

TomatoesTransplant · May 10 – May 24

Peppers, eggplantStart indoors · Feb 22 – Mar 8

Peppers, eggplantTransplant · May 17 – May 31

BeansSow outdoors · May 10 – May 24

Sweet cornSow outdoors · May 10 – May 24

Cucumbers, squashSow outdoors · May 10 – May 24

Fall lettuce, spinachSow outdoors · Aug 11 – Aug 25

GarlicPlant cloves · Sep 22 – Oct 27

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 3) and first fall frost (Oct 6) 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 50112 came on Mar 25 — 13 days earlier than the 1991–2020 average (Apr 7).

average
Apr 72026
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 Grinnell, IA.

Soil in Grinnell, IA

The soil mapped at the center of ZIP 50112 is Tama — silty clay loam, well drained.

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

pH6.5neutral; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter3.5%moderate

Hydrologic groupCwater soaks in slowly

Farmland classprime farmlandUSDA classification

Map unit: Tama silty clay loam, 2 to 5 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 50112; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Grinnell, IA

Summer high83 °Faverage high, July

Winter low10 °Faverage low, January

Hot days11days a year at 90 °F or more

Freezing nights157nights a year at 32 °F or less

Growing degree days2,803base 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)

Precipitation38.2 inper year, rain and melted snow

Snowfall31 inper year

Average high and low, by month

Precipitation by month inches · ❄ = snowfall

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Grinnell 3 Sw, IA (2 mi away); precipitation: Grinnell 3 Sw, IA (2 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

Searsboro, IA · 12 mi

501535b

Lynnville, IA · 12 mi

Iowa planting zones →

Questions about 50112

What planting zone is ZIP code 50112?

ZIP code 50112 (Grinnell, 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 Grinnell, IA?

The median last spring frost (32 °F) near 50112 is May 3; after May 17 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Grinnell 3 Sw station.

When is the first frost in Grinnell, IA?

The median first fall frost near 50112 is October 6, giving a median frost-free growing season of 155 days.

When should I plant tomatoes in 50112?

Start tomato seeds indoors around Mar 8 – Mar 22 and set transplants outside around May 10 – May 24, one to three weeks after the median last frost.

How much rain and snow does 50112 get?

About 38.2 inches of precipitation a year and 31 inches of snowfall (NOAA 1991–2020 normals, Grinnell 3 Sw station).

How hot do summers get in Grinnell, IA?

The average high peaks at 83 °F in July, with about 11 days a year at 90 °F or more.

What kind of soil does 50112 have?

The USDA soil survey maps Tama at the center of ZIP 50112: silty clay loam, well drained, pH about 6.5 (neutral).

Did spring come early in Grinnell, IA in 2026?

Spring leaf-out near 50112 came on Mar 25 in 2026, 13 days earlier than the 1991–2020 average (Apr 7), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Grinnell, IA?

Long-day onions: the longest day near 50112 is 15 h 13 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 50112 change?

Yes. The 2012 USDA map put 50112 in zone 5a; the 2023 map puts it in zone 5b. The newer map uses 1991–2020 data and more weather stations.