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

Planting zone for 52231 — Harper, IA

ZIP code 52231 (Harper, 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 52231 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 zone5b

Same zone as on the 2012 map. 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 Harper, 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 Harper, 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 — 9 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 52231

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 52231 came on Mar 20 — 14 days earlier than the 1991–2020 average (Apr 3).

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.

Trees 49

American basswood100 ft · blooms late spring · low shade tolerance · to −46 °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 °Fbalsam fir60 ft · blooms mid summer · low shade tolerance · to −43 °F

Shrubs 77

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 hornbeam30 ft · blooms mid spring · low shade tolerance · to −34 °F

Wildflowers & perennials 191

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 136

arctic brome3 ft · blooms early summer · low shade tolerance · to −33 °Fawlfruit sedge4 ft · blooms mid spring · low shade tolerance · to −38 °FBebb's sedge3 ft · blooms late spring · low shade tolerance · to −38 °FBicknell's sedge3 ft · blooms late spring · low shade tolerance · to −38 °Fbig bluestem6 ft · blooms summer · high shade tolerance · to −46 °Fblue grama1 ft · blooms early summer · high shade tolerance · to −43 °F

Vines 7

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °Fdevil's darning needles15 ft · blooms mid summer · medium shade tolerance · to −38 °Ffrost grape83 ft · blooms late spring · medium shade tolerance · to −23 °Friverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Fsummer grape30 ft · blooms late spring · low shade tolerance · to −23 °Ftrumpet honeysuckle16 ft · blooms late summer · low shade tolerance · to −33 °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 Harper, IA.

Soil in Harper, IA

The soil mapped at the center of ZIP 52231 is Olmitz — silt loam, moderately well drained.

TextureSilt loama good balance for most vegetables
pH6.2slightly acidic; most vegetables like 6.0–7.0
DrainageModerately well drainednatural drainage class
Organic matter4.1%moderate
Hydrologic groupCwater soaks in slowly
Farmland classPrime farmland if drainedUSDA classification

Map unit: Olmitz-Zook-Vesser complex, 0 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 52231; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Harper, 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 10 mindaylight on June 21
Shortest day9 h 11 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

0°10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 30 °F, low 12 °F 30°12° JanFebruary: average high 35 °F, low 15 °F 35°15° FebMarch: average high 48 °F, low 26 °F 48°26° MarApril: average high 61 °F, low 37 °F 61°37° AprMay: average high 71 °F, low 49 °F 71°49° MayJune: average high 81 °F, low 59 °F 81°59° JunJuly: average high 85 °F, low 63 °F 85°63° JulAugust: average high 83 °F, low 60 °F 83°60° AugSeptember: average high 76 °F, low 52 °F 76°52° SepOctober: average high 63 °F, low 40 °F 63°40° OctNovember: average high 48 °F, low 28 °F 48°28° NovDecember: average high 35 °F, low 18 °F 35°18° Dec

Precipitation by month inches

0″1″2″3″4″5″6″January: 1.18 in precipitation 1.2 JanFebruary: 1.47 in precipitation 1.5 FebMarch: 2.34 in precipitation 2.3 MarApril: 3.70 in precipitation 3.7 AprMay: 4.95 in precipitation 5.0 MayJune: 5.11 in precipitation 5.1 JunJuly: 3.93 in precipitation 3.9 JulAugust: 4.76 in precipitation 4.8 AugSeptember: 3.47 in precipitation 3.5 SepOctober: 2.78 in precipitation 2.8 OctNovember: 2.18 in precipitation 2.2 NovDecember: 1.54 in precipitation 1.5 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Sigourney, IA (9 mi away); precipitation: Sigourney, IA (9 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 5b.

522485b
Keota, IA · 6 mi
525915b
Sigourney, IA · 7 mi
523355b
South English, IA · 8 mi
523555b
Webster, IA · 10 mi
525765b
Ollie, IA · 10 mi
525625b
Hayesville, IA · 12 mi
525855b
Richland, IA · 12 mi
523595b
West Chester, IA · 13 mi

Iowa planting zones →

Questions about 52231

What planting zone is ZIP code 52231?

ZIP code 52231 (Harper, 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 Harper, IA?

The median last spring frost (32 °F) near 52231 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 Harper, IA?

The median first fall frost near 52231 is October 15, giving a median frost-free growing season of 172 days.

When should I plant tomatoes in 52231?

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 52231 get?

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

How hot do summers get in Harper, 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 52231 have?

The USDA soil survey maps Olmitz at the center of ZIP 52231: silt loam, moderately well drained, pH about 6.2 (slightly acidic).

Did spring come early in Harper, IA in 2026?

Spring leaf-out near 52231 came on Mar 20 in 2026, 14 days earlier than the 1991–2020 average (Apr 3), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Harper, IA?

Long-day onions: the longest day near 52231 is 15 h 10 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).