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

Planting zone for Harlan, IA

Harlan, IA is in USDA zone 5b across its 2 ZIP codes. The median last spring frost is April 27 and the first fall frost October 9.

Harlan on the zone map

Outlined: its 1 ZIP code areas · 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
ZIP codes2
Last frostApr 27
First frostOct 9
Growing season165 days

5b 2 ZIPs

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Harlan, 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 Harlan, IA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 27median — 50% chance of a later frost
Safe for tender plantsMay 9only 10% chance of frost after
First fall frostOctober 9median — 10% chance by Sep 24
Growing season165 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 17 and returns around October 20.

From the NOAA station Harlan, IA, elevation 1,270 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 Harlan

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 16 – Mar 30
SpinachSow outdoors · Mar 16 – Apr 6
Broccoli, cabbageStart indoors · Feb 16 – Mar 2
Broccoli, cabbageTransplant · Apr 6 – Apr 20
Onion setsPlant outdoors · Mar 30 – Apr 13
PotatoesPlant outdoors · Apr 6 – Apr 20
LettuceSow outdoors · Mar 30 – Apr 13
Carrots, beetsSow outdoors · Apr 6 – Apr 20
TomatoesStart indoors · Mar 2 – Mar 16
TomatoesTransplant · May 4 – May 18
Peppers, eggplantStart indoors · Feb 16 – Mar 2
Peppers, eggplantTransplant · May 11 – May 25
BeansSow outdoors · May 4 – May 18
Sweet cornSow outdoors · May 4 – May 18
Cucumbers, squashSow outdoors · May 4 – May 18
Fall lettuce, spinachSow outdoors · Aug 14 – Aug 28
GarlicPlant cloves · Sep 25 – Oct 30
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 27) and first fall frost (Oct 9) 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 51537 came on Mar 23 — 12 days earlier than the 1991–2020 average (Apr 4).

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 Harlan, IA.

Soil in Harlan, IA

The soil mapped at the center of ZIP 51537 is Judson — silty clay loam, well drained.

TextureSilty clay loamholds water and nutrients; slow to drain and warm up
pH5.8moderately acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter3.8%moderate
Hydrologic groupCwater soaks in slowly
Farmland classprime farmlandUSDA classification

Map unit: Judson silty clay loam, deep loess, 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 51537; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Harlan, IA

Summer high86 °Faverage high, July
Winter low10 °Faverage low, January
Hot days19days a year at 90 °F or more
Freezing nights148nights a year at 32 °F or less
Growing degree days3,280base 50 °F — the season’s usable heat
Longest day15 h 12 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)
Precipitation34.3 inper year, rain and melted snow
Snowfall31 inper year; 53 days with snow on the ground

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 31 °F, low 10 °F 31°10° JanFebruary: average high 36 °F, low 14 °F 36°14° FebMarch: average high 50 °F, low 25 °F 50°25° MarApril: average high 63 °F, low 36 °F 63°36° AprMay: average high 74 °F, low 49 °F 74°49° MayJune: average high 84 °F, low 60 °F 84°60° JunJuly: average high 86 °F, low 64 °F 86°64° JulAugust: average high 84 °F, low 61 °F 84°61° AugSeptember: average high 79 °F, low 52 °F 79°52° SepOctober: average high 65 °F, low 39 °F 65°39° OctNovember: average high 49 °F, low 26 °F 49°26° NovDecember: average high 36 °F, low 16 °F 36°16° Dec

Precipitation by month inches · ❄ = snowfall

0″1″2″3″4″5″6″January: 0.90 in precipitation, 7.6 in snowfall 0.9❄8 JanFebruary: 0.98 in precipitation, 7.6 in snowfall 1.0❄8 FebMarch: 2.05 in precipitation, 4 in snowfall 2.0❄4 MarApril: 3.34 in precipitation, 1.5 in snowfall 3.3❄2 AprMay: 4.74 in precipitation 4.7 MayJune: 5.17 in precipitation 5.2 JunJuly: 3.72 in precipitation 3.7 JulAugust: 4.32 in precipitation 4.3 AugSeptember: 4.00 in precipitation 4.0 SepOctober: 2.37 in precipitation, 0.9 in snowfall 2.4❄1 OctNovember: 1.43 in precipitation, 2.2 in snowfall 1.4❄2 NovDecember: 1.31 in precipitation, 6.7 in snowfall 1.3❄7 Dec

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

ZIP codes in Harlan

515375b
−15 to −10 °F
515935b
−15 to −10 °F

Iowa planting zones →

Questions about Harlan

What planting zone is Harlan, IA?

Harlan, IA is in USDA hardiness zone 5b on the 2023 map (5b: −15 to −10 °F).

When is the last frost in Harlan, IA?

The median last spring frost is April 27 and the median first fall frost is October 9 (NOAA 1991–2020 normals, Harlan station).

When can I plant a garden in Harlan, IA?

Cool-season crops like peas and spinach go in around Mar 16 – Apr 6; tomatoes, peppers, beans and squash after about May 4, once frost risk has passed.

What kind of onions grow in Harlan, IA?

Long-day onions: the longest day is 15 h 12 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

How much rain and snow does Harlan, IA get?

About 34.3 inches of precipitation a year and 31 inches of snowfall (NOAA 1991–2020 normals).