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

Planting zone for 50668 — Readlyn, IA

ZIP code 50668 (Readlyn, Bremer County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is April 29 and the first fall frost October 9.

Where ZIP 50668 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.

Zone5a−20 to −15 °F
Coldest night−28.9 to −26.1 °C
Last frostApr 29
First frostOct 9
2012 zone5a

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 5a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Readlyn, 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 Readlyn, IA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 29median — 50% chance of a later frost
Safe for tender plantsMay 9only 10% chance of frost after
First fall frostOctober 9median — 10% chance by Sep 25
Growing season162 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 18 and returns around October 20.

From the NOAA station Tripoli, IA — 8 miles away, elevation 1,056 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 50668

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 18 – Apr 1
SpinachSow outdoors · Mar 18 – Apr 8
Broccoli, cabbageStart indoors · Feb 18 – Mar 4
Broccoli, cabbageTransplant · Apr 8 – Apr 22
Onion setsPlant outdoors · Apr 1 – Apr 15
PotatoesPlant outdoors · Apr 8 – Apr 22
LettuceSow outdoors · Apr 1 – Apr 15
Carrots, beetsSow outdoors · Apr 8 – Apr 22
TomatoesStart indoors · Mar 4 – Mar 18
TomatoesTransplant · May 6 – May 20
Peppers, eggplantStart indoors · Feb 18 – Mar 4
Peppers, eggplantTransplant · May 13 – May 27
BeansSow outdoors · May 6 – May 20
Sweet cornSow outdoors · May 6 – May 20
Cucumbers, squashSow outdoors · May 6 – May 20
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 29) 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 50668 came on Mar 28 — 15 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 5a in Iowa

461 plants native to the lower 48 states, recorded in Iowa, and rated by the USDA to survive −20 °F or colder — the bottom of zone 5a. Each links to its USDA PLANTS profile.

Trees 46

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 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 133

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 Readlyn, IA.

Soil in Readlyn, IA

The soil mapped at the center of ZIP 50668 is Marquis — loam, moderately well drained.

TextureLoama good balance for most vegetables
pH5.8moderately acidic; most vegetables like 6.0–7.0
DrainageModerately well drainednatural drainage class
Organic matter2.9%moderate
Hydrologic groupCwater soaks in slowly
Farmland classprime farmlandUSDA classification

Map unit: Marquis 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 50668; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Readlyn, IA

Summer high83 °Faverage high, July
Winter low7 °Faverage low, January
Hot days11days a year at 90 °F or more
Freezing nights158nights a year at 32 °F or less
Growing degree days2,778base 50 °F — the season’s usable heat
Longest day15 h 19 mindaylight on June 21
Shortest day9 h 02 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation38.6 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 25 °F, low 7 °F 25°7° JanFebruary: average high 29 °F, low 12 °F 29°12° FebMarch: average high 43 °F, low 24 °F 43°24° MarApril: average high 58 °F, low 36 °F 58°36° AprMay: average high 70 °F, low 48 °F 70°48° MayJune: average high 80 °F, low 59 °F 80°59° JunJuly: average high 83 °F, low 62 °F 83°62° JulAugust: average high 81 °F, low 59 °F 81°59° AugSeptember: average high 75 °F, low 50 °F 75°50° SepOctober: average high 61 °F, low 38 °F 61°38° OctNovember: average high 44 °F, low 26 °F 44°26° NovDecember: average high 31 °F, low 15 °F 31°15° Dec

Precipitation by month inches

0″2″4″6″January: 1.23 in precipitation 1.2 JanFebruary: 1.09 in precipitation 1.1 FebMarch: 2.00 in precipitation 2.0 MarApril: 3.93 in precipitation 3.9 AprMay: 5.11 in precipitation 5.1 MayJune: 6.03 in precipitation 6.0 JunJuly: 4.80 in precipitation 4.8 JulAugust: 4.29 in precipitation 4.3 AugSeptember: 3.75 in precipitation 3.8 SepOctober: 2.96 in precipitation 3.0 OctNovember: 1.98 in precipitation 2.0 NovDecember: 1.47 in precipitation 1.5 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Tripoli, IA (8 mi away); precipitation: Tripoli, IA (8 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 5a.

506225a
Denver, IA · 5 mi
506765a
Tripoli, IA · 8 mi
506645a
Oran, IA · 8 mi
506265a
Dunkerton, IA · 8 mi
506295a
Fairbank, IA · 9 mi
507035a
Waterloo, IA · 10 mi
506235a
Dewar, IA · 12 mi
506745a
Sumner, IA · 12 mi

Iowa planting zones →

Questions about 50668

What planting zone is ZIP code 50668?

ZIP code 50668 (Readlyn, IA) is in USDA hardiness zone 5a on the 2023 map: the coldest night of an average winter reaches −20 to −15 °F (−28.9 to −26.1 °C).

When is the last frost in Readlyn, IA?

The median last spring frost (32 °F) near 50668 is April 29; after May 9 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Tripoli station.

When is the first frost in Readlyn, IA?

The median first fall frost near 50668 is October 9, giving a median frost-free growing season of 162 days.

When should I plant tomatoes in 50668?

Start tomato seeds indoors around Mar 4 – Mar 18 and set transplants outside around May 6 – May 20, one to three weeks after the median last frost.

How much rain and snow does 50668 get?

About 38.6 inches of precipitation a year (NOAA 1991–2020 normals, Tripoli station).

How hot do summers get in Readlyn, 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 50668 have?

The USDA soil survey maps Marquis at the center of ZIP 50668: loam, moderately well drained, pH about 5.8 (moderately acidic).

Did spring come early in Readlyn, IA in 2026?

Spring leaf-out near 50668 came on Mar 28 in 2026, 15 days earlier than the 1991–2020 average (Apr 12), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Readlyn, IA?

Long-day onions: the longest day near 50668 is 15 h 19 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).