Onawa, Iowa, sits in USDA hardiness zone 5a — a zone band wide enough that plant choice, not possibility, is the interesting question.
A short list that earns its place here — cabbage, cranberry, lavender, and collard greens — with any one site's soil, sun, and drainage making the final cut.
Even in Onawa, no two yards are alike.
A low spot, a south-facing slope, or a stand of trees moves the frost date and sun across a single Onawa lot. Enter your address and we'll score 1,086 plants against your land's actual soil, sun, and frost.
We read public map data around Onawa — soil, climate, flood, and parcel records. How we handle your address.
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Onawa — the short answer
Onawa, Iowa: USDA zone 5a, about 190 days between hard freezes, across 3K acres of city.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 13
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 20
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~190
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 3K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are this town’s own 28°F normals: light frosts run a few weeks past them, and tender transplants usually wait 2–3 weeks after the last hard freeze.
Hardiness Zone Range
Zone 5a is Onawa's average, not your lot's — a low corner or a tree line moves it. Against Onawa's 190-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Onawa
Each reading below is measured for Onawa itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredOnawa runs about 190 days between hard freezes — a moderate growing season.
There is room for the full run of cool-season crops plus a warm-season main crop, but the calendar is tight enough that a late spring or early fall frost still decides how much finishes.
Time warm-season transplants to your last hard freeze, and a few weeks of season extension turns a near-miss harvest into a sure one.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredOnawa sits in USDA hardiness zone 5a.
The zone sets which perennials, shrubs, and fruit trees live through an average winter here — a plant rated for a colder zone is a safe bet, one rated warmer needs winter protection or gets treated as an annual.
Match perennials and fruit to the zone before you buy, so nothing you plant is a gamble against the cold.
USDA PHZM 2023
The kind of country Onawa is in
Onawa sits in the Western Corn Belt Plains — EPA Level III, inside the wider Great Plains. That is read at Onawa's own centroid rather than borrowed from Monona County.
What Grows in Onawa
Every plant below cleared Onawa's own zone 5a and its 190-day season. Each card opens that plant's Iowa profile. How we score.
Is it too late to plant in Onawa?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Mar 16; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 13 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 20 — long-season crops need about 90 days of runway, quick greens only 30. Even past midsummer there is room for a true fall garden here, and garlic planted near the close carries the momentum into next year.
What's on the federal record near Onawa
Closest first, within ~10 miles of Onawa. Listing isn't leaking, and distance isn't exposure.
- MONONA COUNTY SHERIFF OFFICEUnderground Storage Tanks · 400 ft
- 083N45W04CCAA 1918Onawa 2Nitrate · 1000 ft
- 083N45W04CCAB 00094 1905OnawaNitrate · 1000 ft
- ONAWA MUNICIPAL LIGHT PLANTUnderground Storage Tanks · 1000 ft
- 083N45W04CBDB 1964Onawa 6Nitrate · 1100 ft
- 083N45W04CBDB 1964Onawa 5Nitrate · 1100 ft
- IDOT-MAINT GARAGEUnderground Storage Tanks · 1700 ft
- ONAWA COUNTRY STOREUnderground Storage Tanks · 1700 ft
- OIL EXPRESS TEXACOUnderground Storage Tanks · 1700 ft
- 083N45W04CBC 02389 1946Onawa 3Nitrate · 1800 ft
- 083N45W04CBC 03148 1947Onawa 2Nitrate · 1800 ft
- SPARKY''S ONE STOP #21Underground Storage Tanks · 1900 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Onawa's center, not your beds. Run your own address for what's upwind of you specifically.
Environmental Intelligence
The counts behind that headline — every source type on the record near Onawa.
Total Sites
107
within ~10 miles of Onawa
Risk Level
Moderate
Highest-severity
5 Toxics Release Inventory facilities
Sources Checked
within ~10 miles of Onawa
Severity Distribution
within ~10 miles of Onawa
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Onawa, Nitrate runs higher than the national average — 44 sites within ~10 miles of Onawa. It's not cause for alarm — it's worth knowing, and there's a sensible way to grow around it.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Check your specific parcel in Onawa
Get exact proximity distances to contamination sources for your specific parcel — plus soil, sun, drainage, and 1,086 plant recommendations.
25+ data sources read at your address, not the Onawa average
Your land is one point inside Onawa
Everything above resolves to Onawa as a whole — one PHZM band and one 28°F frost record, read across 3K acres. Your parcel is one point inside that footprint. Slope, tree cover, a low spot that pools cold air, and what is in the soil are all resolved at the address, and none of them are visible at city scale. That is the honest boundary of this page, and the reason the next step is a reading of your own ground rather than a better local average.
Your Lot vs the Onawa Average
Onawa Average
- ●USDA Zones 5a, averaged across Onawa
- ●One soil character for all of Onawa
- ●Frost dates read at Onawa's scale: Apr 13 to Oct 20
YOUR Parcel
- ✓Your exact zone, not Onawa's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Onawa
Pull a site-specific report for your exact address in Onawa, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Onawa averages can't show:
We read public map data around Onawa — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Onawa average
How we know this about Onawa
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this city’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Onawa’s own record; boundaries and acreage from US Census TIGER/Line 2025; the contamination records from the federal sources named in their beats above. Soil is not read at city grain anywhere on this page — there is no city-resolution survey in our data, which is why you will find no soil grid here rather than a statewide one wearing a local label. The resolution ledger below names every figure this page inherited.
Common questions about Onawa
When does frost risk typically end in Onawa?
The last hard freeze (28°F) in Onawa typically lands around Apr 13, per NOAA 1991–2020 climate normals — earlier than the light-frost dates most charts quote. That marks the hard freeze, not the last light frost — lighter frosts keep biting past it, which is why tender transplants here wait until about May 4.
When is the first frost in Onawa?
The first hard freeze (28°F) in Onawa typically arrives around Oct 20, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 6, so the watching starts about Sep 20 — about 190 days after the spring freeze cleared.
How long is the growing season in Onawa?
Measured between 28°F hard freezes, Onawa sees about 190 frost-free days — roughly Apr 13 through Oct 20, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 4 to Oct 6, about 155 days.
Is the soil safe to grow vegetables in Onawa?
The federal record around Onawa shows 107 documented sites, mostly Underground Storage Tanks entries (50), the nearest Underground Storage Tanks record under a tenth of a mile out — a typical footprint for a growing area, and proximity to a documented site is information, not a diagnosis of any one yard. It's worth seeing which recorded sites sit closest to where you grow, and testing the soil before new food beds near any of them.
How do I protect my plants from frost in Onawa?
From about Sep 20 the job here turns from planting to protecting — the last start even a 30-day crop can finish. As the season closes around the first 28°F hard freeze near Oct 20 (NOAA 1991–2020 climate normals), three moves buy time: row cover or an old sheet over tender plants on still, clear nights; water the soil the afternoon before a freeze, so it holds warmth overnight; and harvest tomatoes, peppers and basil before Oct 20. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Onawa's scale — its 5a band and its 190-day season. Your yard writes its own version of Onawa's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
What this page reads at Onawa’s own scale
6 of the readings below were taken at Onawa's own scale — its own ~190-day season between 28°F freezes, the 3,372 acres it covers and 107 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Onawa's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Onawa's own 28°F hard-freeze dates, about Apr 13 and Oct 20 and the ~190-day span between themNOAA 1991–2020 Climate Normals
- 107 documented contamination sites on the federal record near Onawa, the closest Underground Storage Tanks record under a tenth of a mile out from the town centreFederal environmental registers
- Onawa's boundary and the 3,372 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~190-day seasonGrowable Ground crop-matching engine
- The ecoregion at Onawa's own centroid, the Western Corn Belt PlainsUS EPA Level III Ecoregions
Inherited from a wider read
- Soil character — no town-scale survey exists, so Onawa's 3,372 acres read through its county's SSURGO summaryfrom Monona County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Monona County
- The growing challenges every Iowa grower plans around — written at state scale, so they stay on the state guidefrom Iowa
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Monona County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Onawa's 3,372 acreswhy one lot differs from the next →
- Which side of Onawa you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 3,372 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record under a tenth of a mile out, measured from the town centrehow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — measured at your address, and averaged away by any Onawa-wide figureread your own land →
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