St. Marys, Iowa, sits in USDA hardiness zone 5b — a band that supports both cool-season staples and warm-season crops chosen to fit the local frost window.
Growers here do well with cabbage, cranberry, lavender, and collard greens — with the usual caveat that any single yard's soil, sun, and drainage cast the deciding vote.
Even in St. Marys, 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 St. Marys 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 St. Marys — soil, climate, flood, and parcel records. How we handle your address.
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St. Marys — the short answer
St. Marys, Iowa: USDA zone 5b, about 195 days between hard freezes, across 90 acres of city.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 11
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 23
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~195
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 90 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 5b is St. Marys's average, not your lot's — a low corner or a tree line moves it. Against St. Marys's 195-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in St. Marys
Each reading below is measured for St. Marys itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredSt. Marys runs about 195 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
MeasuredSt. Marys sits in USDA hardiness zone 5b.
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 St. Marys is in
St. Marys sits in the Western Corn Belt Plains — EPA Level III, inside the wider Great Plains. That is read at St. Marys's own centroid rather than borrowed from Warren County.
What Grows in St. Marys
Every plant below cleared St. Marys's own zone 5b and its 195-day season. Each card opens that plant's Iowa profile. How we score.
Is it too late to plant in St. Marys?
For most of the year, no — what changes is which crops still fit the days remaining. Cool-season crops can go in from around Mar 14; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 11 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 23 — long-season crops need about 90 days of runway, quick greens only 30. Late in the year the fall bench takes over — quick greens, radishes, and garlic that repays you next summer.
What's on the federal record near St. Marys
Closest first, within ~10 miles of St. Marys. Listing isn't leaking, and distance isn't exposure.
- JO-RO GAS N GOUnderground Storage Tanks · 700 ft
- 075N25W16ABA 1959Nitrate · 1200 ft
- 075N25W16ADCA 1979Nitrate · 2200 ft
- 075N25W09BAC 1958Nitrate · 1.0 mi
- 075N25W03DDD 1943Nitrate · 1.0 mi
- 075N25W03ABC 1965Nitrate · 1.9 mi
- 075N25W28AAA 1958Nitrate · 2.1 mi
- 076N25W33DDANitrate · 2.1 mi
- 076N25W35CDD 1954Nitrate · 2.6 mi
- 076N25W34ABCNitrate · 2.9 mi
- RO BANKS WELL CO INCBrownfields · 3.1 mi
- 076N25W35BBAB 01401 1940Nitrate · 3.3 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from St. Marys'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 St. Marys.
Sources Checked
within ~10 miles of St. Marys
Severity Distribution
within ~10 miles of St. Marys
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around St. Marys, Nitrate runs higher than the national average — 66 sites within ~10 miles of St. Marys. 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 St. Marys
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 St. Marys average
Your land is one point inside St. Marys
Everything above resolves to St. Marys as a whole — one PHZM band and one 28°F frost record, read across 90 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 St. Marys Average
St. Marys Average
- ●USDA Zones 5b, averaged across St. Marys
- ●One soil character for all of St. Marys
- ●Frost dates read at St. Marys's scale: Apr 11 to Oct 23
YOUR Parcel
- ✓Your exact zone, not St. Marys's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in St. Marys
Pull a site-specific report for your exact address in St. Marys, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that St. Marys averages can't show:
We read public map data around St. Marys — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the St. Marys average
How we know this about St. Marys
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 St. Marys’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 St. Marys
When does frost risk typically end in St. Marys?
The last hard freeze (28°F) in St. Marys typically lands around Apr 11, 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 2.
When is the first frost in St. Marys?
The first hard freeze (28°F) in St. Marys typically arrives around Oct 23, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 9, so the watching starts about Sep 23 — about 195 days after the spring freeze cleared.
How long is the growing season in St. Marys?
Measured between 28°F hard freezes, St. Marys sees about 195 frost-free days — roughly Apr 11 through Oct 23, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 2 to Oct 9, about 160 days.
Is the soil safe to grow vegetables in St. Marys?
The federal record around St. Marys shows 140 documented sites, mostly Nitrate entries (66), the nearest Underground Storage Tanks record about 0.1 miles 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 St. Marys?
From about Sep 23 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 23 (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 23. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at St. Marys's scale — its 5b band and its 195-day season. Your yard writes its own version of St. Marys'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 St. Marys’s own scale
6 of the readings below were taken at St. Marys's own scale — its own ~195-day season between 28°F freezes, the 90 acres it covers and 140 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- St. Marys's own USDA hardiness band, zones 5bUSDA PHZM 2023
- St. Marys's own 28°F hard-freeze dates, about Apr 11 and Oct 23 and the ~195-day span between themNOAA 1991–2020 Climate Normals
- 140 documented contamination sites on the federal record near St. Marys, the closest Underground Storage Tanks record about 0.1 miles out from the town centreFederal environmental registers
- St. Marys's boundary and the 90 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~195-day seasonGrowable Ground crop-matching engine
- The ecoregion at St. Marys'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 St. Marys's 90 acres read through its county's SSURGO summaryfrom Warren County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Warren 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 Warren County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among St. Marys's 90 acreswhy one lot differs from the next →
- Which side of St. Marys you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 90 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record about 0.1 miles 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 St. Marys-wide figureread your own land →
More in Iowa
Wider growing context
What grows across IowaSt. Marys sits outside our named growing regions, so Iowa as a whole is its closest context here.
Nearby growing guides