St. Thomas, North Dakota, sits in USDA hardiness zone 4a — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
Growers here do well with cabbage, cranberry, turkish rocket, and collard greens — with the usual caveat that any single yard's soil, sun, and drainage cast the deciding vote.
Even in St. Thomas, 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. Thomas 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. Thomas — soil, climate, flood, and parcel records. How we handle your address.
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St. Thomas — the short answer
St. Thomas, North Dakota: USDA zone 4a, about 163 days between hard freezes, across 650 acres of city.
- USDA zone
- 4a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 30
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 10
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~163
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 650 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 4a is St. Thomas's average, not your lot's — a low corner or a tree line moves it. Against St. Thomas's 163-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in St. Thomas
Each reading below is measured for St. Thomas itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredSt. Thomas runs about 163 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. Thomas sits in USDA hardiness zone 4a.
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. Thomas is in
St. Thomas sits in the Lake Agassiz Plain — EPA Level III, inside the wider Great Plains. That is read at St. Thomas's own centroid rather than borrowed from Pembina County.
What Grows in St. Thomas
Every plant below cleared St. Thomas's own zone 4a and its 163-day season. Each card opens that plant's North Dakota profile. How we score.
Is it too late to plant in St. Thomas?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Apr 2; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 30 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 10 — 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. Thomas
Closest first, within ~10 miles of St. Thomas. Listing isn't leaking, and distance isn't exposure.
- Agri Sales IncUnderground Storage Tanks · 350 ft
- Saint Thomas StandardUnderground Storage Tanks · 500 ft
- Saint Thomas Standard OldUnderground Storage Tanks · 650 ft
- 159-053-02CBBNitrate · 850 ft
- St Thomas Municipal AirportUnderground Storage Tanks · 2300 ft
- 159-053-03ANitrate · 0.6 mi
- Grand Forks Seed CompanyUnderground Storage Tanks · 0.7 mi
- 160-053-23BCCNitrate · 3.3 mi
- North Dakota State Dept of TransportationUnderground Storage Tanks · 5.0 mi
- 160-052-07DDDNitrate · 5.5 mi
- 160-054-13AAANitrate · 6.2 mi
- 160-054-22CBDNitrate · 7.5 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from St. Thomas'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. Thomas.
Severity Distribution
within ~10 miles of St. Thomas
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around St. Thomas, Nitrate runs higher than the national average — 24 sites within ~10 miles of St. Thomas. 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. Thomas
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. Thomas average
Your land is one point inside St. Thomas
Everything above resolves to St. Thomas as a whole — one PHZM band and one 28°F frost record, read across 650 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. Thomas Average
St. Thomas Average
- ●USDA Zones 4a, averaged across St. Thomas
- ●One soil character for all of St. Thomas
- ●Frost dates read at St. Thomas's scale: Apr 30 to Oct 10
YOUR Parcel
- ✓Your exact zone, not St. Thomas's 4a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in St. Thomas
Pull a site-specific report for your exact address in St. Thomas, North Dakota — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that St. Thomas averages can't show:
We read public map data around St. Thomas — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the St. Thomas average
How we know this about St. Thomas
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. Thomas’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. Thomas
When does frost risk typically end in St. Thomas?
The last hard freeze (28°F) in St. Thomas typically lands around Apr 30, 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 21.
When is the first frost in St. Thomas?
The first hard freeze (28°F) in St. Thomas typically arrives around Oct 10, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Sep 26, so the watching starts about Sep 10 — about 163 days after the spring freeze cleared.
How long is the growing season in St. Thomas?
Measured between 28°F hard freezes, St. Thomas sees about 163 frost-free days — roughly Apr 30 through Oct 10, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 21 to Sep 26, about 128 days.
Is the soil safe to grow vegetables in St. Thomas?
The federal record around St. Thomas is light — 32 documented sites across the 9 federal source types we checked, mostly Nitrate entries (24), the nearest Underground Storage Tanks record under a tenth of a mile out — and proximity to a documented site is information, not a diagnosis of any one yard. Growing food here starts from a strong position; a soil test before new food beds settles any site-specific question.
How do I protect my plants from frost in St. Thomas?
From about Sep 10 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 10 (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 10. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at St. Thomas's scale — its 4a band and its 163-day season. Your yard writes its own version of St. Thomas'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. Thomas’s own scale
6 of the readings below were taken at St. Thomas's own scale — its own ~163-day season between 28°F freezes, the 650 acres it covers and 32 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- St. Thomas's own USDA hardiness band, zones 4aUSDA PHZM 2023
- St. Thomas's own 28°F hard-freeze dates, about Apr 30 and Oct 10 and the ~163-day span between themNOAA 1991–2020 Climate Normals
- 32 documented contamination sites on the federal record near St. Thomas, the closest Underground Storage Tanks record under a tenth of a mile out from the town centreFederal environmental registers
- St. Thomas's boundary and the 650 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 4a and a ~163-day seasonGrowable Ground crop-matching engine
- The ecoregion at St. Thomas's own centroid, the Lake Agassiz PlainUS EPA Level III Ecoregions
Inherited from a wider read
- Soil character — no town-scale survey exists, so St. Thomas's 650 acres read through its county's SSURGO summaryfrom Pembina County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Pembina County
- The growing challenges every North Dakota grower plans around — written at state scale, so they stay on the state guidefrom North Dakota
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Pembina County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among St. Thomas's 650 acreswhy one lot differs from the next →
- Which side of St. Thomas you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 650 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 St. Thomas-wide figureread your own land →
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