Irwin, Idaho, sits in USDA hardiness zone 5a — a zone band wide enough that plant choice, not possibility, is the interesting question.
On paper, cabbage, cranberry, lavender, and collard greens all suit these conditions — on the ground, soil, sun, and drainage make the final call.
Even in Irwin, 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 Irwin 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 Irwin — soil, climate, flood, and parcel records. How we handle your address.
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Irwin — the short answer
Irwin, Idaho: USDA zone 5a, about 140 days between hard freezes, across 2K acres of city.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- May 11
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Sep 28
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~140
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 2K 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 Irwin's average, not your lot's — a low corner or a tree line moves it. Against Irwin's 140-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Irwin
Each reading below is measured for Irwin itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredIrwin runs about 140 days between hard freezes — a short growing season.
Long-maturing crops like melons, sweet potatoes, and winter squash can run out of runway before the first hard freeze, ripening late or not at all.
Lead with fast-maturing varieties, and stretch the season on both ends with cold frames, low tunnels, or a greenhouse.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredIrwin 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 Irwin is in
Irwin sits in the Middle Rockies — EPA Level III, inside the wider Northwestern Forested Mountains. That is read at Irwin's own centroid rather than borrowed from Bonneville County.
What Grows in Irwin
Every plant below cleared Irwin's own zone 5a and its 140-day season. Each card opens that plant's Idaho profile. How we score.
Is it too late to plant in Irwin?
Too late for some crops, right on time for others — a growing season is a sequence, not a deadline. Cool-season crops can go in from around Apr 13; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near May 11 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Sep 28 — long-season crops need about 90 days of runway, quick greens only 30. Short seasons reward decisiveness — quick-maturing varieties late, hardy greens under cover, and next year’s garlic in the ground before it closes.
What's on the federal record near Irwin
Closest first, within ~10 miles of Irwin. Listing isn't leaking, and distance isn't exposure.
- SWAN VALLEY ELEMENTARY SCHOOLUnderground Storage Tanks · 0.7 mi
- IRWIN TRADING POSTUnderground Storage Tanks · 0.8 mi
- 01N 44E 17ADA1Nitrate · 0.8 mi
- THE LODGE AT PALISADES CREEKUnderground Storage Tanks · 2.7 mi
- 01N 44E 05CCD1Nitrate · 2.7 mi
- Buck's Gas & RVUnderground Storage Tanks · 3.3 mi
- 01N 43E 12ACD1Nitrate · 3.4 mi
- 01N 43E 12ACB1Nitrate · 3.5 mi
- USBR PALISADES FIELD DIVISIONUnderground Storage Tanks · 3.8 mi
- PALISADES RDUnderground Storage Tanks · 3.8 mi
- MILLERS SERVICEUnderground Storage Tanks · 3.8 mi
- THE DAM STOREUnderground Storage Tanks · 3.9 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Irwin'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 Irwin.
Total Sites
33
within ~10 miles of Irwin
Risk Level
Elevated
Highest-severity
1 Superfund site (none NPL-listed)
Sources Checked
within ~10 miles of Irwin
Severity Distribution
within ~10 miles of Irwin
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Irwin, Nitrate runs higher than the national average — 16 sites within ~10 miles of Irwin. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
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 Irwin
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 Irwin average
Your land is one point inside Irwin
Everything above resolves to Irwin as a whole — one PHZM band and one 28°F frost record, read across 2K 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 Irwin Average
Irwin Average
- ●USDA Zones 5a, averaged across Irwin
- ●One soil character for all of Irwin
- ●Frost dates read at Irwin's scale: May 11 to Sep 28
YOUR Parcel
- ✓Your exact zone, not Irwin's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Irwin
Pull a site-specific report for your exact address in Irwin, Idaho — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Irwin averages can't show:
We read public map data around Irwin — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Irwin average
How we know this about Irwin
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 Irwin’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 Irwin
When does frost risk typically end in Irwin?
The last hard freeze (28°F) in Irwin typically lands around May 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 Jun 1.
When is the first frost in Irwin?
The first hard freeze (28°F) in Irwin typically arrives around Sep 28, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Sep 14, so the watching starts about Aug 29 — about 140 days after the spring freeze cleared.
How long is the growing season in Irwin?
Measured between 28°F hard freezes, Irwin sees about 140 frost-free days — roughly May 11 through Sep 28, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Jun 1 to Sep 14, about 105 days.
Is the soil safe to grow vegetables in Irwin?
The federal record around Irwin is a meaningful one — 33 documented sites, mostly Nitrate entries (16), the nearest Underground Storage Tanks record about 0.7 miles out — so a soil test before new food beds is a sensible precaution here, not a reason to hold back from growing. Remember that proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what sits where.
How do gardeners stretch the season in Irwin?
With about 140 frost-free days between hard freezes, Irwin rewards the classic extension moves: floating row cover buys roughly two to four extra weeks at each shoulder — on this season that is a working window nearer 168 to 196 days — and cold frames or low tunnels buy more. Starting transplants indoors ahead of the May 11 hard-freeze normal stretches the season without touching the calendar.
Everything above is measured at Irwin's scale — its 5a band and its 140-day season. Your yard writes its own version of Irwin'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 Irwin’s own scale
6 of the readings below were taken at Irwin's own scale — its own ~140-day season between 28°F freezes, the 1,676 acres it covers and 33 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Irwin's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Irwin's own 28°F hard-freeze dates, about May 11 and Sep 28 and the ~140-day span between themNOAA 1991–2020 Climate Normals
- 33 documented contamination sites on the federal record near Irwin, the closest Underground Storage Tanks record about 0.7 miles out from the town centreFederal environmental registers
- Irwin's boundary and the 1,676 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~140-day seasonGrowable Ground crop-matching engine
- The ecoregion at Irwin's own centroid, the Middle RockiesUS EPA Level III Ecoregions
Inherited from a wider read
- Soil character — no town-scale survey exists, so Irwin's 1,676 acres read through its county's SSURGO summaryfrom Bonneville County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Bonneville County
- The growing challenges every Idaho grower plans around — written at state scale, so they stay on the state guidefrom Idaho
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Bonneville County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Irwin's 1,676 acreswhy one lot differs from the next →
- Which side of Irwin you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 1,676 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record about 0.7 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 Irwin-wide figureread your own land →
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