Humboldt County, in Iowa, sits in USDA hardiness zone 5a — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
The conditions favor cabbage, cranberry, lavender, and collard greens, among others — though every individual site edits that list with its own soil, sun, and drainage.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Humboldt County holds more than one microclimate.
Soils and elevations shift across Humboldt County, so your frost dates and drainage aren't the county average. 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 Humboldt County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Humboldt County — the short answer
Humboldt County, Iowa: USDA zone 5a, about 189 days between hard freezes, across 278K acres.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 16
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 22
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~189
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 278K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are county 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 Humboldt County's average, not your lot's — a low corner or a tree line moves it. Against Humboldt County's 189-day season, see what your own ground reads.
Heat in Humboldt County
Across a year, Humboldt County accumulates about 2,571 growing degree days above 50°F, about 173 fewer than the middle Iowa county — ahead of 26 of Iowa’s 99 counties. Degree days are the heat a crop actually gets to work with — more of them is more room to ripen a long-season crop before the freeze closes the window.
Grounded in PRISM Climate Group, Oregon State University — 1991–2020 monthly normals
Soil in Humboldt County
Across Humboldt County, the ground is predominantly Mollisols, where Canisteo, Webster, and Nicollet are the most extensive named soil series. The soil is generally poorly drained with a clay loam surface. Topsoil pH runs about 6.2–7.5, neutral. Rainfall drains through hydrologic group C/D soils.
Soil order
Mollisols
Drainage
Poorly drained
Prime farmland
35%
Hydric soils
55%
Soil still varies lot by lot — soil types explained.
How the ground varies across Humboldt County
The USDA soil survey maps 48% of Humboldt County as poorly drained — but another 51% drains differently: 20% moderately well drained, 13% somewhat poorly drained.
Canisteo is the most extensive named soil at 29% of mapped acres, then Webster (12%) and Nicollet (11%).
Which of them your own lot sits on is a parcel question, not a county one. Check your address for the survey at your boundary.
What this means for growing in Humboldt County
Each reading below is measured for Humboldt County itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Drainage
MeasuredThe dominant soils across Humboldt County are poorly drained, in hydrologic group C/D — ground that sheds water slowly.
Beds can stay saturated for days after heavy rain, and the roots of most vegetables, berries, and fruit trees suffocate or rot in soil that never drains.
Raised beds or mounded rows lift roots above the wet zone; choose water-tolerant crops for the lowest spots.
USDA NRCS SSURGO
Surface pH
MeasuredTopsoil across Humboldt County runs about pH 6.2–7.5, close to neutral.
This is the range most vegetables prefer — one correction you probably do not have to make, which is genuinely good news for a new bed.
Keep it there with compost rather than lime or sulfur, and only amend if your own soil test says otherwise.
USDA NRCS SSURGO
Season length
MeasuredHumboldt County runs about 189 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
MeasuredHumboldt County 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 Humboldt County is in
Humboldt County sits in the Western Corn Belt Plains — EPA Level III, inside the wider Great Plains. One consistent ecological setting across the whole county.
What Grows in Humboldt County
We scored all 1,086 plants against Humboldt County's own USDA zone 5a and its own 189-day season between hard freezes, using the same matching engine that runs a full parcel report. These cleared both. Sun, soil pH and drainage vary lot by lot — those are the parcel questions below. Each card opens that plant's Iowa profile.
Is it too late to plant in Humboldt County?
Almost never — the real question is what to plant next. Cool-season crops can go in from around Mar 19; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 16 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 22 — long-season crops need about 90 days of runway, quick greens only 30. The tail of the season still works: sixty-day crops into late summer, quick greens after, garlic last of all.
Documented around Humboldt County
- monarch butterfly4 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Humboldt County
USGS estimates 225,462 lb of Glyphosate on Humboldt County farmland in 2019, its largest by weight, ahead of Acetochlor (123,332 lb) and Metolachlor & Metolachlor-s (87,338 lb).
Farmland across the whole county, not any one lot. Check your address for the site-by-site record.
Safe to Grow Here?
What the federal record shows across Humboldt County — and how to grow with it.
We checked the federal record across Humboldt County — 206 documented sites across 5 of the 9 source types we track.
The most significant on record: 9 Toxics Release Inventory facilities. Active industrial facilities reporting chemical releases to air, water, and land.
There's a meaningful federal record across Humboldt County — worth a look before you plant food, not a reason to hold back from growing. Proximity to a documented site is information, not a diagnosis of any one yard. A soil test before new food beds is the sensible precaution here, and the map shows exactly which sites sit where, so you can see what's actually near you.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
What's on the federal record near Humboldt County
Closest first, across Humboldt County. Listing isn't leaking, and distance isn't exposure.
- 092N28W05DBC 02166 1945Nitrate · 2.3 mi
- FRIT INDUSTRIES INCBrownfields · 2.8 mi
- 092N29W36BBC 06670 1954Nitrate · 2.9 mi
- W & H COOPERATIVE OIL COMPANYBrownfields · 3.2 mi
- SANDE CONSTRUCTION AND SUPPLYUnderground Storage Tanks · 3.2 mi
- HBF INVESTMENTS, LLCToxics Release Inventory · 3.3 mi
- DODGEN INDUSTRIES, INCToxics Release Inventory · 3.3 mi
- DODGEN INDUSTRIES, INCBrownfields · 3.3 mi
- HAWKEYE LEISURE TRAILERS, LTDToxics Release Inventory · 3.3 mi
- LIGURIA FOODS INCToxics Release Inventory · 3.3 mi
- FIBERGLASS FABRICATORS INC - FORMER SITE OFBrownfields · 3.3 mi
- PRECISION TANK & EQUIPMENT COToxics Release Inventory · 3.3 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Humboldt County'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 Humboldt County.
Total Sites
206
across Humboldt County
Risk Level
Elevated
Highest-severity
9 Toxics Release Inventory facilities
Sources Checked
across Humboldt County
Severity Distribution
across Humboldt County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Humboldt County, Nitrate runs higher than the national average — 94 sites across Humboldt County. That's not a problem with your land — it's information about 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 Humboldt County
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 Humboldt County average
Your land is one point inside Humboldt County
Everything above resolves to Humboldt County as a whole — one PHZM band and one county frost record, read across 278K 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 county 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 county average.
Your Lot vs the Humboldt County Average
Humboldt County Average
- ●USDA Zones 5a, averaged across Humboldt County
- ●One soil character for all of Humboldt County
- ●Frost dates read at Humboldt County's scale: Apr 16 to Oct 22
YOUR Parcel
- ✓Your exact zone, not Humboldt County's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Humboldt County
Pull a site-specific report for your exact address in Humboldt County, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Humboldt County averages can't show:
We read public map data around Humboldt County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Humboldt County average
How we know this about Humboldt County
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at county grain; hard-freeze dates from NOAA 1991–2020 28°F normals at this county’s own record where one resolves, and the labelled statewide window where none does; soils from the USDA NRCS SSURGO survey, area-weighted across the county’s own map units; boundaries and acreage from US Census TIGER/Line 2025; the contamination and pesticide records from the federal sources named in their beats above. Where Humboldt County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Humboldt County
When does frost risk typically end in Humboldt County?
The last hard freeze (28°F) in Humboldt County typically lands around Apr 16, 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 7.
How long is the growing season in Humboldt County?
Measured between 28°F hard freezes, Humboldt County sees about 189 frost-free days — roughly Apr 16 through Oct 22, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 7 to Oct 8, about 154 days.
Is the soil safe to grow vegetables in Humboldt County?
The federal record around Humboldt County is a meaningful one — 206 documented sites, mostly Nitrate entries (94), the nearest Nitrate record about 2.3 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.
Everything above is measured at Humboldt County's scale — its 5a band and its 189-day season. Your yard writes its own version of Humboldt County's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
Will It Grow Here?
Zone fit is the first question — each answer below reads Iowa's frost window, season length, and soil profile against the plant's real requirements.
What this page reads at Humboldt County’s own scale
9 of the readings below were taken at Humboldt County's own scale — its own ~189-day span between 28°F freezes, the 3 most extensive soil series mapped here and 206 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Humboldt County's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Humboldt County's own 28°F hard-freeze dates and its ~189-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Humboldt CountyUSDA NRCS SSURGO
- 206 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Nitrate record about 2.3 miles outFederal environmental registers
- Humboldt County's boundary and the 277,998 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~189-day seasonGrowable Ground crop-matching engine
- Humboldt County sitting wholly inside the Western Corn Belt PlainsUS EPA Level III Ecoregions
- Open occurrence records documented in Humboldt County for a tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Humboldt County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every Iowa grower plans around — written at state scale, so they stay on the state guidefrom Iowa
What this page can’t tell you
- How the season shifts across Humboldt County — the ~189 days above is one reading at its centre; the 12 towns below each carry their owntowns in this county →
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Humboldt County's 277,998 acreswhy one lot differs from the next →
- Which of those 206 documented sites actually reaches your soil — that needs your address, the wind, and the groundwater directionhow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — none of it survives being averaged over Humboldt Countyread your own land →