Mahaska County, in Iowa, sits in USDA hardiness zone 5b — a zone band wide enough that plant choice, not possibility, is the interesting question.
Expect cabbage, cranberry, lavender, and collard greens to be strong candidates here; the deciding factors on any one parcel stay local — soil, sun, and drainage.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Mahaska County holds more than one microclimate.
Soils and elevations shift across Mahaska 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 Mahaska County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Mahaska County — the short answer
Mahaska County, Iowa: USDA zone 5b, about 200 days between hard freezes, across 365K acres.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 9
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 26
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~200
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 365K 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 5b is Mahaska County's average, not your lot's — a low corner or a tree line moves it. Against Mahaska County's 200-day season, see what your own ground reads.
Heat in Mahaska County
Across a year, Mahaska County accumulates about 3,031 growing degree days above 50°F, about 287 more than the middle Iowa county — ahead of 80 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 Mahaska County
Across Mahaska County, the ground is predominantly Mollisols, where Ladoga, Otley, and Clinton are the most extensive named soil series. The soil is generally moderately well drained with a silty clay loam surface. Topsoil pH runs about 6.1–6.5, slightly acidic. Rainfall drains through hydrologic group C soils.
Soil order
Mollisols
Drainage
Moderately well drained
Prime farmland
30%
Hydric soils
18%
Soil still varies lot by lot — soil types explained.
How the ground varies across Mahaska County
The USDA soil survey maps 55% of Mahaska County as moderately well drained — but another 44% drains differently: 19% poorly drained, 14% somewhat poorly drained.
Ladoga is the most extensive named soil at 15% of mapped acres, then Otley (10%) and Clinton (9%).
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 Mahaska County
Each reading below is measured for Mahaska 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 Mahaska County are moderately well drained, in hydrologic group C — roughly the middle of the drainage range.
This is close to the middle most vegetables prefer: it holds enough water to carry a crop between rains without staying waterlogged. The catch is that a wet spring or a low corner can still tip it soggy.
Plant on the site as-is for most crops, but keep the lowest, slowest-draining beds for moisture-tolerant plantings.
USDA NRCS SSURGO
Surface pH
MeasuredTopsoil across Mahaska County runs about pH 6.1–6.5, 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
Wet ground
MeasuredAbout 18% of Mahaska County is mapped as hydric — wetland — soil.
If your lot falls in one of those pockets, a standard in-ground bed will fight waterlogging all season, and roots that need air will struggle.
Check how your own ground drains before you dig; raised beds or moisture-loving plantings turn a wet corner from a problem into a niche.
USDA NRCS SSURGO
Season length
MeasuredMahaska County runs about 200 days between hard freezes — a long growing season.
There is slack for succession plantings and slow-finishing crops: many vegetables can be sown twice, and the heat-lovers have time to ripen fully.
Plan a second sowing of quick crops after the first comes out, and use the fall window most gardeners let sit idle.
NOAA 1991–2020 Climate Normals
The kind of country Mahaska County is in
Mahaska 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 Mahaska County
We scored all 1,086 plants against Mahaska County's own USDA zone 5b and its own 200-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 Mahaska County?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Mar 12; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 9 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 26 — long-season crops need about 90 days of runway, quick greens only 30. Here the season winds down slowly: late sowings, a real autumn harvest, and garlic in the ground before the first hard freeze.
Documented around Mahaska County
- monarch butterfly26 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Mahaska County
USGS estimates 211,735 lb of Glyphosate on Mahaska County farmland in 2019, its largest by weight, ahead of Acetochlor (193,171 lb) and Atrazine (176,560 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 Mahaska County — and how to grow with it.
We checked the federal record across Mahaska County — 248 documented sites across 7 of the 9 source types we track.
The most significant on record: 1 Superfund site (none NPL-listed). Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
There's a meaningful federal record across Mahaska 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 Mahaska County
Closest first, across Mahaska County. Listing isn't leaking, and distance isn't exposure.
- 076N15W32CCB 11840 1959Nitrate · 0.9 mi
- MIDAMERICAN ENERGY COMPANY OSKALOOSA SERVICE CENTERBrownfields · 1.3 mi
- OSKALOOSA SERVICE CENTERUnderground Storage Tanks · 1.3 mi
- 076N16W25CBDB 41312 1988Oskaloosa 27Nitrate · 1.8 mi
- KUM & GO #1273Underground Storage Tanks · 1.8 mi
- FOREST CEMETERY ASSOCIATIONUnderground Storage Tanks · 1.9 mi
- MAHASKA COUNTY HWY DEPTUnderground Storage Tanks · 2.0 mi
- WATER DEPTUnderground Storage Tanks · 2.1 mi
- OSKY BUS BARNBrownfields · 2.2 mi
- OSKALOOSA SCHOOL BUS GARAGEUnderground Storage Tanks · 2.2 mi
- WYNGARDEN OIL COMPANYUnderground Storage Tanks · 2.2 mi
- MAHASKA HEALTHBrownfields · 2.2 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Mahaska 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 Mahaska County.
Total Sites
248
across Mahaska County
Risk Level
Elevated
Highest-severity
1 Superfund site (none NPL-listed)
Sources Checked
across Mahaska County
Severity Distribution
across Mahaska County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Mahaska County, Nitrate runs higher than the national average — 86 sites across Mahaska County. 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 Mahaska 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 Mahaska County average
Your land is one point inside Mahaska County
Everything above resolves to Mahaska County as a whole — one PHZM band and one county frost record, read across 365K 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 Mahaska County Average
Mahaska County Average
- ●USDA Zones 5b, averaged across Mahaska County
- ●One soil character for all of Mahaska County
- ●Frost dates read at Mahaska County's scale: Apr 9 to Oct 26
YOUR Parcel
- ✓Your exact zone, not Mahaska County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Mahaska County
Pull a site-specific report for your exact address in Mahaska County, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Mahaska County averages can't show:
We read public map data around Mahaska County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Mahaska County average
How we know this about Mahaska 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 Mahaska County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Mahaska County
When does frost risk typically end in Mahaska County?
The last hard freeze (28°F) in Mahaska County typically lands around Apr 9, 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 Apr 30.
How long is the growing season in Mahaska County?
Measured between 28°F hard freezes, Mahaska County sees about 200 frost-free days — roughly Apr 9 through Oct 26, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Apr 30 to Oct 12, about 165 days.
Is the soil safe to grow vegetables in Mahaska County?
The federal record around Mahaska County is a meaningful one — 248 documented sites, mostly Underground Storage Tanks entries (103), the nearest Nitrate record about 0.9 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 Mahaska County's scale — its 5b band and its 200-day season. Your yard writes its own version of Mahaska 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 Mahaska County’s own scale
9 of the readings below were taken at Mahaska County's own scale — its own ~200-day span between 28°F freezes, the 3 most extensive soil series mapped here and 248 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Mahaska County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Mahaska County's own 28°F hard-freeze dates and its ~200-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Mahaska CountyUSDA NRCS SSURGO
- 248 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Nitrate record about 0.9 miles outFederal environmental registers
- Mahaska County's boundary and the 365,344 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~200-day seasonGrowable Ground crop-matching engine
- Mahaska County sitting wholly inside the Western Corn Belt PlainsUS EPA Level III Ecoregions
- Open occurrence records documented in Mahaska County for a tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Mahaska 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 Mahaska County — the ~200 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 Mahaska County's 365,344 acreswhy one lot differs from the next →
- Which of those 248 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 Mahaska Countyread your own land →