Clarke County, in Iowa, sits in USDA hardiness zone 5b — a zone band wide enough that plant choice, not possibility, is the interesting question.
Crops well matched to these conditions include cabbage, cranberry, lavender, and collard greens — though what thrives on any one site still turns on its specific soil, sun, and drainage.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Clarke County holds more than one microclimate.
Soils and elevations shift across Clarke 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 Clarke County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Clarke County — the short answer
Clarke County, Iowa: USDA zone 5b, about 191 days between hard freezes, across 276K acres.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 13
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 21
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~191
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 276K 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 Clarke County's average, not your lot's — a low corner or a tree line moves it. Against Clarke County's 191-day season, see what your own ground reads.
Heat in Clarke County
Across a year, Clarke County accumulates about 2,960 growing degree days above 50°F, about 216 more than the middle Iowa county — ahead of 72 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 Clarke County
Across Clarke County, the ground is predominantly Mollisols, where Gara, Grundy, and Lamoni are the most extensive named soil series. The soil is generally somewhat poorly 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
Somewhat poorly drained
Prime farmland
14%
Hydric soils
17%
Soil still varies lot by lot — soil types explained.
How the ground varies across Clarke County
The USDA soil survey maps 49% of Clarke County as somewhat poorly drained — but another 50% drains differently: 23% well drained, 21% poorly drained.
Gara is the most extensive named soil at 11% of mapped acres, then Grundy (8%) and Lamoni (8%).
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 Clarke County
Each reading below is measured for Clarke 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 Clarke County are somewhat poorly drained, in hydrologic group C — 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 Clarke 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
Season length
MeasuredClarke County runs about 191 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
MeasuredClarke County 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 Clarke County is in
Clarke County spans 2 EPA Level III ecoregions — Central Irregular Plains (80%) and Western Corn Belt Plains (20%). Which one your land falls in changes what grows wild there and how the ground behaves, and a county-wide answer cannot tell you which.
What Grows in Clarke County
We scored all 1,086 plants against Clarke County's own USDA zone 5b and its own 191-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 Clarke 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 16; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 13 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 21 — long-season crops need about 90 days of runway, quick greens only 30. Even past midsummer there is room for a true fall garden here, and garlic planted near the close carries the momentum into next year.
Documented around Clarke County
- monarch butterfly2 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Clarke County
USGS estimates 55,159 lb of Glyphosate on Clarke County farmland in 2019, its largest by weight, ahead of Metolachlor & Metolachlor-s (29,346 lb) and Atrazine (22,333 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 Clarke County — and how to grow with it.
We checked the federal record across Clarke County — 117 documented sites across 7 of the 9 source types we track.
The most significant on record: 5 Toxics Release Inventory facilities. Active industrial facilities reporting chemical releases to air, water, and land.
The federal record across Clarke County is a modest one — a typical footprint for a growing area. Nothing here calls for alarm; it's worth knowing which recorded sites sit closest to where you grow, and each one on the map carries its type and location. If one turns out to be a near neighbor, a one-time soil test settles the question.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
What's on the federal record near Clarke County
Closest first, across Clarke County. Listing isn't leaking, and distance isn't exposure.
- GERMAN MACHINE WORKS INCBrownfields · 550 ft
- IOWA DEPARTMENT OF TRANSPORTATIONBrownfields · 650 ft
- TEXACO FOOD MARTUnderground Storage Tanks · 950 ft
- SALFORD INCBrownfields · 1100 ft
- OLIVER MOTORSUnderground Storage Tanks · 1100 ft
- CASEY''S GENERAL STORE 2550Underground Storage Tanks · 1200 ft
- SIMCO DRILLING EQUIPMENT INCBrownfields · 1400 ft
- ALTEC OSCEOLA BODY PLANTToxics Release Inventory · 1700 ft
- ALTEC OSCEOLA BODY PLANTBrownfields · 1700 ft
- OSCEOLA FOODS INCToxics Release Inventory · 1900 ft
- OSCEOLA FOODS INCBrownfields · 1900 ft
- CLARKE COUNTY TIREBrownfields · 2100 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Clarke 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 Clarke County.
Total Sites
117
across Clarke County
Risk Level
Moderate
Highest-severity
5 Toxics Release Inventory facilities
Sources Checked
across Clarke County
Severity Distribution
across Clarke County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Clarke County, Nitrate runs higher than the national average — 42 sites across Clarke County. 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 Clarke 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 Clarke County average
Your land is one point inside Clarke County
Everything above resolves to Clarke County as a whole — one PHZM band and one county frost record, read across 276K 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 Clarke County Average
Clarke County Average
- ●USDA Zones 5b, averaged across Clarke County
- ●One soil character for all of Clarke County
- ●Frost dates read at Clarke County's scale: Apr 13 to Oct 21
YOUR Parcel
- ✓Your exact zone, not Clarke County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Clarke County
Pull a site-specific report for your exact address in Clarke County, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Clarke County averages can't show:
We read public map data around Clarke County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Clarke County average
How we know this about Clarke 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 Clarke County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Clarke County
When does frost risk typically end in Clarke County?
The last hard freeze (28°F) in Clarke County typically lands around Apr 13, 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 4.
How long is the growing season in Clarke County?
Measured between 28°F hard freezes, Clarke County sees about 191 frost-free days — roughly Apr 13 through Oct 21, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 4 to Oct 7, about 156 days.
Is the soil safe to grow vegetables in Clarke County?
The federal record around Clarke County shows 117 documented sites, mostly Nitrate entries (42), the nearest Brownfields 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.
Everything above is measured at Clarke County's scale — its 5b band and its 191-day season. Your yard writes its own version of Clarke 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 Clarke County’s own scale
9 of the readings below were taken at Clarke County's own scale — its own ~191-day span between 28°F freezes, the 3 most extensive soil series mapped here and 117 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Clarke County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Clarke County's own 28°F hard-freeze dates and its ~191-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Clarke CountyUSDA NRCS SSURGO
- 117 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Brownfields record about 0.1 miles outFederal environmental registers
- Clarke County's boundary and the 275,911 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~191-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Clarke County spans, Central Irregular Plains the largestUS EPA Level III Ecoregions
- Open occurrence records documented in Clarke County for a tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Clarke 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 Clarke County — the ~191 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 Clarke County's 275,911 acreswhy one lot differs from the next →
- Which of those 117 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 Clarke Countyread your own land →