Johnson County, in Iowa, sits in USDA hardiness zone 5b — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
Among the crops suited to this profile: cabbage, cranberry, lavender, and collard greens. The site-level story — soil, sun, drainage — decides the rest.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Johnson County holds more than one microclimate.
Soils and elevations shift across Johnson 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 Johnson County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Johnson County — the short answer
Johnson County, Iowa: USDA zone 5b, about 197 days between hard freezes, across 392K acres.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 10
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 24
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~197
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 392K 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 Johnson County's average, not your lot's — a low corner or a tree line moves it. Against Johnson County's 197-day season, see what your own ground reads.
Heat in Johnson County
Across a year, Johnson County accumulates about 3,022 growing degree days above 50°F, about 278 more than the middle Iowa county — ahead of 79 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 Johnson County
Across Johnson County, the ground is predominantly Alfisols, where Fayette, Ladoga, and Colo are the most extensive named soil series. The soil is generally well drained with a silt loam surface. Topsoil pH runs about 6.2–6.5, slightly acidic. Rainfall drains through hydrologic group C soils.
Soil order
Alfisols
Drainage
Well drained
Prime farmland
29%
Hydric soils
19%
Soil still varies lot by lot — soil types explained.
How the ground varies across Johnson County
The USDA soil survey maps 39% of Johnson County as well drained — but another 58% drains differently: 25% moderately well drained, 18% poorly drained.
Fayette is the most extensive named soil at 20% of mapped acres, then Ladoga (8%) and Colo (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 Johnson County
Each reading below is measured for Johnson 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 Johnson County are well drained, in hydrologic group C — ground that drains quickly.
The upside is soil you can work early in spring and roots that never drown; the cost is that fast-draining ground dries out fast and leaches nutrients away before plants can use them.
Work in compost and mulch to hold moisture and feed, and water more often in short doses than deep and rarely.
USDA NRCS SSURGO
Surface pH
MeasuredTopsoil across Johnson County runs about pH 6.2–6.5, 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 19% of Johnson 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
MeasuredJohnson County runs about 197 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
The kind of country Johnson County is in
Johnson County spans 2 EPA Level III ecoregions — Western Corn Belt Plains (86%) and Interior River Valleys and Hills (14%). 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 Johnson County
We scored all 1,086 plants against Johnson County's own USDA zone 5b and its own 197-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 Johnson County?
Rarely: the season closes in stages, not all at once, and each stage has its crops. Cool-season crops can go in from around Mar 13; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 10 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 24 — 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.
Documented around Johnson County
- Japanese beetle7 documented records
- monarch butterfly237 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Johnson County
USGS estimates 186,042 lb of Glyphosate on Johnson County farmland in 2019, its largest by weight, ahead of Atrazine (104,822 lb) and Metolachlor & Metolachlor-s (71,157 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 Johnson County — and how to grow with it.
We checked the federal record across Johnson County — 681 documented sites across 6 of the 9 source types we track.
The most significant on record: 7 Superfund sites (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 Johnson 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 Johnson County
Closest first, across Johnson County. Listing isn't leaking, and distance isn't exposure.
- GROWMARK INCUnderground Storage Tanks · 2500 ft
- CASEY''S GENERAL STORE 2779Underground Storage Tanks · 0.6 mi
- PRONTO MARTUnderground Storage Tanks · 0.6 mi
- KINGS MATERIAL INCUnderground Storage Tanks · 0.6 mi
- CITY OF CORALVILLE WASTE WATER PLANTUnderground Storage Tanks · 0.6 mi
- OFF CAMPUS CONOCOUnderground Storage Tanks · 0.7 mi
- COLONIAL BAKING COUnderground Storage Tanks · 0.7 mi
- S & E CUSTOM CABINETS INCBrownfields · 0.7 mi
- TOYOTA OF IOWA CITYUnderground Storage Tanks · 0.7 mi
- UNIVERSITY OF IA OAKDALE CAMPUSUnderground Storage Tanks · 0.7 mi
- IOWA MEDICAL & CLASSIFICATION CENTERUnderground Storage Tanks · 0.7 mi
- IDOT-OakdaleUnderground Storage Tanks · 0.7 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Johnson 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 Johnson County.
Total Sites
681
across Johnson County
Risk Level
Elevated
Highest-severity
7 Superfund sites (none NPL-listed)
Sources Checked
across Johnson County
Severity Distribution
across Johnson County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Johnson County, Nitrate runs higher than the national average — 198 sites across Johnson 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 Johnson 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 Johnson County average
Your land is one point inside Johnson County
Everything above resolves to Johnson County as a whole — one PHZM band and one county frost record, read across 392K 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 Johnson County Average
Johnson County Average
- ●USDA Zones 5b, averaged across Johnson County
- ●One soil character for all of Johnson County
- ●Frost dates read at Johnson County's scale: Apr 10 to Oct 24
YOUR Parcel
- ✓Your exact zone, not Johnson County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Johnson County
Pull a site-specific report for your exact address in Johnson County, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Johnson County averages can't show:
We read public map data around Johnson County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Johnson County average
How we know this about Johnson 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 Johnson County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Johnson County
When does frost risk typically end in Johnson County?
The last hard freeze (28°F) in Johnson County typically lands around Apr 10, 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 1.
How long is the growing season in Johnson County?
Measured between 28°F hard freezes, Johnson County sees about 197 frost-free days — roughly Apr 10 through Oct 24, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 1 to Oct 10, about 162 days.
Is the soil safe to grow vegetables in Johnson County?
The federal record around Johnson County is a meaningful one — 681 documented sites, mostly Underground Storage Tanks entries (308), the nearest Underground Storage Tanks record about 0.5 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 Johnson County's scale — its 5b band and its 197-day season. Your yard writes its own version of Johnson 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 Johnson County’s own scale
9 of the readings below were taken at Johnson County's own scale — its own ~197-day span between 28°F freezes, the 3 most extensive soil series mapped here and 681 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Johnson County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Johnson County's own 28°F hard-freeze dates and its ~197-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Johnson CountyUSDA NRCS SSURGO
- 681 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Underground Storage Tanks record about 0.5 miles outFederal environmental registers
- Johnson County's boundary and the 392,318 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~197-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Johnson County spans, Western Corn Belt Plains the largestUS EPA Level III Ecoregions
- Open occurrence records documented in Johnson County for 2 tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Johnson 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 Johnson County — the ~197 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 Johnson County's 392,318 acreswhy one lot differs from the next →
- Which of those 681 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 Johnson Countyread your own land →