Jefferson 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.
Reliable performers under these conditions include cabbage, cranberry, lavender, and collard greens; what your own ground favors still comes down to its soil, sun, and drainage.
Jefferson County lies within the Southeast Iowa / Great River Region (Mississippi–Des Moines river corner) — a regional growing area with its own character.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Jefferson County holds more than one microclimate.
Soils and elevations shift across Jefferson 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 Jefferson County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Jefferson County — the short answer
Jefferson County, Iowa: USDA zone 5b, about 204 days between hard freezes, across 279K acres.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 7
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 28
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~204
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 279K 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 Jefferson County's average, not your lot's — a low corner or a tree line moves it. Against Jefferson County's 204-day season, see what your own ground reads.
Heat in Jefferson County
Across a year, Jefferson County accumulates about 3,180 growing degree days above 50°F, about 436 more than the middle Iowa county — ahead of 91 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 Jefferson County
Across Jefferson County, the ground is predominantly Alfisols, where Lindley, Mahaska, and Weller are the most extensive named soil series. The soil is generally moderately well drained with a silty clay loam surface. Topsoil pH runs about 5.9–6.5, slightly acidic. Rainfall drains through hydrologic group D soils.
Soil order
Alfisols
Drainage
Moderately well drained
Prime farmland
23%
Hydric soils
28%
Soil still varies lot by lot — soil types explained.
How the ground varies across Jefferson County
The USDA soil survey maps 34% of Jefferson County as moderately well drained — but another 66% drains differently: 30% poorly drained, 23% somewhat poorly drained.
Lindley is the most extensive named soil at 8% of mapped acres, then Mahaska (8%) and Weller (7%).
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 Jefferson County
Each reading below is measured for Jefferson 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 Jefferson County are moderately well drained, in hydrologic group D — 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 Jefferson County runs about pH 5.9–6.5, 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 28% of Jefferson 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
MeasuredJefferson County runs about 204 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 Jefferson County is in
Jefferson County spans 2 EPA Level III ecoregions — Western Corn Belt Plains (64%) and Central Irregular Plains (36%). 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 Jefferson County
We scored all 1,086 plants against Jefferson County's own USDA zone 5b and its own 204-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 Jefferson 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 10; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 7 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 28 — 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.
What gets applied to farmland in Jefferson County
USGS estimates 126,307 lb of Glyphosate on Jefferson County farmland in 2019, its largest by weight, ahead of Acetochlor (105,795 lb) and Atrazine (95,654 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 Jefferson County — and how to grow with it.
We checked the federal record across Jefferson County — 168 documented sites across 7 of the 9 source types we track.
The most significant on record: 3 Superfund sites. 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 Jefferson 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 Jefferson County
Closest first, across Jefferson County. Listing isn't leaking, and distance isn't exposure.
- FORMER VISTAUnderground Storage Tanks · 75 ft
- SMITHBURG AUTOMOTIVE INCUnderground Storage Tanks · 300 ft
- QUALITY QUICK STOPUnderground Storage Tanks · 350 ft
- KESSELS CONOCOUnderground Storage Tanks · 500 ft
- BOBS SERVICEUnderground Storage Tanks · 600 ft
- IOWA DEPARTMENT OF TRANSPORTATIONUnderground Storage Tanks · 850 ft
- JEFF COUNTY LAW ENFORCEMENT CENTERUnderground Storage Tanks · 850 ft
- JEFFERSON MOTOR COMP INCUnderground Storage Tanks · 850 ft
- MAHARISHI INTERNATIONAL UNIVERSITYUnderground Storage Tanks · 850 ft
- FIRST LUTHERAN CHURCHUnderground Storage Tanks · 900 ft
- FAMILY DOLLAR STORES OF IOWA LLC 10076Brownfields · 950 ft
- O'GRADY CLEANERSBrownfields · 1300 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Jefferson 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 Jefferson County.
Sources Checked
across Jefferson County
Severity Distribution
across Jefferson County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Jefferson County, two things run higher than the national average across Jefferson County — Nitrate (70 sites) and Superfund (3 sites). 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.
Superfund: Superfund sites represent the most severe contamination in the federal system.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Commission professional soil testing before any food production (test for heavy metals, VOCs, and SVOCs).
Check your specific parcel in Jefferson 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 Jefferson County average
Your land is one point inside Jefferson County
Everything above resolves to Jefferson County as a whole — one PHZM band and one county frost record, read across 279K 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 Jefferson County Average
Jefferson County Average
- ●USDA Zones 5b, averaged across Jefferson County
- ●One soil character for all of Jefferson County
- ●Frost dates read at Jefferson County's scale: Apr 7 to Oct 28
YOUR Parcel
- ✓Your exact zone, not Jefferson County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Jefferson County
Pull a site-specific report for your exact address in Jefferson County, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Jefferson County averages can't show:
We read public map data around Jefferson County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Jefferson County average
How we know this about Jefferson 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 Jefferson County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Jefferson County
When does frost risk typically end in Jefferson County?
The last hard freeze (28°F) in Jefferson County typically lands around Apr 7, 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 28.
How long is the growing season in Jefferson County?
Measured between 28°F hard freezes, Jefferson County sees about 204 frost-free days — roughly Apr 7 through Oct 28, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Apr 28 to Oct 14, about 169 days.
Is the soil safe to grow vegetables in Jefferson County?
The federal record around Jefferson County is a meaningful one — 168 documented sites, mostly Nitrate entries (70), the nearest Underground Storage Tanks record under a tenth of a mile 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 Jefferson County's scale — its 5b band and its 204-day season. Your yard writes its own version of Jefferson 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 Jefferson County’s own scale
8 of the readings below were taken at Jefferson County's own scale — its own ~204-day span between 28°F freezes, the 3 most extensive soil series mapped here and 168 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Jefferson County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Jefferson County's own 28°F hard-freeze dates and its ~204-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Jefferson CountyUSDA NRCS SSURGO
- 168 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Underground Storage Tanks record under a tenth of a mile outFederal environmental registers
- Jefferson County's boundary and the 278,735 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~204-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Jefferson County spans, Western Corn Belt Plains the largestUS EPA Level III Ecoregions
- Which agricultural compounds go onto Jefferson 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 Jefferson County — the ~204 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 Jefferson County's 278,735 acreswhy one lot differs from the next →
- Which of those 168 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 Jefferson Countyread your own land →