Stephenson County, in Illinois, sits in USDA hardiness zone 5a — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
These conditions suit cabbage, cranberry, lavender, and collard greens — a starting list any specific site will trim or extend with its own soil, sun, and drainage.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Stephenson County holds more than one microclimate.
Soils and elevations shift across Stephenson 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 Stephenson County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Stephenson County — the short answer
Stephenson County, Illinois: USDA zone 5a, about 188 days between hard freezes, across 361K 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 21
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~188
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 361K 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 Stephenson County's average, not your lot's — a low corner or a tree line moves it. Against Stephenson County's 188-day season, see what your own ground reads.
Heat in Stephenson County
Across a year, Stephenson County accumulates about 2,567 growing degree days above 50°F, about 816 fewer than the middle Illinois county — ahead of 1 of Illinois’s 102 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 Stephenson County
Across Stephenson County, the ground is predominantly Alfisols, where Osco, Fayette, and Greenbush are the most extensive named soil series. The soil is generally well drained with a silt loam surface. Topsoil pH runs about 6.2–6.7, slightly acidic. Rainfall drains through hydrologic group B soils.
Soil order
Alfisols
Drainage
Well drained
Prime farmland
42%
Hydric soils
4%
Soil still varies lot by lot — soil types explained.
How the ground varies across Stephenson County
The USDA soil survey maps 76% of Stephenson County as well drained — but another 23% drains differently: 14% somewhat poorly drained, 4% moderately well drained.
Osco is the most extensive named soil at 12% of mapped acres, then Fayette (8%) and Greenbush (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 Stephenson County
Each reading below is measured for Stephenson 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 Stephenson County are well drained, in hydrologic group B — 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 Stephenson County runs about pH 6.2–6.7, 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
MeasuredStephenson County runs about 188 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
MeasuredStephenson 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 Stephenson County is in
Stephenson County spans 2 EPA Level III ecoregions — Central Corn Belt Plains (57%) and Southeastern Wisconsin Till Plains (42%). 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 Stephenson County
We scored all 1,086 plants against Stephenson County's own USDA zone 5a and its own 188-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 Illinois profile.
Is it too late to plant in Stephenson 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 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 21 — 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 Stephenson County
- Japanese beetle9 documented records
- monarch butterfly10 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Stephenson County
USGS estimates 253,782 lb of Glyphosate on Stephenson County farmland in 2019, its largest by weight, ahead of Atrazine (109,831 lb) and Metolachlor & Metolachlor-s (94,005 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 Stephenson County — and how to grow with it.
We checked the federal record across Stephenson County — 280 documented sites across 8 of the 9 source types we track.
The most significant on record: 5 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 Stephenson 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 Stephenson County
Closest first, across Stephenson County. Listing isn't leaking, and distance isn't exposure.
- FARM & FLEET OF FREEPORTBrownfields · 1900 ft
- Freeport Vault CoUnderground Storage Tanks · 1.8 mi
- ROBERTSON AUTOMOTIVEBrownfields · 1.8 mi
- Kelley Williamson MobilUnderground Storage Tanks · 2.4 mi
- Reed SerUnderground Storage Tanks · 2.4 mi
- Cedarville Village OfUnderground Storage Tanks · 2.5 mi
- FREEPORTPFAS · 2.6 mi
- COMMONWEALTH EDISON CO-FREEPORTBrownfields · 2.6 mi
- Trucks IncUnderground Storage Tanks · 2.7 mi
- Johnson Oil CompanyUnderground Storage Tanks · 3.2 mi
- Natl By Products IncUnderground Storage Tanks · 3.2 mi
- R S & D IncUnderground Storage Tanks · 3.2 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Stephenson 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 Stephenson County.
Total Sites
280
across Stephenson County
Risk Level
Elevated
Highest-severity
5 Superfund sites (none NPL-listed)
Sources Checked
across Stephenson County
Severity Distribution
across Stephenson County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Stephenson County, two things run higher than the national average across Stephenson County — CAFO (20 sites) and Superfund (5 sites). That's not a problem with your land — it's information about it.
CAFO: CAFOs pose a different contamination profile than chemical sources.
Superfund: Superfund sites represent the most severe contamination in the federal system.
Wash all produce consumed raw thoroughly, especially leafy greens grown near CAFOs.
Commission professional soil testing before any food production (test for heavy metals, VOCs, and SVOCs).
Check your specific parcel in Stephenson 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 Stephenson County average
Your land is one point inside Stephenson County
Everything above resolves to Stephenson County as a whole — one PHZM band and one county frost record, read across 361K 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 Stephenson County Average
Stephenson County Average
- ●USDA Zones 5a, averaged across Stephenson County
- ●One soil character for all of Stephenson County
- ●Frost dates read at Stephenson County's scale: Apr 16 to Oct 21
YOUR Parcel
- ✓Your exact zone, not Stephenson County's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Stephenson County
Pull a site-specific report for your exact address in Stephenson County, Illinois — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Stephenson County averages can't show:
We read public map data around Stephenson County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Stephenson County average
How we know this about Stephenson 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 Stephenson County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Stephenson County
When does frost risk typically end in Stephenson County?
The last hard freeze (28°F) in Stephenson 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 Stephenson County?
Measured between 28°F hard freezes, Stephenson County sees about 188 frost-free days — roughly Apr 16 through Oct 21, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 7 to Oct 7, about 153 days.
Is the soil safe to grow vegetables in Stephenson County?
The federal record around Stephenson County is a meaningful one — 280 documented sites, mostly Underground Storage Tanks entries (145), the nearest Brownfields record about 0.4 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 Stephenson County's scale — its 5a band and its 188-day season. Your yard writes its own version of Stephenson 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 Illinois's frost window, season length, and soil profile against the plant's real requirements.
What this page reads at Stephenson County’s own scale
9 of the readings below were taken at Stephenson County's own scale — its own ~188-day span between 28°F freezes, the 3 most extensive soil series mapped here and 280 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Stephenson County's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Stephenson County's own 28°F hard-freeze dates and its ~188-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Stephenson CountyUSDA NRCS SSURGO
- 280 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Brownfields record about 0.4 miles outFederal environmental registers
- Stephenson County's boundary and the 361,118 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~188-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Stephenson County spans, Central Corn Belt Plains the largestUS EPA Level III Ecoregions
- Open occurrence records documented in Stephenson County for 2 tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Stephenson County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every Illinois grower plans around — written at state scale, so they stay on the state guidefrom Illinois
What this page can’t tell you
- How the season shifts across Stephenson County — the ~188 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 Stephenson County's 361,118 acreswhy one lot differs from the next →
- Which of those 280 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 Stephenson Countyread your own land →