Johnson County, in Illinois, sits in USDA hardiness zone 7a — 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, hazelnut, california poppy, and collard greens. The site-level story — soil, sun, drainage — decides the rest.
Johnson County lies within the Shawnee Hills / Southern Illinois (Little Egypt) — a regional growing area with its own character.
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, Illinois: USDA zone 7a, about 226 days between hard freezes, across 220K acres.
- USDA zone
- 7a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Mar 28
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Nov 9
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~226
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 220K 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 7a is Johnson County's average, not your lot's — a low corner or a tree line moves it. Against Johnson County's 226-day season, see what your own ground reads.
Heat in Johnson County
Across a year, Johnson County accumulates about 4,024 growing degree days above 50°F, about 641 more than the middle Illinois county — ahead of 93 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 Johnson County
Across Johnson County, the ground is predominantly Alfisols, where Hosmer, Grantsburg, and Zanesville are the most extensive named soil series. The soil is generally moderately well drained with a silt loam surface. Topsoil pH runs about 5.3–5.8, strongly acidic. Rainfall drains through hydrologic group C soils.
Soil order
Alfisols
Drainage
Moderately well drained
Prime farmland
18%
Hydric soils
8%
Soil still varies lot by lot — soil types explained.
How the ground varies across Johnson County
The USDA soil survey maps 62% of Johnson County as moderately well drained — but another 37% drains differently: 21% well drained, 9% somewhat poorly drained.
Hosmer is the most extensive named soil at 30% of mapped acres, then Grantsburg (17%) and Zanesville (11%).
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 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 Johnson County runs about pH 5.3–5.8, on the acidic side.
Blueberries, potatoes, and rhododendrons thrive in it, but most vegetables start locking out nutrients below about pH 6.0 and grow pale and stunted no matter how much you feed them.
A measured lime application raises pH for the crops that need it — but test first, because the right rate depends on your own soil, not the county average.
USDA NRCS SSURGO
Season length
MeasuredJohnson County runs about 226 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
Winter hardiness
MeasuredJohnson County sits in USDA hardiness zone 7a.
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 Johnson County is in
Johnson County sits in the Interior Plateau — EPA Level III, inside the wider Eastern Temperate Forests. One consistent ecological setting across the whole county.
What Grows in Johnson County
We scored all 1,086 plants against Johnson County's own USDA zone 7a and its own 226-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 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 Feb 28; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 28 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 9 — long-season crops need about 90 days of runway, quick greens only 30. There is slack in a calendar like this — late plantings, second rounds of favorites, and a fall bench that keeps beds working.
Documented around Johnson County
- Japanese beetle1 documented records
- monarch butterfly4 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Johnson County
USGS estimates 32,316 lb of Glyphosate on Johnson County farmland in 2019, its largest by weight, ahead of Metolachlor & Metolachlor-s (27,584 lb) and Metolachlor-s (20,565 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 — 66 documented sites across 5 of the 9 source types we track.
The most significant on record: 1 mining site (prospects, none worked). Historic and active mines that may leach heavy metals like arsenic, lead, and cadmium.
The federal record across Johnson 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 Johnson County
Closest first, across Johnson County. Listing isn't leaking, and distance isn't exposure.
- IDOT VIENNA TEAM SECTION 923VBrownfields · 1.8 mi
- Knapp MartUnderground Storage Tanks · 2.1 mi
- Vinsons StoreUnderground Storage Tanks · 2.1 mi
- Johnson County Ambulance/Sheriff's Dept.Underground Storage Tanks · 2.2 mi
- Vienna High SchoolUnderground Storage Tanks · 2.3 mi
- Hudges Robert OUnderground Storage Tanks · 2.8 mi
- MILLSTONE PWDPFAS · 2.8 mi
- LAKE OF EGYPT PWDPFAS · 2.8 mi
- DOLLAR GENERAL STORE 1129Brownfields · 3.0 mi
- Thomas LiquorUnderground Storage Tanks · 3.0 mi
- ROC One Stop #11Underground Storage Tanks · 3.0 mi
- AceesUnderground Storage Tanks · 3.1 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
66
across Johnson County
Risk Level
Moderate
Highest-severity
1 mining site (prospects, none worked)
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, Underground Storage Tanks runs higher than the national average — 50 sites across Johnson County. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
Underground Storage Tanks: Underground storage tanks are the single most common source of soil contamination near homes and gardens.
Use raised beds with imported soil — this eliminates the primary soil-contact pathway.
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 220K 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 7a, averaged across Johnson County
- ●One soil character for all of Johnson County
- ●Frost dates read at Johnson County's scale: Mar 28 to Nov 9
YOUR Parcel
- ✓Your exact zone, not Johnson County's 7a
- ✓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, Illinois — 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 Mar 28, 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 18.
How long is the growing season in Johnson County?
Measured between 28°F hard freezes, Johnson County sees about 226 frost-free days — roughly Mar 28 through Nov 9, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Apr 18 to Oct 26, about 191 days.
Is the soil safe to grow vegetables in Johnson County?
The federal record around Johnson County shows 66 documented sites, mostly Underground Storage Tanks entries (50), the nearest Brownfields record about 1.8 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 Johnson County's scale — its 7a band and its 226-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 Illinois'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 ~226-day span between 28°F freezes, the 3 most extensive soil series mapped here and 66 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 7aUSDA PHZM 2023
- Johnson County's own 28°F hard-freeze dates and its ~226-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
- 66 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Brownfields record about 1.8 miles outFederal environmental registers
- Johnson County's boundary and the 219,991 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 7a and a ~226-day seasonGrowable Ground crop-matching engine
- Johnson County sitting wholly inside the Interior PlateauUS 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 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 Johnson County — the ~226 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 219,991 acreswhy one lot differs from the next →
- Which of those 66 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 →