Williamson County, in Illinois, sits in USDA hardiness zone 7a — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
The conditions favor cabbage, hazelnut, california poppy, and collard greens, among others — though every individual site edits that list with its own soil, sun, and drainage.
Williamson 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
Williamson County holds more than one microclimate.
Soils and elevations shift across Williamson 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 Williamson County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Williamson County — the short answer
Williamson County, Illinois: USDA zone 7a, about 224 days between hard freezes, across 269K acres.
- USDA zone
- 7a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Mar 27
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Nov 6
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~224
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 269K 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 Williamson County's average, not your lot's — a low corner or a tree line moves it. Against Williamson County's 224-day season, see what your own ground reads.
Heat in Williamson County
Across a year, Williamson County accumulates about 3,933 growing degree days above 50°F, about 550 more than the middle Illinois county — ahead of 85 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 Williamson County
Across Williamson County, the ground is predominantly Alfisols, where Ava, Belknap, and Bluford are the most extensive named soil series. The soil is generally moderately well drained with a silt loam surface. Topsoil pH runs about 5.6–6.3, moderately acidic. Rainfall drains through hydrologic group C soils.
Soil order
Alfisols
Drainage
Moderately well drained
Prime farmland
24%
Hydric soils
9%
Soil still varies lot by lot — soil types explained.
How the ground varies across Williamson County
The USDA soil survey maps 43% of Williamson County as moderately well drained — but another 51% drains differently: 23% somewhat poorly drained, 20% well drained.
Ava is the most extensive named soil at 14% of mapped acres, then Belknap (11%) and Bluford (9%).
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 Williamson County
Each reading below is measured for Williamson 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 Williamson 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 Williamson County runs about pH 5.6–6.3, 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
MeasuredWilliamson County runs about 224 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
MeasuredWilliamson 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 Williamson County is in
Williamson County spans 2 EPA Level III ecoregions — Interior River Valleys and Hills (86%) and Interior Plateau (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 Williamson County
We scored all 1,086 plants against Williamson County's own USDA zone 7a and its own 224-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 Williamson 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 27; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 27 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 6 — long-season crops need about 90 days of runway, quick greens only 30. A long window like this one runs successions deep into fall — and even its last weeks take quick greens and garlic.
Documented around Williamson County
- monarch butterfly25 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Williamson County
USGS estimates 60,671 lb of Glyphosate on Williamson County farmland in 2019, its largest by weight, ahead of Metolachlor & Metolachlor-s (56,493 lb) and Metolachlor-s (43,595 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 Williamson County — and how to grow with it.
We checked the federal record across Williamson County — 341 documented sites across 5 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 Williamson 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 Williamson County
Closest first, across Williamson County. Listing isn't leaking, and distance isn't exposure.
- Ideal LawnmowerUnderground Storage Tanks · 200 ft
- Stanley Gary LUnderground Storage Tanks · 200 ft
- P&S Grain Co IncUnderground Storage Tanks · 200 ft
- Samples Plumbing & HeatingUnderground Storage Tanks · 200 ft
- Hertz Car RentalUnderground Storage Tanks · 200 ft
- Southern Il Power CoopUnderground Storage Tanks · 200 ft
- General Dynamics Ordnance & Tectical SystemsUnderground Storage Tanks · 200 ft
- E BLANKENSHIP & CO INCBrownfields · 550 ft
- GTE North Inc., IllinoisUnderground Storage Tanks · 550 ft
- Knapp Oil CompanyUnderground Storage Tanks · 700 ft
- Williamson County Court HouseUnderground Storage Tanks · 750 ft
- Marion Courts Motel CorpUnderground Storage Tanks · 800 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Williamson 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 Williamson County.
Sources Checked
across Williamson County
Severity Distribution
across Williamson County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Williamson County, two things run higher than the national average across Williamson County — PFAS (7 sites) and Toxics Release Inventory (22 sites). Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
PFAS: PFAS (per- and polyfluoroalkyl substances) are called "forever chemicals" because they do not biodegrade.
Toxics Release Inventory: TRI facilities report annual chemical releases to air, water, and land.
Test irrigation water source — this is the primary pathway for PFAS to reach garden crops.
Check prevailing wind direction — downwind parcels face higher exposure than upwind or crosswind locations.
Check your specific parcel in Williamson 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 Williamson County average
Your land is one point inside Williamson County
Everything above resolves to Williamson County as a whole — one PHZM band and one county frost record, read across 269K 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 Williamson County Average
Williamson County Average
- ●USDA Zones 7a, averaged across Williamson County
- ●One soil character for all of Williamson County
- ●Frost dates read at Williamson County's scale: Mar 27 to Nov 6
YOUR Parcel
- ✓Your exact zone, not Williamson County's 7a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Williamson County
Pull a site-specific report for your exact address in Williamson County, Illinois — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Williamson County averages can't show:
We read public map data around Williamson County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Williamson County average
How we know this about Williamson 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 Williamson County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Williamson County
When does frost risk typically end in Williamson County?
The last hard freeze (28°F) in Williamson County typically lands around Mar 27, 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 17.
How long is the growing season in Williamson County?
Measured between 28°F hard freezes, Williamson County sees about 224 frost-free days — roughly Mar 27 through Nov 6, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Apr 17 to Oct 23, about 189 days.
Is the soil safe to grow vegetables in Williamson County?
The federal record around Williamson County is a meaningful one — 341 documented sites, mostly Underground Storage Tanks entries (211), 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 Williamson County's scale — its 7a band and its 224-day season. Your yard writes its own version of Williamson 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 Williamson County’s own scale
9 of the readings below were taken at Williamson County's own scale — its own ~224-day span between 28°F freezes, the 3 most extensive soil series mapped here and 341 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Williamson County's own USDA hardiness band, zones 7aUSDA PHZM 2023
- Williamson County's own 28°F hard-freeze dates and its ~224-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Williamson CountyUSDA NRCS SSURGO
- 341 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
- Williamson County's boundary and the 268,938 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 7a and a ~224-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Williamson County spans, Interior River Valleys and Hills the largestUS EPA Level III Ecoregions
- Open occurrence records documented in Williamson County for a tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Williamson 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 Williamson County — the ~224 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 Williamson County's 268,938 acreswhy one lot differs from the next →
- Which of those 341 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 Williamson Countyread your own land →