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 sweet corn, tomato, pumpkin, and apple, among others — though every individual site edits that list with its own soil, sun, and drainage.
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 1,112 plants against your land's actual soil, sun, and frost.
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
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Quick Facts
USDA Zones
7a
Last Hard Freeze (28°F)
Mar 2
County normal — light frosts run a few weeks later
First Hard Freeze (28°F)
Nov 30
County normal — light frosts arrive a few weeks earlier
County Area
269K acres
Hardiness Zone Range
Zone maps are averages across Williamson County. Your yard's slope, trees, and frost pockets shift what actually grows — see your land's exact reading.
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.
What Grows in Williamson County
Plants matched to Williamson County's USDA zones 7a — each links to its full growing 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 2; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 2 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 30 — long-season crops need about 90 days of runway, quick greens only 30. Here the calendar nearly circles: cool-season crops take the winter shift, and the next window is always close.
Growing Challenges in Illinois
What an experienced grower plans around here — each one has a move.
Heavy clay soils in northern IL drain poorly
A raised bed solves the standing-water problem in a weekend; fall compost keeps improving the clay beneath it.
Extreme temperature swings between summer and winter
Wide swings reward truly hardy varieties and a deep mulch blanket — insulation smooths what the weather won't.
Japanese beetles are a major garden pest
Hand-pick into soapy water early and often, and skip the traps (they attract more than they catch) — extension IPM guides have the rest.
For cultivar selection, pest pressure, and planting-time guidance specific to Illinois, the University of Illinois Extension is the authoritative local source.
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.
Williamson County carries one of the heavier federal records we track — and that's not a verdict on your yard. Proximity to a documented site is information, not a diagnosis: nothing here says any particular parcel is affected. It does earn one concrete step — before food beds go in the ground, a professional soil test tells you exactly what you're working with, and raised beds with clean imported soil grow well almost anywhere in the meantime.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
Environmental Intelligence
Understanding what's nearby helps you make informed decisions about where and how to grow.
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 — PFAS (7 sites) and Toxic 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.
Toxic 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 1,112 plant recommendations.
Three things about your exact spot that zone averages miss:
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources analyzed in seconds
Your Specific Parcel Matters
Williamson County Average
- ●USDA Zones 7a
- ●Generic soil type for the area
- ●State-average frost dates
YOUR Parcel
- ✓Your exact hardiness zone
- ✓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 exact spot that zone averages miss:
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources analyzed in seconds
Key Growing Facts for Williamson County, Illinois
- USDA Hardiness Zones: 7a (USDA PHZM 2023)
- Last Hard Freeze (28°F): Mar 2 (county normal, NOAA 1991–2020 Climate Normals — light frosts can follow for a few weeks)
- First Hard Freeze (28°F): Nov 30 (county normal, NOAA 1991–2020 Climate Normals — light frosts can arrive a few weeks earlier)
- Days Between Hard Freezes: ~273 (county normal, NOAA 1991–2020 Climate Normals)
- County Land Area: 269K acres (US Census TIGER 2025)
Zone data: USDA ARS Plant Hardiness Zone Map 2023. Climate data: NOAA NCEI. County boundaries: US Census TIGER/Line 2025.
Frost dates here are the Williamson County average. Low spots and tree cover move them by days on any one yard — see your exact frost windows.
Frequently Asked Questions
What hardiness zone is Williamson County, Illinois?
Williamson County sits in USDA hardiness zone 7a, per the USDA Plant Hardiness Zone Map 2023. Zones reflect average annual extreme minimum temperatures from 1991–2020 weather data.
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 2; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 2 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 30 — long-season crops need about 90 days of runway, quick greens only 30. Here the calendar nearly circles: cool-season crops take the winter shift, and the next window is always close.
When does frost risk typically end in Williamson County?
The last hard freeze (28°F) in Williamson County typically lands around Mar 2, per NOAA 1991–2020 climate normals — an earlier marker than the light-frost dates many planting charts quote. That marks the hard freeze, not the last light frost — light frosts can still bite for a few more weeks, so tender transplants usually wait another 2–3 weeks.
How long is the growing season in Williamson County?
Measured between 28°F hard freezes, Williamson County sees about 273 frost-free days — roughly Mar 2 through Nov 30, per NOAA 1991–2020 climate normals. Tender crops get a somewhat shorter practical window, since lighter frosts reach a few weeks past the hard-freeze dates on both ends.
What vegetables grow in Williamson County?
Williamson County's zone 7a supports a wide range — strong performers include Sweet Corn, Tomato, Pumpkin, Apple, and Coneflower. What actually takes on any one site comes down to its soil, sun, and drainage, and we score each plant against the real conditions at your address.
Which hardiness zone is Williamson County, really?
Officially, Williamson County sits in USDA zone 7a (USDA PHZM 2023) — but a zone is a 30-year average of winter's coldest night across an area, and it can't see any one yard. A south-facing slope, a tree line, or a low frost pocket can shift a single site by half a zone either way, which is why neighboring gardeners often quote different numbers. We read the conditions at your exact address — soil, sun, slope, and frost — and score 1,112 plants against what's actually there.
Is the soil safe to grow vegetables in Williamson County?
The federal record around Williamson County runs heavier than most — 341 documented sites — so test the soil before planting food in the ground, and raised beds with clean imported soil grow well in the meantime. Even here, proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what's recorded and where.
Just moved to Williamson County — what should I know before planting?
Start with three facts. Williamson County sits in USDA zone 7a, which sets what survives winter; the last 28°F hard freeze typically clears around Mar 2, with about 273 frost-free days to work with (NOAA 1991–2020 climate normals); and 341 documented sites sit on the federal record here, so a soil test before food beds is the smart first step. From there, matching plants to your actual soil and sun is the fun part.
Everything on this page is a Williamson County average. Your yard writes its own version — 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.