Knox County, in Illinois, sits in USDA hardiness zone 5b — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
A short list that earns its place here — sweet corn, tomato, pumpkin, and apple — with any one site's soil, sun, and drainage making the final cut.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Knox County holds more than one microclimate.
Soils and elevations shift across Knox 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
5b
Last Hard Freeze (28°F)
Mar 26
County normal — light frosts run a few weeks later
First Hard Freeze (28°F)
Nov 14
County normal — light frosts arrive a few weeks earlier
County Area
458K acres
Hardiness Zone Range
Zone maps are averages across Knox County. Your yard's slope, trees, and frost pockets shift what actually grows — see your land's exact reading.
Soil in Knox County
Across Knox County, the ground is predominantly Alfisols, where Ipava, Osco, and Rozetta are the most extensive named soil series. The soil is generally well drained with a silt loam surface. Topsoil pH runs about 6.0–6.6, slightly acidic. Rainfall drains through hydrologic group B soils.
Soil order
Alfisols
Drainage
Well drained
Prime farmland
48%
Hydric soils
5%
Soil still varies lot by lot — soil types explained.
What Grows in Knox County
Plants matched to Knox County's USDA zones 5b — each links to its full growing profile.
Is it too late to plant in Knox County?
Almost never — the real question is what to plant next. Cool-season crops can go in from around Feb 26; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 26 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 14 — long-season crops need about 90 days of runway, quick greens only 30. With a season this long, “too late” mostly means “switch crops” — second sowings and a full fall garden are the norm, with garlic closing the year.
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 Knox County — and how to grow with it.
We checked the federal record across Knox County — 346 documented sites across 7 of the 9 source types we track.
The most significant on record: 5 Superfund sites. Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
Knox 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.
Severity Distribution
across Knox County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Knox County, two things run higher than the national average — CAFO (17 sites) and Superfund (5 sites). Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
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 Knox 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
Knox County Average
- ●USDA Zones 5b
- ●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 Knox County
Pull a site-specific report for your exact address in Knox 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 Knox County, Illinois
- USDA Hardiness Zones: 5b (USDA PHZM 2023)
- Last Hard Freeze (28°F): Mar 26 (county normal, NOAA 1991–2020 Climate Normals — light frosts can follow for a few weeks)
- First Hard Freeze (28°F): Nov 14 (county normal, NOAA 1991–2020 Climate Normals — light frosts can arrive a few weeks earlier)
- Days Between Hard Freezes: ~233 (county normal, NOAA 1991–2020 Climate Normals)
- County Land Area: 458K 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 Knox 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 Knox County, Illinois?
Knox County sits in USDA hardiness zone 5b, 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 Knox County?
Almost never — the real question is what to plant next. Cool-season crops can go in from around Feb 26; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Mar 26 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 14 — long-season crops need about 90 days of runway, quick greens only 30. With a season this long, “too late” mostly means “switch crops” — second sowings and a full fall garden are the norm, with garlic closing the year.
When does frost risk typically end in Knox County?
The last hard freeze (28°F) in Knox County typically lands around Mar 26, 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 Knox County?
Measured between 28°F hard freezes, Knox County sees about 233 frost-free days — roughly Mar 26 through Nov 14, 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 Knox County?
Knox County's zone 5b 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 Knox County, really?
Officially, Knox County sits in USDA zone 5b (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 Knox County?
The federal record around Knox County runs heavier than most — 346 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 Knox County — what should I know before planting?
Start with three facts. Knox County sits in USDA zone 5b, which sets what survives winter; the last 28°F hard freeze typically clears around Mar 26, with about 233 frost-free days to work with (NOAA 1991–2020 climate normals); and 346 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 Knox 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.