Knoxville, Iowa, sits in USDA hardiness zone 5b — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
Reliable performers under these conditions include cabbage, cranberry, lavender, and collard greens; what your own ground favors still comes down to its soil, sun, and drainage.
Even in Knoxville, no two yards are alike.
A low spot, a south-facing slope, or a stand of trees moves the frost date and sun across a single Knoxville lot. 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 Knoxville — soil, climate, flood, and parcel records. How we handle your address.
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Knoxville — the short answer
Knoxville, Iowa: USDA zone 5b, about 204 days between hard freezes, across 3K acres of city.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 7
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 28
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~204
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 3K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are this town’s own 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 5b is Knoxville's average, not your lot's — a low corner or a tree line moves it. Against Knoxville's 204-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Knoxville
Each reading below is measured for Knoxville itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredKnoxville runs about 204 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
MeasuredKnoxville sits in USDA hardiness zone 5b.
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 Knoxville is in
Knoxville sits in the Western Corn Belt Plains — EPA Level III, inside the wider Great Plains. That is read at Knoxville's own centroid rather than borrowed from Marion County.
What Grows in Knoxville
Every plant below cleared Knoxville's own zone 5b and its 204-day season. Each card opens that plant's Iowa profile. How we score.
Is it too late to plant in Knoxville?
For most of the year, no — what changes is which crops still fit the days remaining. Cool-season crops can go in from around Mar 10; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 7 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 28 — 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.
What's on the federal record near Knoxville
Closest first, within ~10 miles of Knoxville. Listing isn't leaking, and distance isn't exposure.
- COASTAL MART #1064Underground Storage Tanks · 800 ft
- Not KnownUnderground Storage Tanks · 850 ft
- KNOXVILLE 66Underground Storage Tanks · 850 ft
- MARION COUNTYUnderground Storage Tanks · 950 ft
- VETERANS ADMINISTRATIONUnderground Storage Tanks · 950 ft
- PHILIP W CLARKUnderground Storage Tanks · 950 ft
- SYNHORST LIVESTOCK & GRAINUnderground Storage Tanks · 950 ft
- HUNTER OIL COUnderground Storage Tanks · 950 ft
- GOODYEAR ASCUnderground Storage Tanks · 950 ft
- KIRKWOOD RADIATOR SHOPUnderground Storage Tanks · 950 ft
- U S WESTUnderground Storage Tanks · 1000 ft
- PENNINGTON ENGINE REBUILDERSBrownfields · 1000 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Knoxville'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 Knoxville.
Total Sites
215
within ~10 miles of Knoxville
Risk Level
Elevated
Highest-severity
2 Toxics Release Inventory facilities
Sources Checked
within ~10 miles of Knoxville
Severity Distribution
within ~10 miles of Knoxville
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Knoxville, two things run higher than the national average within ~10 miles of Knoxville — Nitrate (106 sites) and PFAS (3 sites). Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
PFAS: PFAS (per- and polyfluoroalkyl substances) are called "forever chemicals" because they do not biodegrade.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Test irrigation water source — this is the primary pathway for PFAS to reach garden crops.
Check your specific parcel in Knoxville
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 Knoxville average
Your land is one point inside Knoxville
Everything above resolves to Knoxville as a whole — one PHZM band and one 28°F frost record, read across 3K 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 city 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 local average.
Your Lot vs the Knoxville Average
Knoxville Average
- ●USDA Zones 5b, averaged across Knoxville
- ●One soil character for all of Knoxville
- ●Frost dates read at Knoxville's scale: Apr 7 to Oct 28
YOUR Parcel
- ✓Your exact zone, not Knoxville's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Knoxville
Pull a site-specific report for your exact address in Knoxville, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Knoxville averages can't show:
We read public map data around Knoxville — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Knoxville average
How we know this about Knoxville
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this city’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Knoxville’s own record; boundaries and acreage from US Census TIGER/Line 2025; the contamination records from the federal sources named in their beats above. Soil is not read at city grain anywhere on this page — there is no city-resolution survey in our data, which is why you will find no soil grid here rather than a statewide one wearing a local label. The resolution ledger below names every figure this page inherited.
Common questions about Knoxville
When does frost risk typically end in Knoxville?
The last hard freeze (28°F) in Knoxville typically lands around Apr 7, 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 28.
When is the first frost in Knoxville?
The first hard freeze (28°F) in Knoxville typically arrives around Oct 28, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 14, so the watching starts about Sep 28 — about 204 days after the spring freeze cleared.
How long is the growing season in Knoxville?
Measured between 28°F hard freezes, Knoxville sees about 204 frost-free days — roughly Apr 7 through Oct 28, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Apr 28 to Oct 14, about 169 days.
Is the soil safe to grow vegetables in Knoxville?
The federal record around Knoxville is a meaningful one — 215 documented sites, mostly Nitrate entries (106), the nearest Underground Storage Tanks record about 0.2 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.
How do I protect my plants from frost in Knoxville?
From about Sep 28 the job here turns from planting to protecting — the last start even a 30-day crop can finish. As the season closes around the first 28°F hard freeze near Oct 28 (NOAA 1991–2020 climate normals), three moves buy time: row cover or an old sheet over tender plants on still, clear nights; water the soil the afternoon before a freeze, so it holds warmth overnight; and harvest tomatoes, peppers and basil before Oct 28. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Knoxville's scale — its 5b band and its 204-day season. Your yard writes its own version of Knoxville's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
What this page reads at Knoxville’s own scale
6 of the readings below were taken at Knoxville's own scale — its own ~204-day season between 28°F freezes, the 2,956 acres it covers and 215 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Knoxville's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Knoxville's own 28°F hard-freeze dates, about Apr 7 and Oct 28 and the ~204-day span between themNOAA 1991–2020 Climate Normals
- 215 documented contamination sites on the federal record near Knoxville, the closest Underground Storage Tanks record about 0.2 miles out from the town centreFederal environmental registers
- Knoxville's boundary and the 2,956 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~204-day seasonGrowable Ground crop-matching engine
- The ecoregion at Knoxville's own centroid, the Western Corn Belt PlainsUS EPA Level III Ecoregions
Inherited from a wider read
- Soil character — no town-scale survey exists, so Knoxville's 2,956 acres read through its county's SSURGO summaryfrom Marion County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Marion County
- The growing challenges every Iowa grower plans around — written at state scale, so they stay on the state guidefrom Iowa
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Marion County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Knoxville's 2,956 acreswhy one lot differs from the next →
- Which side of Knoxville you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 2,956 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record about 0.2 miles out, measured from the town centrehow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — measured at your address, and averaged away by any Knoxville-wide figureread your own land →
More in Iowa
Wider growing context
What grows across IowaKnoxville sits outside our named growing regions, so Iowa as a whole is its closest context here.
Nearby growing guides