Denison, Iowa, sits in USDA hardiness zone 5a — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
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 Denison, 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 Denison 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 Denison — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Denison — the short answer
Denison, Iowa: USDA zone 5a, about 191 days between hard freezes, across 4K acres of city.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 12
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 20
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~191
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 4K 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 5a is Denison's average, not your lot's — a low corner or a tree line moves it. Against Denison's 191-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Denison
Each reading below is measured for Denison itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredDenison runs about 191 days between hard freezes — a moderate growing season.
There is room for the full run of cool-season crops plus a warm-season main crop, but the calendar is tight enough that a late spring or early fall frost still decides how much finishes.
Time warm-season transplants to your last hard freeze, and a few weeks of season extension turns a near-miss harvest into a sure one.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredDenison sits in USDA hardiness zone 5a.
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 Denison is in
Denison sits in the Western Corn Belt Plains — EPA Level III, inside the wider Great Plains. That is read at Denison's own centroid rather than borrowed from Crawford County.
What Grows in Denison
Every plant below cleared Denison's own zone 5a and its 191-day season. Each card opens that plant's Iowa profile. How we score.
Is it too late to plant in Denison?
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 15; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 12 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 20 — long-season crops need about 90 days of runway, quick greens only 30. As the window narrows, the plantings just get faster — fall brassicas, then greens, then garlic to finish.
What's on the federal record near Denison
Closest first, within ~10 miles of Denison. Listing isn't leaking, and distance isn't exposure.
- DENISON NEWSPAPERS INCBrownfields · 400 ft
- IBPUnderground Storage Tanks · 550 ft
- DENISON WASTE WATER PLANTUnderground Storage Tanks · 550 ft
- EASY STREET REPAIRUnderground Storage Tanks · 1100 ft
- D & L SERVICEUnderground Storage Tanks · 1100 ft
- PETERSEN MFG COBrownfields · 1400 ft
- G & R AUTOMOTIVE LOTUnderground Storage Tanks · 1400 ft
- AUTO SHOP PLUSUnderground Storage Tanks · 1400 ft
- CASEY''S GENERAL STORE 2489Underground Storage Tanks · 1500 ft
- RIES PROPERTYUnderground Storage Tanks · 1500 ft
- 4 M AUTO REPAIRBrownfields · 1500 ft
- CRAWFORD COUNTY SECONDARY ROAD DEPARTMENTBrownfields · 1600 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Denison'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 Denison.
Total Sites
160
within ~10 miles of Denison
Risk Level
Moderate
Highest-severity
14 Toxics Release Inventory facilities
Sources Checked
within ~10 miles of Denison
Severity Distribution
within ~10 miles of Denison
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Denison, two things run higher than the national average within ~10 miles of Denison — Nitrate (66 sites) and Toxics Release Inventory (14 sites). It's not cause for alarm — it's worth knowing, and there's a sensible way to grow around it.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
Toxics Release Inventory: TRI facilities report annual chemical releases to air, water, and land.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Check prevailing wind direction — downwind parcels face higher exposure than upwind or crosswind locations.
Check your specific parcel in Denison
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 Denison average
Your land is one point inside Denison
Everything above resolves to Denison as a whole — one PHZM band and one 28°F frost record, read across 4K 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 Denison Average
Denison Average
- ●USDA Zones 5a, averaged across Denison
- ●One soil character for all of Denison
- ●Frost dates read at Denison's scale: Apr 12 to Oct 20
YOUR Parcel
- ✓Your exact zone, not Denison's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Denison
Pull a site-specific report for your exact address in Denison, Iowa — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Denison averages can't show:
We read public map data around Denison — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Denison average
How we know this about Denison
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 Denison’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 Denison
When does frost risk typically end in Denison?
The last hard freeze (28°F) in Denison typically lands around Apr 12, 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 May 3.
When is the first frost in Denison?
The first hard freeze (28°F) in Denison typically arrives around Oct 20, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 6, so the watching starts about Sep 20 — about 191 days after the spring freeze cleared.
How long is the growing season in Denison?
Measured between 28°F hard freezes, Denison sees about 191 frost-free days — roughly Apr 12 through Oct 20, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 3 to Oct 6, about 156 days.
Is the soil safe to grow vegetables in Denison?
The federal record around Denison shows 160 documented sites, mostly Nitrate entries (66), the nearest Brownfields record under a tenth of a mile 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.
How do I protect my plants from frost in Denison?
From about Sep 20 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 20 (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 20. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Denison's scale — its 5a band and its 191-day season. Your yard writes its own version of Denison'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 Denison’s own scale
6 of the readings below were taken at Denison's own scale — its own ~191-day season between 28°F freezes, the 4,186 acres it covers and 160 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Denison's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Denison's own 28°F hard-freeze dates, about Apr 12 and Oct 20 and the ~191-day span between themNOAA 1991–2020 Climate Normals
- 160 documented contamination sites on the federal record near Denison, the closest Brownfields record under a tenth of a mile out from the town centreFederal environmental registers
- Denison's boundary and the 4,186 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~191-day seasonGrowable Ground crop-matching engine
- The ecoregion at Denison'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 Denison's 4,186 acres read through its county's SSURGO summaryfrom Crawford County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Crawford 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 Crawford County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Denison's 4,186 acreswhy one lot differs from the next →
- Which side of Denison you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 4,186 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Brownfields record under a tenth of a mile 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 Denison-wide figureread your own land →
More in Iowa
Wider growing context
What grows across IowaDenison sits outside our named growing regions, so Iowa as a whole is its closest context here.
Nearby growing guides