Sully County, in South Dakota, sits in USDA hardiness zone 4b — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
The conditions favor tomato, black hills spruce, potato, and rhubarb, 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
Sully County holds more than one microclimate.
Soils and elevations shift across Sully 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
4b
Last Hard Freeze (28°F)
Apr 17
County normal — light frosts run a few weeks later
First Hard Freeze (28°F)
Oct 24
County normal — light frosts arrive a few weeks earlier
County Area
644K acres
Hardiness Zone Range
Zone maps are averages across Sully County. Your yard's slope, trees, and frost pockets shift what actually grows — see your land's exact reading.
Soil in Sully County
Across Sully County, the ground is predominantly Mollisols, where Agar, Highmore, and Mobridge are the most extensive named soil series. The soil is generally well drained with a silt loam surface. Topsoil pH runs about 6.4–6.7, slightly acidic. Rainfall drains through hydrologic group C soils.
Soil order
Mollisols
Drainage
Well drained
Prime farmland
12%
Hydric soils
7%
Soil still varies lot by lot — soil types explained.
What Grows in Sully County
Plants matched to Sully County's USDA zones 4b — each links to its full growing profile.
Is it too late to plant in Sully County?
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 20; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 17 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 24 — long-season crops need about 90 days of runway, quick greens only 30. Late in the year the fall bench takes over — quick greens, radishes, and garlic that repays you next summer.
Growing Challenges in South Dakota
What an experienced grower plans around here — each one has a move.
Extreme cold and short growing season
Cold-proven varieties and a high tunnel turn a short prairie season into a reliable one — the northern-plains standard.
Low rainfall in western SD
West-river gardens run on drip and mulch — putting the water plan first makes the dry summers routine.
Wind exposure on the open prairie
A windbreak is the best structure you can plant on the prairie — even a shrub row shifts the microclimate.
For cultivar selection, pest pressure, and planting-time guidance specific to South Dakota, the SDSU Extension is the authoritative local source.
Safe to Grow Here?
What the federal record shows across Sully County — and how to grow with it.
We checked the federal record across Sully County — 61 documented sites across 5 of the 9 source types we track.
The most significant on record: 1 Toxics Release Inventory facility. Active industrial facilities reporting chemical releases to air, water, and land.
The federal record across Sully County is a modest one — a typical footprint for a growing area. Nothing here calls for alarm; it's worth knowing which recorded sites sit closest to where you grow, and each one on the map carries its type and location. If one turns out to be a near neighbor, a one-time soil test settles the question.
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.
Total Sites
61
across Sully County
Risk Level
Moderate
Highest-severity
1 Toxics Release Inventory facility
Sources Checked
across Sully County
Severity Distribution
across Sully County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Sully County, Nitrate runs higher than the national average — 40 sites nearby. 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.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Check your specific parcel in Sully 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
Sully County Average
- ●USDA Zones 4b
- ●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 Sully County
Pull a site-specific report for your exact address in Sully County, South Dakota — 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 Sully County, South Dakota
- USDA Hardiness Zones: 4b (USDA PHZM 2023)
- Last Hard Freeze (28°F): Apr 17 (county normal, NOAA 1991–2020 Climate Normals — light frosts can follow for a few weeks)
- First Hard Freeze (28°F): Oct 24 (county normal, NOAA 1991–2020 Climate Normals — light frosts can arrive a few weeks earlier)
- Days Between Hard Freezes: ~190 (county normal, NOAA 1991–2020 Climate Normals)
- County Land Area: 644K 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 Sully 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 Sully County, South Dakota?
Sully County sits in USDA hardiness zone 4b, 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 Sully County?
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 20; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 17 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 24 — long-season crops need about 90 days of runway, quick greens only 30. Late in the year the fall bench takes over — quick greens, radishes, and garlic that repays you next summer.
When does frost risk typically end in Sully County?
The last hard freeze (28°F) in Sully County typically lands around Apr 17, 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 Sully County?
Measured between 28°F hard freezes, Sully County sees about 190 frost-free days — roughly Apr 17 through Oct 24, 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 Sully County?
Sully County's zone 4b supports a wide range — strong performers include Tomato, Black Hills Spruce, Potato, and Rhubarb. 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 Sully County, really?
Officially, Sully County sits in USDA zone 4b (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 Sully County?
The federal record around Sully County shows 61 documented sites — 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.
Just moved to Sully County — what should I know before planting?
Start with three facts. Sully County sits in USDA zone 4b, which sets what survives winter; the last 28°F hard freeze typically clears around Apr 17, with about 190 frost-free days to work with (NOAA 1991–2020 climate normals); and 61 documented sites sit on the federal record — a typical footprint for a growing area, worth a look on the contamination map before food beds. From there, matching plants to your actual soil and sun is the fun part.
Everything on this page is a Sully 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 South Dakota's frost window, season length, and soil profile against the plant's real requirements.