Union County, in South Dakota, sits in USDA hardiness zone 5a — a band that supports both cool-season staples and warm-season crops chosen to fit the local frost window.
On paper, tomato, black hills spruce, potato, and rhubarb all suit these conditions — on the ground, soil, sun, and drainage make the final call.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Union County holds more than one microclimate.
Soils and elevations shift across Union 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
5a
Last Hard Freeze (28°F)
Apr 4
County normal — light frosts run a few weeks later
First Hard Freeze (28°F)
Nov 1
County normal — light frosts arrive a few weeks earlier
County Area
295K acres
Hardiness Zone Range
Zone maps are averages across Union County. Your yard's slope, trees, and frost pockets shift what actually grows — see your land's exact reading.
Soil in Union County
Across Union County, the ground is predominantly Mollisols, where Crofton, Alcester, and Moody are the most extensive named soil series. The soil is generally well drained with a silty clay loam surface. Topsoil pH runs about 6.7–7.9, neutral. Rainfall drains through hydrologic group B soils.
Soil order
Mollisols
Drainage
Well drained
Prime farmland
44%
Hydric soils
25%
Soil still varies lot by lot — soil types explained.
What Grows in Union County
Plants matched to Union County's USDA zones 5a — each links to its full growing profile.
Is it too late to plant in Union County?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Mar 7; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 4 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 1 — long-season crops need about 90 days of runway, quick greens only 30. There is slack in a calendar like this — late plantings, second rounds of favorites, and a fall bench that keeps beds working.
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 Union County — and how to grow with it.
We checked the federal record across Union County — 281 documented sites across 4 of the 9 source types we track.
The most significant on record: 6 Toxics Release Inventory facilities. Active industrial facilities reporting chemical releases to air, water, and land.
There's a meaningful federal record across Union County — worth a look before you plant food, not a reason to hold back from growing. Proximity to a documented site is information, not a diagnosis of any one yard. A soil test before new food beds is the sensible precaution here, and the map shows exactly which sites sit where, so you can see what's actually near you.
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
281
across Union County
Risk Level
Elevated
Highest-severity
6 Toxics Release Inventory facilities
Sources Checked
across Union County
Severity Distribution
across Union County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Union County, Nitrate runs higher than the national average — 196 sites nearby. 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.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Check your specific parcel in Union 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
Union County Average
- ●USDA Zones 5a
- ●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 Union County
Pull a site-specific report for your exact address in Union 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 Union County, South Dakota
- USDA Hardiness Zones: 5a (USDA PHZM 2023)
- Last Hard Freeze (28°F): Apr 4 (county normal, NOAA 1991–2020 Climate Normals — light frosts can follow for a few weeks)
- First Hard Freeze (28°F): Nov 1 (county normal, NOAA 1991–2020 Climate Normals — light frosts can arrive a few weeks earlier)
- Days Between Hard Freezes: ~211 (county normal, NOAA 1991–2020 Climate Normals)
- County Land Area: 295K 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 Union 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 Union County, South Dakota?
Union County sits in USDA hardiness zone 5a, 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 Union County?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Mar 7; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 4 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Nov 1 — long-season crops need about 90 days of runway, quick greens only 30. There is slack in a calendar like this — late plantings, second rounds of favorites, and a fall bench that keeps beds working.
When does frost risk typically end in Union County?
The last hard freeze (28°F) in Union County typically lands around Apr 4, 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 Union County?
Measured between 28°F hard freezes, Union County sees about 211 frost-free days — roughly Apr 4 through Nov 1, 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 Union County?
Union County's zone 5a 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 Union County, really?
Officially, Union County sits in USDA zone 5a (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 Union County?
The federal record around Union County is a meaningful one — 281 documented sites — 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.
Just moved to Union County — what should I know before planting?
Start with three facts. Union County sits in USDA zone 5a, which sets what survives winter; the last 28°F hard freeze typically clears around Apr 4, with about 211 frost-free days to work with (NOAA 1991–2020 climate normals); and 281 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 Union 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.