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, cabbage, cranberry, lavender, and collard greens all suit these conditions — on the ground, soil, sun, and drainage make the final call.
Union County lies within the Southeastern South Dakota Corn Belt (Sioux Falls / Big Sioux) — a regional growing area with its own character.
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 1,086 plants against your land's actual soil, sun, and frost.
We read public map data around Union County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Union County — the short answer
Union County, South Dakota: USDA zone 5a, about 178 days between hard freezes, across 295K acres.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 20
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 15
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~178
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 295K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are county 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 Union County's average, not your lot's — a low corner or a tree line moves it. Against Union County's 178-day season, see what your own ground reads.
Heat in Union County
Across a year, Union County accumulates about 2,706 growing degree days above 50°F, about 280 more than the middle South Dakota county — ahead of 57 of South Dakota’s 66 counties. Degree days are the heat a crop actually gets to work with — more of them is more room to ripen a long-season crop before the freeze closes the window.
Grounded in PRISM Climate Group, Oregon State University — 1991–2020 monthly normals
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.
How the ground varies across Union County
The USDA soil survey maps 54% of Union County as well drained — but another 44% drains differently: 17% poorly drained, 13% moderately well drained.
Crofton is the most extensive named soil at 13% of mapped acres, then Alcester (9%) and Moody (7%).
Which of them your own lot sits on is a parcel question, not a county one. Check your address for the survey at your boundary.
What this means for growing in Union County
Each reading below is measured for Union County itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Drainage
MeasuredThe dominant soils across Union County are well drained, in hydrologic group B — ground that drains quickly.
The upside is soil you can work early in spring and roots that never drown; the cost is that fast-draining ground dries out fast and leaches nutrients away before plants can use them.
Work in compost and mulch to hold moisture and feed, and water more often in short doses than deep and rarely.
USDA NRCS SSURGO
Surface pH
MeasuredTopsoil across Union County runs about pH 6.7–7.9, on the alkaline side.
Many vegetables cope, but acid-lovers like blueberries struggle, and iron and manganese lock up above about pH 7.5 — leaves yellow between the veins even in rich soil.
Elemental sulfur and generous organic matter lower pH slowly for the beds that need it; test first so you add only what the soil actually calls for.
USDA NRCS SSURGO
Wet ground
MeasuredAbout 25% of Union County is mapped as hydric — wetland — soil.
If your lot falls in one of those pockets, a standard in-ground bed will fight waterlogging all season, and roots that need air will struggle.
Check how your own ground drains before you dig; raised beds or moisture-loving plantings turn a wet corner from a problem into a niche.
USDA NRCS SSURGO
Season length
MeasuredUnion County runs about 178 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
The kind of country Union County is in
Union County spans 2 EPA Level III ecoregions — Western Corn Belt Plains (86%) and Northern Glaciated Plains (14%). Which one your land falls in changes what grows wild there and how the ground behaves, and a county-wide answer cannot tell you which.
What Grows in Union County
We scored all 1,086 plants against Union County's own USDA zone 5a and its own 178-day season between hard freezes, using the same matching engine that runs a full parcel report. These cleared both. Sun, soil pH and drainage vary lot by lot — those are the parcel questions below. Each card opens that plant's South Dakota 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 23; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 20 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 15 — 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.
What gets applied to farmland in Union County
USGS estimates 266,668 lb of Glyphosate on Union County farmland in 2019, its largest by weight, ahead of Acetochlor (68,104 lb) and Metolachlor & Metolachlor-s (46,170 lb).
Farmland across the whole county, not any one lot. Check your address for the site-by-site record.
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.
What's on the federal record near Union County
Closest first, across Union County. Listing isn't leaking, and distance isn't exposure.
- 92N49W17ACCCNitrate · 3.0 mi
- 92N49W 2CDDD2Nitrate · 3.1 mi
- KERVYN HEERENUnderground Storage Tanks · 4.2 mi
- 93N49W24DAAANitrate · 4.6 mi
- 92N49W28ACBNitrate · 4.8 mi
- SPINK OILUnderground Storage Tanks · 5.0 mi
- CONNIES 76 TRUCK STOPUnderground Storage Tanks · 5.5 mi
- ARDEN HANSONUnderground Storage Tanks · 5.6 mi
- 92N50W22DDDDNitrate · 6.0 mi
- 92N50W 4CCCCNitrate · 6.0 mi
- 93N50W 4DAA2Nitrate · 6.9 mi
- 91N49W 5DDCCNitrate · 7.5 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Union County'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 Union County.
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 across Union County. 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 1,086 plant recommendations.
25+ data sources read at your address, not the Union County average
Your land is one point inside Union County
Everything above resolves to Union County as a whole — one PHZM band and one county frost record, read across 295K 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 county 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 county average.
Your Lot vs the Union County Average
Union County Average
- ●USDA Zones 5a, averaged across Union County
- ●One soil character for all of Union County
- ●Frost dates read at Union County's scale: Apr 20 to Oct 15
YOUR Parcel
- ✓Your exact zone, not Union County's 5a
- ✓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 lot that Union County averages can't show:
We read public map data around Union County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Union County average
How we know this about Union County
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at county grain; hard-freeze dates from NOAA 1991–2020 28°F normals at this county’s own record where one resolves, and the labelled statewide window where none does; soils from the USDA NRCS SSURGO survey, area-weighted across the county’s own map units; boundaries and acreage from US Census TIGER/Line 2025; the contamination and pesticide records from the federal sources named in their beats above. Where Union County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Union County
When does frost risk typically end in Union County?
The last hard freeze (28°F) in Union County typically lands around Apr 20, 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 11.
How long is the growing season in Union County?
Measured between 28°F hard freezes, Union County sees about 178 frost-free days — roughly Apr 20 through Oct 15, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 11 to Oct 1, about 143 days.
Is the soil safe to grow vegetables in Union County?
The federal record around Union County is a meaningful one — 281 documented sites, mostly Nitrate entries (196), the nearest Nitrate record about 3.0 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.
Everything above is measured at Union County's scale — its 5a band and its 178-day season. Your yard writes its own version of Union County's numbers: 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.
What this page reads at Union County’s own scale
8 of the readings below were taken at Union County's own scale — its own ~178-day span between 28°F freezes, the 3 most extensive soil series mapped here and 281 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Union County's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Union County's own 28°F hard-freeze dates and its ~178-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Union CountyUSDA NRCS SSURGO
- 281 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Nitrate record about 3.0 miles outFederal environmental registers
- Union County's boundary and the 294,884 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~178-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Union County spans, Western Corn Belt Plains the largestUS EPA Level III Ecoregions
- Which agricultural compounds go onto Union County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every South Dakota grower plans around — written at state scale, so they stay on the state guidefrom South Dakota
What this page can’t tell you
- How the season shifts across Union County — the ~178 days above is one reading at its centre; the 12 towns below each carry their owntowns in this county →
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Union County's 294,884 acreswhy one lot differs from the next →
- Which of those 281 documented sites actually reaches your soil — that needs your address, the wind, and the groundwater directionhow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — none of it survives being averaged over Union Countyread your own land →