Clark County, in Idaho, sits in USDA hardiness zone 5b — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
The conditions favor cabbage, cranberry, lavender, and collard greens, among others — though every individual site edits that list with its own soil, sun, and drainage.
Clark County lies within the Eastern Idaho (Upper Snake River Valley) and the Snake River Plain — a regional growing area with its own character.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Clark County holds more than one microclimate.
Soils and elevations shift across Clark 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 Clark County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Clark County — the short answer
Clark County, Idaho: USDA zone 5b, about 142 days between hard freezes, across 1.1M acres.
- USDA zone
- 5b
- USDA PHZM 2023
- Last hard freeze (28°F)
- May 13
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Oct 2
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~142
- NOAA 1991–2020 Climate Normals, 28°F
- County area
- 1.1M 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 5b is Clark County's average, not your lot's — a low corner or a tree line moves it. Against Clark County's 142-day season, see what your own ground reads.
Heat in Clark County
Across a year, Clark County accumulates about 1,445 growing degree days above 50°F, about 46 fewer than the middle Idaho county — ahead of 20 of Idaho’s 44 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 Clark County
Across Clark County, the ground is predominantly Mollisols, where Zeebar, Simeroi, and Edgway are the most extensive named soil series. The soil is generally well drained with a gravelly loam surface. Topsoil pH runs about 6.7–7.9, neutral. Rainfall drains through hydrologic group B soils.
Soil order
Mollisols
Drainage
Well drained
Hydric soils
2%
Soil still varies lot by lot — soil types explained.
How the ground varies across Clark County
The USDA soil survey maps 90% of Clark County as well drained — but another 9% drains differently: 4% somewhat excessively drained, 2% somewhat poorly drained.
Zeebar is the most extensive named soil at 6% of mapped acres, then Simeroi (5%) and Edgway (5%).
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 Clark County
Each reading below is measured for Clark 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 Clark 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 Clark 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
Season length
MeasuredClark County runs about 142 days between hard freezes — a short growing season.
Long-maturing crops like melons, sweet potatoes, and winter squash can run out of runway before the first hard freeze, ripening late or not at all.
Lead with fast-maturing varieties, and stretch the season on both ends with cold frames, low tunnels, or a greenhouse.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredClark County 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 Clark County is in
Clark County spans 2 EPA Level III ecoregions — Middle Rockies (63%) and Snake River Plain (37%). 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 Clark County
We scored all 1,086 plants against Clark County's own USDA zone 5b and its own 142-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 Idaho profile.
Is it too late to plant in Clark County?
Too late for some crops, right on time for others — a growing season is a sequence, not a deadline. Cool-season crops can go in from around Apr 15; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near May 13 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 2 — long-season crops need about 90 days of runway, quick greens only 30. When the window is tight, the fall moves are quick ones — baby greens, radishes, and garlic set for next season.
What gets applied to farmland in Clark County
USGS estimates 8,591 lb of Glyphosate on Clark County farmland in 2019, its largest by weight, ahead of Metolachlor-s (3,340 lb) and Metolachlor & Metolachlor-s (3,340 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 Clark County — and how to grow with it.
We checked the federal record across Clark County — 121 documented sites across 6 of the 9 source types we track.
The most significant on record: 1 Superfund site (none NPL-listed). Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
There's a meaningful federal record across Clark 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 Clark County
Closest first, across Clark County. Listing isn't leaking, and distance isn't exposure.
- MEDICINE LODGE RANCH - BUFFALO RANCHCAFO · 3.0 mi
- 11N 34E 25DDA1 KILLIANNitrate · 3.5 mi
- Viola MineMining · 6.0 mi
- 10N 36E 08DBD1 HB-1Nitrate · 8.0 mi
- 11N 36E 34AAC1Nitrate · 8.4 mi
- USDA ARS ID DUBOIS SHEEP EXPERIMENT STATIONSuperfund · 8.5 mi
- USDA ARS ID DUBOIS SHEEP EXPERIMENT STATIONBrownfields · 8.5 mi
- 10N 35E 32DBA1Nitrate · 9.5 mi
- Ikes 66Underground Storage Tanks · 9.7 mi
- EUGENE R HUNTERUnderground Storage Tanks · 9.7 mi
- SPENCER SECTIONUnderground Storage Tanks · 9.8 mi
- AT&T Communications Spencer ""J""Underground Storage Tanks · 9.8 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Clark 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 Clark County.
Total Sites
121
across Clark County
Risk Level
Elevated
Highest-severity
1 Superfund site (none NPL-listed)
Sources Checked
across Clark County
Severity Distribution
across Clark County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Clark County, two things run higher than the national average across Clark County — Nitrate (78 sites) and Mining (15 sites). That's not a problem with your land — it's information about it.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
Mining: Mining sites — both historic and active — can leach heavy metals (arsenic, lead, cadmium, mercury) into soil and water for centuries after operations cease.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Test soil for heavy metals (lead, arsenic, cadmium, mercury) — this is essential near any mining site.
Check your specific parcel in Clark 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 Clark County average
Your land is one point inside Clark County
Everything above resolves to Clark County as a whole — one PHZM band and one county frost record, read across 1.1M 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 Clark County Average
Clark County Average
- ●USDA Zones 5b, averaged across Clark County
- ●One soil character for all of Clark County
- ●Frost dates read at Clark County's scale: May 13 to Oct 2
YOUR Parcel
- ✓Your exact zone, not Clark County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Clark County
Pull a site-specific report for your exact address in Clark County, Idaho — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Clark County averages can't show:
We read public map data around Clark County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Clark County average
How we know this about Clark 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 Clark County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Clark County
When does frost risk typically end in Clark County?
The last hard freeze (28°F) in Clark County typically lands around May 13, 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 Jun 3.
How long is the growing season in Clark County?
Measured between 28°F hard freezes, Clark County sees about 142 frost-free days — roughly May 13 through Oct 2, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Jun 3 to Sep 18, about 107 days.
Is the soil safe to grow vegetables in Clark County?
The federal record around Clark County is a meaningful one — 121 documented sites, mostly Nitrate entries (78), the nearest CAFO 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.
How do gardeners stretch the season in Clark County?
With about 142 frost-free days between hard freezes, Clark County rewards the classic extension moves: floating row cover buys roughly two to four extra weeks at each shoulder — on this season that is a working window nearer 170 to 198 days — and cold frames or low tunnels buy more. Starting transplants indoors ahead of the May 13 hard-freeze normal stretches the season without touching the calendar.
Everything above is measured at Clark County's scale — its 5b band and its 142-day season. Your yard writes its own version of Clark 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 Idaho's frost window, season length, and soil profile against the plant's real requirements.
What this page reads at Clark County’s own scale
8 of the readings below were taken at Clark County's own scale — its own ~142-day span between 28°F freezes, the 3 most extensive soil series mapped here and 121 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Clark County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Clark County's own 28°F hard-freeze dates and its ~142-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Clark CountyUSDA NRCS SSURGO
- 121 documented contamination sites inside the county boundary, distance-sorted from its center — the closest CAFO record about 3.0 miles outFederal environmental registers
- Clark County's boundary and the 1,128,412 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5b and a ~142-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Clark County spans, Middle Rockies the largestUS EPA Level III Ecoregions
- Which agricultural compounds go onto Clark County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every Idaho grower plans around — written at state scale, so they stay on the state guidefrom Idaho
What this page can’t tell you
- How the season shifts across Clark County — the ~142 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 Clark County's 1,128,412 acreswhy one lot differs from the next →
- Which of those 121 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 Clark Countyread your own land →