Washington County, in Idaho, sits in USDA hardiness zone 6a — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
Expect cabbage, blackberry, california poppy, and collard greens to be strong candidates here; the deciding factors on any one parcel stay local — soil, sun, and drainage.
Washington County lies within 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
Washington County holds more than one microclimate.
Soils and elevations shift across Washington 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 Washington County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Washington County — the short answer
Washington County, Idaho: USDA zone 6a, about 178 days between hard freezes, across 930K acres.
- USDA zone
- 6a
- 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
- 930K 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 6a is Washington County's average, not your lot's — a low corner or a tree line moves it. Against Washington County's 178-day season, see what your own ground reads.
Heat in Washington County
Across a year, Washington County accumulates about 2,384 growing degree days above 50°F, about 893 more than the middle Idaho county — ahead of 39 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 Washington County
Across Washington County, the ground is predominantly Mollisols, where Gem, Reywat, and Bakeoven are the most extensive named soil series. The soil is generally well drained with a extremely stony loam surface. Topsoil pH runs about 6.7–7.0, neutral. Rainfall drains through hydrologic group D soils.
Soil order
Mollisols
Drainage
Well drained
Hydric soils
0%
Soil still varies lot by lot — soil types explained.
How the ground varies across Washington County
The USDA soil survey maps 94% of Washington County as well drained — but another 3% drains differently: 3% somewhat poorly drained.
Gem is the most extensive named soil at 14% of mapped acres, then Reywat (11%) and Bakeoven (10%).
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 Washington County
Each reading below is measured for Washington 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 Washington County are well drained, in hydrologic group D — 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 Washington County runs about pH 6.7–7.0, close to neutral.
This is the range most vegetables prefer — one correction you probably do not have to make, which is genuinely good news for a new bed.
Keep it there with compost rather than lime or sulfur, and only amend if your own soil test says otherwise.
USDA NRCS SSURGO
Season length
MeasuredWashington 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
Winter hardiness
MeasuredWashington County sits in USDA hardiness zone 6a.
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 Washington County is in
Washington County spans 2 EPA Level III ecoregions — Snake River Plain (54%) and Blue Mountains (46%). 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 Washington County
We scored all 1,086 plants against Washington County's own USDA zone 6a 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 Idaho profile.
Is it too late to plant in Washington County?
Almost never — the real question is what to plant next. 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. As the window narrows, the plantings just get faster — fall brassicas, then greens, then garlic to finish.
What gets applied to farmland in Washington County
USGS estimates 21,615 lb of Glyphosate on Washington County farmland in 2019, its largest by weight, ahead of 2,4-d (4,233 lb) and Chlorpyrifos (2,293 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 Washington County — and how to grow with it.
We checked the federal record across Washington County — 236 documented sites across 7 of the 9 source types we track.
The most significant on record: 2 Toxics Release Inventory facilities. Active industrial facilities reporting chemical releases to air, water, and land.
Washington County carries one of the heavier federal records we track — and that's not a verdict on your yard. Proximity to a documented site is information, not a diagnosis: nothing here says any particular parcel is affected. It does earn one concrete step — before food beds go in the ground, a professional soil test tells you exactly what you're working with, and raised beds with clean imported soil grow well almost anywhere in the meantime.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
What's on the federal record near Washington County
Closest first, across Washington County. Listing isn't leaking, and distance isn't exposure.
- Taylor CinnabarMining · 0.7 mi
- 13N 04W 13BAC1Nitrate · 1.9 mi
- Unnamed MineMining · 3.3 mi
- JOHN G PIPERUnderground Storage Tanks · 3.5 mi
- LEE WILLIAMS MEMORIAL AIRPORTUnderground Storage Tanks · 3.5 mi
- MIDVALE STATIONUnderground Storage Tanks · 3.5 mi
- 13N 03W 08CCC1Nitrate · 3.5 mi
- 13N 03W 17DBB1Nitrate · 3.7 mi
- 14N 04W 36DDC1Nitrate · 4.0 mi
- 13N 03W 10CDA1Nitrate · 5.8 mi
- 14N 03W 19DDA1Nitrate · 6.4 mi
- 12N 04W 19CACC1Nitrate · 7.1 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Washington 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 Washington County.
Total Sites
236
across Washington County
Risk Level
High
Highest-severity
2 Toxics Release Inventory facilities
Sources Checked
across Washington County
Severity Distribution
across Washington County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Washington County, two things run higher than the national average across Washington County — Mining (71 sites) and Nitrate (98 sites). It's not cause for alarm — it's worth knowing, and there's a sensible way to grow around it.
Mining: Mining sites — both historic and active — can leach heavy metals (arsenic, lead, cadmium, mercury) into soil and water for centuries after operations cease.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
Test soil for heavy metals (lead, arsenic, cadmium, mercury) — this is essential near any mining site.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Check your specific parcel in Washington 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 Washington County average
Your land is one point inside Washington County
Everything above resolves to Washington County as a whole — one PHZM band and one county frost record, read across 930K 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 Washington County Average
Washington County Average
- ●USDA Zones 6a, averaged across Washington County
- ●One soil character for all of Washington County
- ●Frost dates read at Washington County's scale: Apr 20 to Oct 15
YOUR Parcel
- ✓Your exact zone, not Washington County's 6a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Washington County
Pull a site-specific report for your exact address in Washington County, Idaho — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Washington County averages can't show:
We read public map data around Washington County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Washington County average
How we know this about Washington 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 Washington County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Washington County
When does frost risk typically end in Washington County?
The last hard freeze (28°F) in Washington 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 Washington County?
Measured between 28°F hard freezes, Washington 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 Washington County?
The federal record around Washington County runs heavier than most — 236 documented sites, mostly Nitrate entries (98), the nearest Mining record about 0.7 miles out — so test the soil before planting food in the ground, and raised beds with clean imported soil grow well in the meantime. Even here, proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what's recorded and where.
Everything above is measured at Washington County's scale — its 6a band and its 178-day season. Your yard writes its own version of Washington 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 Washington County’s own scale
8 of the readings below were taken at Washington County's own scale — its own ~178-day span between 28°F freezes, the 3 most extensive soil series mapped here and 236 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Washington County's own USDA hardiness band, zones 6aUSDA PHZM 2023
- Washington 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 Washington CountyUSDA NRCS SSURGO
- 236 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Mining record about 0.7 miles outFederal environmental registers
- Washington County's boundary and the 929,826 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 6a and a ~178-day seasonGrowable Ground crop-matching engine
- The 2 ecoregions Washington County spans, Snake River Plain the largestUS EPA Level III Ecoregions
- Which agricultural compounds go onto Washington 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 Washington 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 Washington County's 929,826 acreswhy one lot differs from the next →
- Which of those 236 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 Washington Countyread your own land →