Jefferson County, in Montana, sits in USDA hardiness zone 5a — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
Reliable performers under these conditions include cabbage, cranberry, lavender, and collard greens; what your own ground favors still comes down to its soil, sun, and drainage.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Jefferson County holds more than one microclimate.
Soils and elevations shift across Jefferson 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 Jefferson County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Jefferson County — the short answer
Jefferson County, Montana: USDA zone 5a, about 123 days between hard freezes, across 1.1M acres.
- USDA zone
- 5a
- USDA PHZM 2023
- Last hard freeze (28°F)
- May 24
- NOAA 1991–2020 Climate Normals, 28°F
- First hard freeze (28°F)
- Sep 24
- NOAA 1991–2020 Climate Normals, 28°F
- Days between freezes
- ~123
- 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 5a is Jefferson County's average, not your lot's — a low corner or a tree line moves it. Against Jefferson County's 123-day season, see what your own ground reads.
Heat in Jefferson County
Across a year, Jefferson County accumulates about 783 growing degree days above 50°F, about 887 fewer than the middle Montana county — ahead of 7 of Montana’s 56 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 Jefferson County
Across Jefferson County, the ground is predominantly Inceptisols, where Bronec, Amesha, and Sappington are the most extensive named soil series. The soil is generally well drained with a gravelly loam surface. Topsoil pH runs about 6.5–7.2, neutral. Rainfall drains through hydrologic group B soils.
Soil order
Inceptisols
Drainage
Well drained
Hydric soils
2%
Soil still varies lot by lot — soil types explained.
How the ground varies across Jefferson County
The USDA soil survey maps 57% of Jefferson County as well drained — but another 16% drains differently: 10% somewhat excessively drained, 2% excessively drained.
Bronec is the most extensive named soil at 3% of mapped acres, then Amesha (3%) and Sappington (2%).
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 Jefferson County
Each reading below is measured for Jefferson 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 Jefferson 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 Jefferson County runs about pH 6.5–7.2, 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
MeasuredJefferson County runs about 123 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
MeasuredJefferson County sits in USDA hardiness zone 5a.
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 Jefferson County is in
Jefferson County sits in the Middle Rockies — EPA Level III, inside the wider Northwestern Forested Mountains. One consistent ecological setting across the whole county.
What Grows in Jefferson County
We scored all 1,086 plants against Jefferson County's own USDA zone 5a and its own 123-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 Montana profile.
Is it too late to plant in Jefferson County?
Rarely: the season closes in stages, not all at once, and each stage has its crops. Cool-season crops can go in from around Apr 26; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near May 24 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Sep 24 — long-season crops need about 90 days of runway, quick greens only 30. Even here the calendar’s edges hold value: thirty-day greens late in the window, then garlic and a rested bed for spring.
What gets applied to farmland in Jefferson County
USGS estimates 18,392 lb of Glyphosate on Jefferson County farmland in 2019, its largest by weight, ahead of 2,4-d (12,394 lb) and Atrazine (4,030 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 Jefferson County — and how to grow with it.
We checked the federal record across Jefferson County — 923 documented sites across 6 of the 9 source types we track.
The most significant on record: 10 Superfund sites. Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
Jefferson 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 Jefferson County
Closest first, across Jefferson County. Listing isn't leaking, and distance isn't exposure.
- Dora LeighMining · 1.8 mi
- Mccauley Lead MineMining · 1.8 mi
- Boulder Mercury GroupMining · 2.1 mi
- 04N04W17AAAB01Nitrate · 3.2 mi
- 05N03W18CDCD01Nitrate · 4.0 mi
- Unknown Ag Mine On State CreekMining · 5.4 mi
- Big FourMining · 5.6 mi
- 05N03W22CAAA01Nitrate · 5.6 mi
- 05N03W22DBBB01Nitrate · 5.6 mi
- Unnamed Lead & CopperMining · 5.8 mi
- 05N04W03CCCB01Nitrate · 6.1 mi
- AjaxMining · 6.1 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Jefferson 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 Jefferson County.
Sources Checked
across Jefferson County
Severity Distribution
across Jefferson County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Jefferson County, Mining runs higher than the national average — 638 sites across Jefferson County. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
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 soil for heavy metals (lead, arsenic, cadmium, mercury) — this is essential near any mining site.
Check your specific parcel in Jefferson 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 Jefferson County average
Your land is one point inside Jefferson County
Everything above resolves to Jefferson 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 Jefferson County Average
Jefferson County Average
- ●USDA Zones 5a, averaged across Jefferson County
- ●One soil character for all of Jefferson County
- ●Frost dates read at Jefferson County's scale: May 24 to Sep 24
YOUR Parcel
- ✓Your exact zone, not Jefferson County's 5a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Jefferson County
Pull a site-specific report for your exact address in Jefferson County, Montana — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Jefferson County averages can't show:
We read public map data around Jefferson County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Jefferson County average
How we know this about Jefferson 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 Jefferson County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Jefferson County
When does frost risk typically end in Jefferson County?
The last hard freeze (28°F) in Jefferson County typically lands around May 24, 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 14.
How long is the growing season in Jefferson County?
Measured between 28°F hard freezes, Jefferson County sees about 123 frost-free days — roughly May 24 through Sep 24, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Jun 14 to Sep 10, about 88 days.
Is the soil safe to grow vegetables in Jefferson County?
The federal record around Jefferson County runs heavier than most — 923 documented sites, mostly Mining entries (638), the nearest Mining record about 1.8 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.
How do gardeners stretch the season in Jefferson County?
With about 123 frost-free days between hard freezes, Jefferson 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 151 to 179 days — and cold frames or low tunnels buy more. Starting transplants indoors ahead of the May 24 hard-freeze normal stretches the season without touching the calendar.
Everything above is measured at Jefferson County's scale — its 5a band and its 123-day season. Your yard writes its own version of Jefferson 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 Montana's frost window, season length, and soil profile against the plant's real requirements.
What this page reads at Jefferson County’s own scale
8 of the readings below were taken at Jefferson County's own scale — its own ~123-day span between 28°F freezes, the 3 most extensive soil series mapped here and 923 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Jefferson County's own USDA hardiness band, zones 5aUSDA PHZM 2023
- Jefferson County's own 28°F hard-freeze dates and its ~123-day span between themNOAA 1991–2020 Climate Normals
- The soil order, drainage, surface pH, and the 3 most extensive named soil series mapped in Jefferson CountyUSDA NRCS SSURGO
- 923 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Mining record about 1.8 miles outFederal environmental registers
- Jefferson County's boundary and the 1,060,477 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 5a and a ~123-day seasonGrowable Ground crop-matching engine
- Jefferson County sitting wholly inside the Middle RockiesUS EPA Level III Ecoregions
- Which agricultural compounds go onto Jefferson County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every Montana grower plans around — written at state scale, so they stay on the state guidefrom Montana
What this page can’t tell you
- How the season shifts across Jefferson County — the ~123 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 Jefferson County's 1,060,477 acreswhy one lot differs from the next →
- Which of those 923 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 Jefferson Countyread your own land →