Generally — Most Areas
black mustard (zones 6-10) suits most of Idaho (3b-8a) — zones 6-8 are in range, and the rest of the state is the exception.
Black Mustard in Idaho — the short answer
Black Mustard in Idaho: hardy in zones 6–10, full sun, against a state median of ~160 frost-free days.
- Plant zones
- 6-10
- Manual horticultural correction (source not retained); NC State Extension and Missouri Botanical Garden, with the remaining rows from other published sources or Growable Ground compilation
- Idaho zones
- 3b-8a
- USDA PHZM 2023
- Frost window
- Apr 26 - May 17 – Oct 1 - Oct 17
- NOAA RCC ACIS 1991-2020, 28°F
- Shortest county season
- 94 days
- NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze
- Days to maturity
- 60 days
- Growable Ground Plant Database
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS and university Extension sources — per-field provenance varies and is published with the dataset.
Your yard isn't the whole zone.
Black Mustard is grown as an annual, so your winter zone isn't the deciding factor — your frost-free window is. A typical Idaho site runs ~160 frost-free days, but that is a state median, not your yard: slope, trees, and low spots move the last-frost date across a single parcel. Enter your address and we'll score black mustard against your land's actual frost dates, sun, and soil.
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Idaho is not one climate
A single Idaho answer averages 44 counties whose growing seasons are 122 days apart — from 94 frost-free days in Valley County to 216 in Nez Perce County. Which end of that your land sits nearer changes what black mustard can finish before the first hard freeze.
The middle half of 44 counties runs 133–174 days, with a median of 159 days — a 122-day span end to end.
Measured across 44 Idaho counties. NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze. Quartiles are order statistics — every figure shown is a value a real county recorded, not an interpolation. Your own site sits somewhere inside this range; a parcel report resolves where.
Where in Idaho Fits Best
Black Mustard is rated for zones 6–10 — a cold floor USDA puts at -10 to 0°F. Against Idaho’s 44 counties that band holds in 33 of them, and the line runs at Adams County (zone 6a), the coldest county still inside it.
Inside zones 6–10
33 counties
- Boundary County · zone 6a
- Butte County · zone 6a
- Custer County · zone 6a
3 of 33 shown
Below the zone 6 floor
11 counties
- Bear Lake County · zone 5a
- Caribou County · zone 5a
- Fremont County · zone 5a
3 of 11 shown
Winter cold is the limit here — the same plant works in a container that comes under cover.
County hardiness zones: USDA Plant Hardiness Zone Map (PHZM) 2023, per county. A zone is a 10°F band of average annual extreme minimum temperature, so the floor above is the band the plant's rating denotes, not a single measured kill point. Slope, shelter and cold-air drainage routinely move a site half a zone either way from its county figure — which is the part only your address resolves.
Even within Idaho's zones 3b-8a, county microclimates differ enough to change what thrives. These counties carry the closest zone match for black mustard (USDA PHZM 2023):
Zone Comparison
Black Mustard Needs
- USDA Zones: 6-10
- Frost-Free Days: 120+
- Sun: Full Sun
- Mature size: 4 ft tall
Idaho Has
- USDA Zones: 3b-8a
- Last Hard Freeze (28°F): Apr 26 - May 17
- First Hard Freeze (28°F): Oct 1 - Oct 17
- Annual Rainfall: 8-35 inches
- Common Soils: Volcanic ash, Silt loam, Sandy loam
Plant Zone Range (zones 6-10)
Preferred Soil pH
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS, and university Extension sources — per-field provenance varies and is published with the dataset. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Black Mustard in Idaho
The frost window
Across Idaho, the last spring hard freeze (28°F) clears between Apr 26 and May 17, and the first fall hard freeze lands between Oct 1 and Oct 17 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 137-day window you can count on — up to 174 days on a mild site in a kind year. That marks the hard freeze, not the last light frost, so tender transplants usually wait another 2-3 weeks.
Days to maturity vs. the window
At 60 days to maturity (Growable Ground Plant Database), counting from the last hard freeze it ripens with 77 days to spare even in Idaho's tightest freeze scenario — room for a later start or a second sowing.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: Growable Ground Plant Database — frost-free days from FAO ECOCROP, days to harvest from university Extension production guides, and growing-degree-day and chill-hour minimums that are compilation estimates rather than published figures. Your site's own frost dates can run earlier or later than the state range — a parcel report pins them down.
Growing Season Fit
The zone check says black mustard survives winter in part of Idaho, not across all of it. Whether the season is long enough — and warm enough — is a different question.
Frost-free days
Black Mustard wants 120+ frost-free days; a typical Idaho site sees ~160 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves a workable window — start indoors to bank time.
GDD and chill-hour figures are computed from PRISM 1991-2020 monthly climate normals; frost-free days from NOAA Regional Climate Centers ACIS, 1991-2020. Each is the state median across its counties (P25/P75 span the within-state spread). Hardiness-zone range is USDA PHZM 2023.
Soil + Drainage Fit
Black Mustard likes near-neutral soil (pH 5.3-7.8). That's the common-ground band across Idaho's volcanic ash and silt loam — a soil test confirms it for your site. It will grow across that whole span, but it does best between pH 6 and 7 — worth knowing before you decide whether a Idaho soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Idaho site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Plant pH requirements: FAO ECOCROP and USDA PLANTS, with the remaining rows from other published sources or Growable Ground compilation. Drainage: FAO ECOCROP, with the remaining rows from other published sources or Growable Ground compilation. Idaho soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
What Else to Consider for Black Mustard in Idaho
Zone compatibility tells you about winter cold survival — but Idaho growers also need to think about:
Short growing season at higher elevations
Arid conditions require irrigation in most of the state
Cold winter snaps can reach -30F in mountain valleys
What to actually do about these — and the constraints that apply to every Idaho grower, not just black mustard — is covered on the Idaho growing guide.
Timing shifts within Idaho
Black Mustard needs about 120 days of frost-free season to complete its cycle. 39 of Idaho’s 44 counties give it that outright. The tightest that does is Butte County at 123 frost-free days — 3 to spare. Below that line you are transplanting or extending the season, not direct-sowing.
Across Idaho’s 44 counties the frost-free season runs from 94 days in Valley County to 216 days in Nez Perce County — a spread of 122 days, which is why a single statewide planting date misleads. The median Idaho county sees 160 frost-free days, leaving black mustard about 40 days of slack on a typical parcel — room for a second sowing where the season runs long.
County last-freeze dates and frost-free day counts: NOAA Regional Climate Centers ACIS (1991-2020), county 28°F freeze (earlier than the folk 32°F "last frost"). Black Mustard's 120-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
Pollinator + Wildlife Value
Black Mustard draws honeybees and native bees (moderate value, Growable Ground Plant Database). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, black mustard can only work that bloom inside Idaho's frost window — roughly 160 days, opening after the last 28°F freeze (Apr 26 - May 17) and closing at the first (Oct 1 - Oct 17), per NOAA Regional Climate Centers ACIS, 1991-2020.
Idaho Cooperative Extension
For Idaho-specific cultivar recommendations, planting calendars, and pest pressure for black mustard, the canonical source is University of Idaho Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Black Mustard native to Idaho?
No — the USDA PLANTS Database lists Black Mustard as introduced rather than native in the Lower 48, so it is not part of Idaho's native flora. It grows here as a garden plant; pairing it with a few Idaho natives keeps local pollinators fed too.
Looking for plants that belong here? The Idaho growing guide lists USDA-documented natives for the state.
Native-range data: USDA PLANTS Database state-distribution records, accessed 2026-07-01.
Common Questions About Growing Black Mustard in Idaho
When can I plant Black Mustard in Idaho?
Idaho's last spring hard freeze (28°F) clears between Apr 26 and May 17, and the first fall hard freeze lands between Oct 1 and Oct 17 (NOAA Regional Climate Centers ACIS, 1991-2020). Time outdoor planting to after the last hard freeze for your specific site — a further two to three weeks for anything tender — and count back from those dates for transplant scheduling.
Can Black Mustard mature before first frost in Idaho?
Yes — Black Mustard matures in 60 days (Growable Ground Plant Database), and Idaho's dependable hard-freeze-free window runs 137 days (NOAA Regional Climate Centers ACIS, 1991-2020), leaving 77 days of margin. Plant once the last hard freeze has passed — two to three weeks later for anything tender — and it ripens ahead of the first fall freeze.
What hardiness zone is Black Mustard grown in across Idaho?
Idaho spans USDA hardiness zones 3b-8a (USDA ARS PHZM 2023). Black Mustard carries a range of zones 6-10, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Idaho site have?
A typical Idaho site sees ~160 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Black Mustard needs 120+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date.
Is Black Mustard native to Idaho?
No — the USDA PLANTS Database lists Black Mustard as introduced rather than native in the Lower 48, so it is not part of Idaho's native flora. It grows here as a garden plant; pairing it with a few Idaho natives keeps local pollinators fed too.
How should I amend the soil for Black Mustard in Idaho?
Black Mustard prefers pH 5.3-7.8 and well (dry spells) drainage (FAO ECOCROP). That sits in the common-ground band across Idaho soils — a 30-minute soil test from a local Extension lab confirms it for your specific site.
Will Black Mustard actually grow on my specific land in Idaho?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores black mustard against your address's exact soil pH, drainage, sun, and frost-date data drawn from USDA SSURGO, NOAA, and PRISM — not state averages.
Check your specific parcel in Idaho
State-level data is a sketch. Your Growable Ground report scores black mustard against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Idaho black mustard averages can't show:
We read public map data around Idaho — soil, climate, flood, and parcel records. How we handle your address.
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