Boulder, Colorado, sits in USDA hardiness zone 6a — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
Among the crops suited to this profile: cabbage, hazelnut, lavender, and collard greens. The site-level story — soil, sun, drainage — decides the rest.
Even in Boulder, no two yards are alike.
A low spot, a south-facing slope, or a stand of trees moves the frost date and sun across a single Boulder lot. 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 Boulder — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Boulder — the short answer
Boulder, Colorado: USDA zone 6a, about 184 days between hard freezes, across 17K acres of city.
- USDA zone
- 6a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 18
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 19
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~184
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 17K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are this town’s own 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 Boulder's average, not your lot's — a low corner or a tree line moves it. Against Boulder's 184-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Boulder
Each reading below is measured for Boulder itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredBoulder runs about 184 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
MeasuredBoulder 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 Boulder is in
Boulder sits in the High Plains — EPA Level III, inside the wider Great Plains. That is read at Boulder's own centroid, and it is not what its county reads: Boulder County is mostly Southern Rockies. The wider page above you is answering a different question about different ground.
What Grows in Boulder
Every plant below cleared Boulder's own zone 6a and its 184-day season. Each card opens that plant's Colorado profile. How we score.
Is it too late to plant in Boulder?
Usually not — gardeners here simply switch what goes in the ground as the season moves. Cool-season crops can go in from around Mar 21; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 18 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 19 — long-season crops need about 90 days of runway, quick greens only 30. The tail of the season still works: sixty-day crops into late summer, quick greens after, garlic last of all.
What's on the federal record near Boulder
Closest first, within ~10 miles of Boulder. Listing isn't leaking, and distance isn't exposure.
- National Construction CoUnderground Storage Tanks · 400 ft
- Conoco Bulk PlantUnderground Storage Tanks · 450 ft
- Pollard Friendly MotorsUnderground Storage Tanks · 550 ft
- SHERWIN WILLIAMS AUTOMOTIVE FINISHESBrownfields · 550 ft
- COAL CREEK - ABC AUTOBODY BOULDERBrownfields · 550 ft
- GEBHARDT BUICK & GMCBrownfields · 650 ft
- GLACIER PARK COMPANYBrownfields · 750 ft
- Boulder Valley Dodge And ChrystlerUnderground Storage Tanks · 850 ft
- Bartkus Oil CompanyUnderground Storage Tanks · 850 ft
- CIRCULAR TECHNOLOGIESBrownfields · 900 ft
- Hollister DodgeUnderground Storage Tanks · 1000 ft
- Mcpherson Trucking CoUnderground Storage Tanks · 1100 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Boulder'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 Boulder.
Sources Checked
within ~10 miles of Boulder
Severity Distribution
within ~10 miles of Boulder
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Boulder, two things run higher than the national average within ~10 miles of Boulder — Superfund (13 sites) and Mining (52 sites). It's not cause for alarm — it's worth knowing, and there's a sensible way to grow around it.
Superfund: Superfund sites represent the most severe contamination in the federal system.
Mining: Mining sites — both historic and active — can leach heavy metals (arsenic, lead, cadmium, mercury) into soil and water for centuries after operations cease.
Commission professional soil testing before any food production (test for heavy metals, VOCs, and SVOCs).
Test soil for heavy metals (lead, arsenic, cadmium, mercury) — this is essential near any mining site.
Check your specific parcel in Boulder
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 Boulder average
Your land is one point inside Boulder
Everything above resolves to Boulder as a whole — one PHZM band and one 28°F frost record, read across 17K 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 city 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 local average.
Your Lot vs the Boulder Average
Boulder Average
- ●USDA Zones 6a, averaged across Boulder
- ●One soil character for all of Boulder
- ●Frost dates read at Boulder's scale: Apr 18 to Oct 19
YOUR Parcel
- ✓Your exact zone, not Boulder's 6a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Boulder
Pull a site-specific report for your exact address in Boulder, Colorado — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Boulder averages can't show:
We read public map data around Boulder — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Boulder average
How we know this about Boulder
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this city’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Boulder’s own record; boundaries and acreage from US Census TIGER/Line 2025; the contamination records from the federal sources named in their beats above. Soil is not read at city grain anywhere on this page — there is no city-resolution survey in our data, which is why you will find no soil grid here rather than a statewide one wearing a local label. The resolution ledger below names every figure this page inherited.
Common questions about Boulder
When does frost risk typically end in Boulder?
The last hard freeze (28°F) in Boulder typically lands around Apr 18, 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 9.
When is the first frost in Boulder?
The first hard freeze (28°F) in Boulder typically arrives around Oct 19, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 5, so the watching starts about Sep 19 — about 184 days after the spring freeze cleared.
How long is the growing season in Boulder?
Measured between 28°F hard freezes, Boulder sees about 184 frost-free days — roughly Apr 18 through Oct 19, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 9 to Oct 5, about 149 days.
Is the soil safe to grow vegetables in Boulder?
The federal record around Boulder runs heavier than most — 821 documented sites, mostly Brownfields entries (289), the nearest Underground Storage Tanks record under a tenth of a mile 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 I protect my plants from frost in Boulder?
From about Sep 19 the job here turns from planting to protecting — the last start even a 30-day crop can finish. As the season closes around the first 28°F hard freeze near Oct 19 (NOAA 1991–2020 climate normals), three moves buy time: row cover or an old sheet over tender plants on still, clear nights; water the soil the afternoon before a freeze, so it holds warmth overnight; and harvest tomatoes, peppers and basil before Oct 19. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Boulder's scale — its 6a band and its 184-day season. Your yard writes its own version of Boulder's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
What this page reads at Boulder’s own scale
6 of the readings below were taken at Boulder's own scale — its own ~184-day season between 28°F freezes, the 17,202 acres it covers and 821 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Boulder's own USDA hardiness band, zones 6aUSDA PHZM 2023
- Boulder's own 28°F hard-freeze dates, about Apr 18 and Oct 19 and the ~184-day span between themNOAA 1991–2020 Climate Normals
- 821 documented contamination sites on the federal record near Boulder, the closest Underground Storage Tanks record under a tenth of a mile out from the town centreFederal environmental registers
- Boulder's boundary and the 17,202 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 6a and a ~184-day seasonGrowable Ground crop-matching engine
- Boulder sits in the High Plains — not the ecoregion its county mostly readsUS EPA Level III Ecoregions
Inherited from a wider read
- Soil character — no town-scale survey exists, so Boulder's 17,202 acres read through its county's SSURGO summaryfrom Boulder County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Boulder County
- The growing challenges every Colorado grower plans around — written at state scale, so they stay on the state guidefrom Colorado
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Boulder County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Boulder's 17,202 acreswhy one lot differs from the next →
- Which side of Boulder you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 17,202 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record under a tenth of a mile out, measured from the town centrehow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — measured at your address, and averaged away by any Boulder-wide figureread your own land →
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