Boulder County, in Colorado, sits in USDA hardiness zone 5b — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
On paper, cabbage, cranberry, lavender, and collard greens all suit these conditions — on the ground, soil, sun, and drainage make the final call.
Boulder County lies within the Front Range — a regional growing area with its own character.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring · NOAA 1991–2020 Climate Normals
Boulder County holds more than one microclimate.
Soils and elevations shift across Boulder 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 Boulder County — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Boulder County — the short answer
Boulder County, Colorado: USDA zone 5b, about 142 days between hard freezes, across 465K 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
- 465K 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 Boulder County's average, not your lot's — a low corner or a tree line moves it. Against Boulder County's 142-day season, see what your own ground reads.
One county, several growing seasons
Boulder County's hard-freeze record runs about 142 days between freezes — but that is the county-wide average, and several towns inside it carry a markedly different season in their own 30-year record: Louisville at about 186 days, Superior at 186, Eldorado Springs at 184, Gunbarrel at 180 and Niwot at 180 (against the county's ~142).
Those are measured records, not estimates. If you garden in one of these pockets, plant against your own town's season rather than the county's ~142-day average.
NOAA Regional Climate Centers ACIS (1991–2020), 30-year daily minimum record. Only held-out towns carrying their own record are named — a town with no record of its own is absent rather than estimated, and a county with nothing qualifying shows no spread at all.
Heat in Boulder County
Across a year, Boulder County accumulates about 1,128 growing degree days above 50°F, about 704 fewer than the middle Colorado county — ahead of 19 of Colorado’s 64 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 Boulder County
Across Boulder County, the ground is predominantly Mollisols, where Nunn, Ascalon, and Valmont are the most extensive named soil series. The soil is generally somewhat excessively drained with a very gravelly sandy loam surface. Topsoil pH runs about 6.2–7.0, neutral. Rainfall drains through hydrologic group D soils.
Soil order
Mollisols
Drainage
Somewhat excessively drained
Hydric soils
3%
Soil still varies lot by lot — soil types explained.
How the ground varies across Boulder County
The USDA soil survey maps 43% of Boulder County as somewhat excessively drained — but another 50% drains differently: 40% well drained, 7% poorly drained.
Nunn is the most extensive named soil at 8% of mapped acres, then Ascalon (6%) and Valmont (4%).
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 Boulder County
Each reading below is measured for Boulder 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 Boulder County are somewhat excessively 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 Boulder County runs about pH 6.2–7.0, 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
MeasuredBoulder 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
MeasuredBoulder 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 Boulder County is in
Boulder County spans 2 EPA Level III ecoregions — Southern Rockies (65%) and High Plains (35%). 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 Boulder County
We scored all 1,086 plants against Boulder 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 Colorado profile.
Is it too late to plant in Boulder County?
For most of the year, no — what changes is which crops still fit the days remaining. 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.
Documented around Boulder County
- Japanese beetle53 documented records
- monarch butterfly66 documented records
Sighting records, so they follow observers as much as insects.
What gets applied to farmland in Boulder County
USGS estimates 8,438 lb of Glyphosate on Boulder County farmland in 2019, its largest by weight, ahead of 2,4-d (3,694 lb) and Atrazine (1,993 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 Boulder County — and how to grow with it.
We checked the federal record across Boulder County — 1,249 documented sites across 7 of the 9 source types we track.
The most significant on record: 15 Superfund sites. Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
Boulder 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 Boulder County
Closest first, across Boulder County. Listing isn't leaking, and distance isn't exposure.
- Nugget GroupMining · 1.1 mi
- Humboldt, Sparten, SplendorMining · 1.4 mi
- Beryl No 1 ProspectMining · 1.6 mi
- Beryl #1Mining · 1.6 mi
- Buena MineMining · 1.7 mi
- BuenaMining · 1.7 mi
- SB00207224DACA1 SITE C05Nitrate · 1.7 mi
- Last ChanceMining · 1.8 mi
- Beryl #2Mining · 1.8 mi
- Beryl No.1 & 2 ClaimsMining · 1.8 mi
- Alice MineMining · 1.9 mi
- BuckhornMining · 1.9 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Boulder 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 Boulder County.
Sources Checked
across Boulder County
Severity Distribution
across Boulder County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Boulder County, Mining runs higher than the national average — 123 sites across Boulder County. That's not a problem with your land — it's information about 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.
Test soil for heavy metals (lead, arsenic, cadmium, mercury) — this is essential near any mining site.
Check your specific parcel in Boulder 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 Boulder County average
Your land is one point inside Boulder County
Everything above resolves to Boulder County as a whole — a county whose own towns’ 30-year records run 180 days between freezes in Gunbarrel to 186 in Superior. Your parcel sits at one point in that range, not in the middle of it. 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 Boulder County Average
Boulder County Average
- ●USDA Zones 5b, averaged across Boulder County
- ●One soil character for all of Boulder County
- ●Frost dates read at Boulder County's scale: May 13 to Oct 2
YOUR Parcel
- ✓Your exact zone, not Boulder County's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Boulder County
Pull a site-specific report for your exact address in Boulder County, Colorado — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Boulder County averages can't show:
We read public map data around Boulder County — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Boulder County average
How we know this about Boulder 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 Boulder County carries no record of its own, the figure is inherited and labelled — the resolution ledger below names each.
Common questions about Boulder County
When does frost risk typically end in Boulder County?
The last hard freeze (28°F) in Boulder 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 Boulder County?
Measured between 28°F hard freezes, Boulder 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 Boulder County?
The federal record around Boulder County runs heavier than most — 1,249 documented sites, mostly Underground Storage Tanks entries (417), the nearest Mining record about 1.1 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 Boulder County?
With about 142 frost-free days between hard freezes, Boulder 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 Boulder County's scale — its 5b band and its 142-day season. Your yard writes its own version of Boulder 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 Colorado's frost window, season length, and soil profile against the plant's real requirements.
What this page reads at Boulder County’s own scale
9 of the readings below were taken at Boulder County's own scale — its own ~142-day span between 28°F freezes, the 3 most extensive soil series mapped here and 1,249 documented sites inside its boundary; one is borrowed from a wider read, named and linked below.
Measured here
- Boulder County's own USDA hardiness band, zones 5bUSDA PHZM 2023
- Boulder 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 Boulder CountyUSDA NRCS SSURGO
- 1,249 documented contamination sites inside the county boundary, distance-sorted from its center — the closest Mining record about 1.1 miles outFederal environmental registers
- Boulder County's boundary and the 464,906 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 Boulder County spans, Southern Rockies the largestUS EPA Level III Ecoregions
- Open occurrence records documented in Boulder County for 2 tracked speciesGBIF derived datasets (CC0 / CC BY 4.0)
- Which agricultural compounds go onto Boulder County farmland, and roughly how muchUSGS NAWQA EPest county estimates
Inherited from a wider read
- The growing challenges every Colorado grower plans around — written at state scale, so they stay on the state guidefrom Colorado
What this page can’t tell you
- How the season shifts across Boulder 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 Boulder County's 464,906 acreswhy one lot differs from the next →
- Which of those 1,249 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 Boulder Countyread your own land →