Rio Oso, California, sits in USDA hardiness zone 9b — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
Growers here do well with okra, meyer lemon, california poppy, and artichoke — with the usual caveat that any single yard's soil, sun, and drainage cast the deciding vote.
Even in Rio Oso, 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 Rio Oso 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 Rio Oso — soil, climate, flood, and parcel records. How we handle your address.
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Rio Oso — the short answer
Rio Oso, California: USDA zone 9b, about 330 days between hard freezes, across 4K acres of town.
- USDA zone
- 9b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Jan 16
- Sutter County normal — NOAA 1991–2020, nearest to this town
- First hard freeze (28°F)
- Dec 12
- Sutter County normal — NOAA 1991–2020, nearest to this town
- Days between freezes
- ~330
- Sutter County normal — NOAA 1991–2020, nearest to this town
- Town area
- 4K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. No 30-year normal resolves at this town’s own centroid, so the hard-freeze dates are Sutter County’s — the closest measured record, labelled as such rather than passed off as local.
Hardiness Zone Range
Zone 9b is Rio Oso's average, not your lot's — a low corner or a tree line moves it. Against Rio Oso's 330-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Rio Oso
Each reading below is measured for Rio Oso itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
From county recordsRio Oso runs about 330 days between hard freezes (read from Sutter County, the closest normal to Rio Oso) — close to year-round growing.
Frost is rarely the limit here; winter plantings are as real as summer ones, and the harder question is which crops shrug off the summer heat.
Grow cool-season crops through the mild winter, and choose heat-tolerant varieties for the summer beds.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredRio Oso sits in USDA hardiness zone 9b.
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 Rio Oso is in
Rio Oso sits in the Central California Valley — EPA Level III, inside the wider Mediterranean California. That is read at Rio Oso's own centroid rather than borrowed from Sutter County.
What Grows in Rio Oso
Every plant below cleared Rio Oso's own zone 9b and its 330-day season. Each card opens that plant's California profile. How we score.
Is it too late to plant in Rio Oso?
Too late for some crops, right on time for others — a growing season is a sequence, not a deadline. Cool-season crops can go in from around Jan 1; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Jan 16 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Dec 12 — long-season crops need about 90 days of runway, quick greens only 30. Here the calendar nearly circles: cool-season crops take the winter shift, and the next window is always close.
What's on the federal record near Rio Oso
Closest first, within ~10 miles of Rio Oso. Listing isn't leaking, and distance isn't exposure.
- PARAMJIT RAHUL FARMSBrownfields · 2200 ft
- 013N004E27M001MNitrate · 0.6 mi
- MICHAEL VAN DYKEBrownfields · 0.6 mi
- R & M DE VALENTINE FARMSBrownfields · 0.7 mi
- MARTIN AND SUSAN LUND FARMING COBrownfields · 0.9 mi
- MANUEL KAFKARESBrownfields · 1.0 mi
- 013N004E20B003MNitrate · 1.3 mi
- 013N004E33J001MNitrate · 1.6 mi
- 013N004E26G001MNitrate · 1.8 mi
- DE VALENTINE ORCHARDS INCBrownfields · 2.1 mi
- RECLAMATION DISTRICT 1001Brownfields · 2.1 mi
- NELSON FARMS INCBrownfields · 2.1 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Rio Oso'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 Rio Oso.
Total Sites
353
within ~10 miles of Rio Oso
Risk Level
Elevated
Highest-severity
3 Toxics Release Inventory facilities
Sources Checked
within ~10 miles of Rio Oso
Severity Distribution
within ~10 miles of Rio Oso
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Rio Oso, two things run higher than the national average within ~10 miles of Rio Oso — Nitrate (194 sites) and PFAS (5 sites). That's not a problem with your land — it's information about it.
Nitrate: Nitrate contamination primarily comes from agricultural fertilizer runoff and failing septic systems.
PFAS: PFAS (per- and polyfluoroalkyl substances) are called "forever chemicals" because they do not biodegrade.
Test well water for nitrate if you rely on a private well for irrigation (EPA standard: 10 mg/L).
Test irrigation water source — this is the primary pathway for PFAS to reach garden crops.
Check your specific parcel in Rio Oso
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 Rio Oso average
Your land is one point inside Rio Oso
Everything above resolves to Rio Oso as a whole — one PHZM band, read across 4K acres, with hard-freeze dates borrowed from Sutter County’s record because no 30-year normal resolves at this town’s own centroid. 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 town 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 Rio Oso Average
Rio Oso Average
- ●USDA Zones 9b, averaged across Rio Oso
- ●One soil character for all of Rio Oso
- ●Frost dates read at Rio Oso's scale: Jan 16 to Dec 12
YOUR Parcel
- ✓Your exact zone, not Rio Oso's 9b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Rio Oso
Pull a site-specific report for your exact address in Rio Oso, California — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Rio Oso averages can't show:
We read public map data around Rio Oso — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Rio Oso average
How we know this about Rio Oso
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this town’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Sutter County’s record, because none resolves here; 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 town grain anywhere on this page — there is no town-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 Rio Oso
When does frost risk typically end in Rio Oso?
The last hard freeze (28°F) in Rio Oso typically lands around Jan 16, 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 Feb 6.
When is the first frost in Rio Oso?
The first hard freeze (28°F) in Rio Oso typically arrives around Dec 12, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Nov 28, so the watching starts about Nov 12 — about 330 days after the spring freeze cleared.
How long is the growing season in Rio Oso?
Measured between 28°F hard freezes, Rio Oso sees about 330 frost-free days — roughly Jan 16 through Dec 12, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Feb 6 to Nov 28, about 295 days.
Is the soil safe to grow vegetables in Rio Oso?
The federal record around Rio Oso is a meaningful one — 353 documented sites, mostly Nitrate entries (194), the nearest Brownfields record about 0.4 miles out — so a soil test before new food beds is a sensible precaution here, not a reason to hold back from growing. Remember that proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what sits where.
How do I protect my plants from frost in Rio Oso?
From about Nov 12 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 Dec 12 (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 Dec 12. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Rio Oso's scale — its 9b band and its 330-day season. Your yard writes its own version of Rio Oso'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 Rio Oso’s own scale
5 of the readings below were taken at Rio Oso's own scale — the 4,160 acres it covers and 353 documented sites around it; 5 are borrowed from a wider read, named and linked below.
Measured here
- Rio Oso's own USDA hardiness band, zones 9bUSDA PHZM 2023
- 353 documented contamination sites on the federal record near Rio Oso, the closest Brownfields record about 0.4 miles out from the town centreFederal environmental registers
- Rio Oso's boundary and the 4,160 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 9b and a ~330-day seasonGrowable Ground crop-matching engine
- The ecoregion at Rio Oso's own centroid, the Central California ValleyUS EPA Level III Ecoregions
Inherited from a wider read
- The 28°F hard-freeze dates — Rio Oso has no normal of its own, so its county's is the closest measured readfrom Sutter County
- Soil character — no town-scale survey exists, so Rio Oso's 4,160 acres read through its county's SSURGO summaryfrom Sutter County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Sutter County
- The growing challenges every California grower plans around — written at state scale, so they stay on the state guidefrom California
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Sutter County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Rio Oso's 4,160 acreswhy one lot differs from the next →
- Which side of Rio Oso you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 4,160 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Brownfields record about 0.4 miles 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 Rio Oso-wide figureread your own land →