Alto, California, sits in USDA hardiness zone 10a — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
Well-matched crops include okra, meyer lemon, california poppy, and artichoke, and the gap between "grows in the area" and "grows in your yard" is closed by soil, sun, and drainage.
Even in Alto, 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 Alto 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 Alto — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Alto — the short answer
Alto, California: USDA zone 10a, about 336 days between hard freezes, across 81 acres of town.
- USDA zone
- 10a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Jan 9
- Marin County normal — NOAA 1991–2020, nearest to this town
- First hard freeze (28°F)
- Dec 11
- Marin County normal — NOAA 1991–2020, nearest to this town
- Days between freezes
- ~336
- Marin County normal — NOAA 1991–2020, nearest to this town
- Town area
- 81 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 Marin County’s — the closest measured record, labelled as such rather than passed off as local.
Hardiness Zone Range
Zone 10a is Alto's average, not your lot's — a low corner or a tree line moves it. Against Alto's 336-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Alto
Each reading below is measured for Alto itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
From county recordsAlto runs about 336 days between hard freezes (read from Marin County, the closest normal to Alto) — 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
MeasuredAlto sits in USDA hardiness zone 10a.
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 Alto is in
Alto sits in the Central California Foothills and Coastal Mountains — EPA Level III, inside the wider Mediterranean California. That is read at Alto's own centroid rather than borrowed from Marin County.
What Grows in Alto
Every plant below cleared Alto's own zone 10a and its 336-day season. Each card opens that plant's California profile. How we score.
Is it too late to plant in Alto?
Almost never — the real question is what to plant next. 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 9 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Dec 11 — long-season crops need about 90 days of runway, quick greens only 30. In a climate this gentle, “too late” hardly applies — the question becomes which crops prefer the cooler months ahead.
Documented around Alto
Spotted lanternfly documented 19 miles from Alto, first recorded 2023. What to watch for.
What's on the federal record near Alto
Closest first, within ~10 miles of Alto. Listing isn't leaking, and distance isn't exposure.
- COONEY FAMILY TRUSTBrownfields · 1100 ft
- SALHI FAMILY HOLDINGS INC DBA MILL VALLEY CHEVRONBrownfields · 1400 ft
- Grand Mill ValleyUnderground Storage Tanks · 1400 ft
- SALLY MAHEBrownfields · 1500 ft
- EXPRESS 1 HOUR PHOTOBrownfields · 1500 ft
- HOUSE PROPERTIESBrownfields · 1600 ft
- GUION CHILDRESSBrownfields · 1800 ft
- CVS PHARMACY # 9913Brownfields · 1900 ft
- THOMAS WINSLOWBrownfields · 1900 ft
- CLEAN LOOKBrownfields · 2100 ft
- MILL VALLEY IMPORTSBrownfields · 2200 ft
- SONNEN PORSCHEBrownfields · 2200 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Alto'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 Alto.
Sources Checked
within ~10 miles of Alto
Severity Distribution
within ~10 miles of Alto
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Alto, Brownfields runs higher than the national average — 2,881 sites within ~10 miles of Alto. It's not cause for alarm — it's worth knowing, and there's a sensible way to grow around it.
Brownfields: Brownfield sites are former commercial or industrial properties where legacy soil contamination (heavy metals, PAHs, petroleum compounds) may persist.
Check EPA brownfield remediation status — many sites have completed cleanup with institutional controls.
Check your specific parcel in Alto
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 Alto average
Your land is one point inside Alto
Everything above resolves to Alto as a whole — one PHZM band, read across 81 acres, with hard-freeze dates borrowed from Marin 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 Alto Average
Alto Average
- ●USDA Zones 10a, averaged across Alto
- ●One soil character for all of Alto
- ●Frost dates read at Alto's scale: Jan 9 to Dec 11
YOUR Parcel
- ✓Your exact zone, not Alto's 10a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Alto
Pull a site-specific report for your exact address in Alto, California — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Alto averages can't show:
We read public map data around Alto — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Alto average
How we know this about Alto
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 Marin 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 Alto
When does frost risk typically end in Alto?
The last hard freeze (28°F) in Alto typically lands around Jan 9, 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 Jan 30.
When is the first frost in Alto?
The first hard freeze (28°F) in Alto typically arrives around Dec 11, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Nov 27, so the watching starts about Nov 11 — about 336 days after the spring freeze cleared.
How long is the growing season in Alto?
Measured between 28°F hard freezes, Alto sees about 336 frost-free days — roughly Jan 9 through Dec 11, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Jan 30 to Nov 27, about 301 days.
Is the soil safe to grow vegetables in Alto?
The federal record around Alto is a meaningful one — 3,118 documented sites, mostly Brownfields entries (2,881), the nearest Brownfields record about 0.2 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 Alto?
From about Nov 11 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 11 (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 11. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Alto's scale — its 10a band and its 336-day season. Your yard writes its own version of Alto'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 Alto’s own scale
6 of the readings below were taken at Alto's own scale — the 81 acres it covers and 3,118 documented sites around it; 5 are borrowed from a wider read, named and linked below.
Measured here
- Alto's own USDA hardiness band, zones 10aUSDA PHZM 2023
- 3,118 documented contamination sites on the federal record near Alto, the closest Brownfields record about 0.2 miles out from the town centreFederal environmental registers
- Alto's boundary and the 81 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 10a and a ~336-day seasonGrowable Ground crop-matching engine
- The ecoregion at Alto's own centroid, the Central California Foothills and Coastal MountainsUS EPA Level III Ecoregions
- A documented spotted-lanternfly record within reach of Altolydemapr (De Bona & Helmus), 10 km survey lattice
Inherited from a wider read
- The 28°F hard-freeze dates — Alto has no normal of its own, so its county's is the closest measured readfrom Marin County
- Soil character — no town-scale survey exists, so Alto's 81 acres read through its county's SSURGO summaryfrom Marin County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Marin 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 Marin County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Alto's 81 acreswhy one lot differs from the next →
- Which side of Alto you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 81 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Brownfields record about 0.2 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 Alto-wide figureread your own land →