What does USDA hardiness zone 6a mean in California?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 4 locations in California fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in California's zone 6a is the growing season longest?

Buck Meadows runs the longest season on this page at about 208 days; Madeline is shortest at about 94 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in California's zone 6a?

Using Bridgeport's frost dates (this page's median-season location): start seeds indoors about Mar 28 – Apr 11, then transplant outside about May 30 – Jun 6. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in California's zone 6a have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 4 locations listed (see the table above for each location's own dates).

USDA zone 6a in California

−10 to −5 °F avg. annual extreme minimum · 4 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 4 locations across California (average annual extreme minimum −10 to −5 °F): Bridgeport, CA, Buck Meadows, CA, Madeline, CA, McGee Creek, CA. Average last spring frost across this group ranges April 16 (Buck Meadows) to June 14 (Madeline), and growing seasons run 94–208 days (Madeline to Buck Meadows) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every California location's own frost dates; the printable planting calendar uses Bridgeport, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: Apr 16–Jun 14avg, 32°F
First frost range: Sep 17–Nov 12avg, 32°F
Growing season range: 94–208days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Bridgeport *	June Lake	Lee Vining	May 23	Oct 9	139
Buck Meadows	—	Hetch Hetchy	Apr 16	Nov 12	208
Madeline	—	Jess Valley	Jun 14	Sep 18	94
McGee Creek	—	Mammoth Lakes Rs	Jun 10	Sep 17	98

* Bridgeport is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Bridgeport, representative)

Computed from Bridgeport's average frost dates — the median-season location in this Californiazone 6a group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Bridgeport (California, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 28 – Apr 11	May 30 – Jun 6	Jul 29 – Aug 18	—	matures comfortably
Pepper	Very tender	Mar 14 – Mar 28	Jun 6 – Jun 13	Aug 5 – Sep 4	—	matures comfortably
Cucumber	Tender	Apr 25 – May 2	May 30 – Jun 6	Jul 19 – Aug 8	—	matures comfortably
Summer squash / zucchini	Tender	—	May 30 – Jun 6	Jul 14 – Jul 29	—	matures comfortably
Bush bean	Tender	—	May 30 – Jun 6	Jul 19 – Jul 29	Aug 10 – Aug 20	matures comfortably
Sweet corn	Tender	—	May 23 – Jun 6	Jul 22 – Aug 21	—	matures comfortably
Basil	Very tender	Apr 11 – Apr 25	May 30 – Jun 6	Jun 29 – Jul 14	—	matures comfortably
Lettuce	Half-hardy	Apr 11 – Apr 25	Apr 25 – May 9	Jun 9 – Jun 24	Jul 27 – Aug 11	matures comfortably
Pea	Hardy	—	Apr 11 – Apr 25	Jun 5 – Jun 20	Jul 17 – Aug 1	matures comfortably
Spinach	Hardy	—	Apr 11 – Apr 25	May 21 – May 31	Aug 6 – Aug 16	matures comfortably
Carrot	Half-hardy	—	May 2 – May 9	Jul 1 – Jul 21	Jul 7 – Jul 27	matures comfortably
Broccoli	Half-hardy	Mar 28 – Apr 11	Apr 25 – May 9	Jun 19 – Jul 9	Jul 12 – Aug 1	matures comfortably

Which crops fit this growing season

Using Bridgeport's own 139-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Bridgeport)

From NOAA's 1991–2020 Climate Normals at station USC00044881. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Bridgeport.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 5	Jun 19	Sep 28	Oct 12	114
32°F (freeze)	May 23	Jun 9	Oct 9	Oct 24	139
28°F	May 2	May 26	Oct 21	Nov 6	168
24°F	Apr 19	May 6	Nov 2	Nov 17	195

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 4 locations is 1,862; Bridgeport's own reading is shown below.

Annual growing degree days for Bridgeport (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,133	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,201	cool-season crops (brassicas, greens)

Hardiness zone 6a in California

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. California spans 12 hardiness zones in this dataset — see all California locations for the full range, including zones5b, 6b, 7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b, 11a.

Explore zone 6a in other states at zone 6a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6a mean in California?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 4 locations in California fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in California's zone 6a is the growing season longest?: Buck Meadows runs the longest season on this page at about 208 days; Madeline is shortest at about 94 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in California's zone 6a?: Using Bridgeport's frost dates (this page's median-season location): start seeds indoors about Mar 28 – Apr 11, then transplant outside about May 30 – Jun 6. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in California's zone 6a have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 4 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.