When to plant in Union City, CA
USDA Zone 9bHere are the average frost dates, USDA hardiness zone, and a month-by-month planting calendar for Union City, California — all computed from Union City's nearest NOAA weather station.
Union City enjoys a long ~349-day frost-free season — you can succession-sow, fit in a second crop, and grow long-season heat-lovers with room to spare. Zone 9b is warm enough that Union City can grow subtropical perennials, and the short (or absent) frost period barely limits the annual calendar.
The average first fall frost in Union City is now 7 days earlier than in the 1981–2010 normals. See how frost dates are shifting nationwide →
Frost probability
FREMONT · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Union City’s own odds, recorded at FREMONT.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Mar 11 | Feb 6 | Jan 3 | Nov 17 | Dec 5 | Jan 2 |
| 32°F | Feb 7 | Jan 7 | Dec 14 | Dec 2 | Dec 22 | Jan 21 |
| 28°F | Jan 20 | Dec 31 | Dec 13 | Dec 11 | Dec 29 | Jan 17 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Nothing new to sow or transplant outdoors in the next few weeks — a seasonal lull. Check the full-year calendar below for the next window.
Full-year planting calendar
Each bar is the exact window to take a planting action in Union City, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 10 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Union City planting FAQ
When is the last spring frost in Union City, CA?
Union City's average last spring frost falls near January 7 — the 50% mark at 32°F in the 1991–2020 normals. Hold tender transplants until the risk has passed, then plant out.
When is the first fall frost in Union City, CA?
Expect Union City's first fall frost near December 22 — a 50% chance of 32°F by that date. Bring in or cover tender crops ahead of it.
What hardiness zone is Union City in?
Union City is in USDA hardiness zone 9b. In zone 9b, winters are mild — many tender perennials overwinter here.
How long is the growing season in Union City?
There are roughly 349 frost-free days in Union City (a long growing season), running from the average last frost around January 7 to the first fall frost near December 22.
When should I plant tomatoes in Union City?
In Union City, start tomato seeds indoors around November 12–November 26, then transplant seedlings outdoors around January 14 once the danger of frost has passed.
Never miss a window in Union City
An email when it’s time to start seeds, transplant, and sow — timed to Union City’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Hayward · 8 km
- Fairview · 9 km
- Newark · 11 km
- Cherryland · 11 km
- Castro Valley · 12 km
- San Lorenzo · 13 km
- Ashland · 13 km
- Fremont · 14 km
Frost dates recorded at FREMONT, 7 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Union City, CA — Frost Dates & Zone 9b." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00043244. Retrieved from https://blissgarden.com/california/union-city.