Plant Onions in Zone 10b

Seed-starting, transplant & harvest dates for USDA Zone 10b

Start indoors

Nov 6

Transplant

Jan 1

Harvest (~100d)

Apr 11

Zone 10b averages 350 frost-free days, running from a last spring frost around January 15 to a first fall frost around December 31. Onions needs roughly 100 days from planting out to first harvest, so it uses about 29 percent of that window. Onions is a hardy crop, which is what decides where in the window it belongs.

Because onions needs a head start, sow seed indoors around Nov 6, about 10 weeks ahead of the Zone 10b last frost. Transplants go out around Jan 1. Starting earlier than that in Zone 10b usually backfires: plants outgrow their cells and stall when they finally hit cold soil. At 100 days to harvest, a 350-day Zone 10b season has room for about 3 sowings, so stagger them rather than planting everything at once.

Planting onions for a fall harvest in Zone 10b

Yes. Plant onions between November 19 and December 10 in Zone 10b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.

Short-day onion sets planted in fall overwinter in zone 7 and warmer and bulb up months ahead of a spring planting.

Plant from

Nov 19

Plant by

Dec 10

Overwinters, harvest in spring

Spring

Fall dates are counted backward from the Zone 10b average first frost, not forward from a sowing date. See the fall planting calendar by zone for the full method.

Spring vs fall onions in Zone 10b

Planting	Sow or start	In the ground	Harvest
Spring	Nov 6	Jan 1	Apr 11
Fall	by Dec 10	Dec 10	Next spring

Last spring frost

Jan 15

First fall frost

Dec 31

Growing season

350 days

Onions in nearby zones

Onions in Zone 10a (colder) Onions in Zone 11a (warmer) Full Onions grow guide Zone 10b planting calendar

Other crops in Zone 10b

Tomatoes in Zone 10b Peppers in Zone 10b Cucumbers in Zone 10b Lettuce in Zone 10b Zucchini in Zone 10b Carrots in Zone 10b Beans (bush) in Zone 10b Peas in Zone 10b Broccoli in Zone 10b Spinach in Zone 10b Kale in Zone 10b Garlic in Zone 10b Potatoes in Zone 10b Squash (winter) in Zone 10b Corn in Zone 10b Radishes in Zone 10b Beets in Zone 10b Eggplant in Zone 10b Basil in Zone 10b Cilantro in Zone 10b Parsley in Zone 10b Pumpkins in Zone 10b Sunflowers in Zone 10b

Frequently asked questions

When should I start onions in Zone 10b?

Start onions seeds indoors around Nov 6 (Zone 10b).

Can I direct sow onions in Zone 10b?

Onions is best started indoors and transplanted in Zone 10b rather than direct sown.

Can I plant onions in the fall in Zone 10b?

Yes. Plant onions between November 19 and December 10 in Zone 10b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.

Will onions survive frost in Zone 10b?

Onions is a hardy crop and keeps going roughly 21 days past the first light frost in Zone 10b. A row cover or a straw mulch buys more.

How many plantings of onions fit in a Zone 10b season?

About 3. Zone 10b averages 350 frost-free days and onions needs roughly 100 days plus a couple of weeks of turnaround, so sowing every 57 days keeps a supply coming.