š§ When to Plant Onion in USDA Zone 10b (2026 Guide)
Long season. Start from seed indoors very early or use sets. Here is the cool-season planting schedule for onion in zone 10b (covering areas like Key West FL, coastal So-Cal). All dates are based on 2026 frost data for your zone.
Start Seeds Indoors
Nov 6
Transplant
Dec 18
Harvest From
Mar 28
Because onion is a cool-season crop, it tolerates light frost and can go out around Dec 18, before zone 10b's average last frost on Jan 15.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Zone 10b frost: Jan 15 ā Dec 31
- Season length: ~350 days
- Companions: carrot, lettuce, beets
Growing onion in zone 10b
Zone 10b has an average last spring frost around Jan 15 and a first fall frost around Dec 31, giving roughly 350 frost-free growing days. That window comfortably fits onion's 100-day cycle, with room for a second succession planting if you time it right.
Get a head start by sowing seeds indoors around Nov 6, roughly 10 weeks before your last frost. Move the seedlings outside around Dec 18. Give each plant about 4 inches of space in full sun.
Zone 10b specific growing notes
In zone 10b (Key West FL, coastal So-Cal), your 350-day growing season means onion should be one of your first spring plantings. Cool-season crops thrive in the mild part of your growing window. Monitor soil temperature ā onion germinates best when soil is 40-75Ā°F.
Your first fall frost around Dec 31 means your last possible harvest is roughly late Dec. For a continuous supply, consider succession planting every 2-3 weeks from Dec 18 through mid-season.
Pro tip for onion
Day-length determines bulb formationāplant long-day varieties north of 37Ā°N latitude and short-day varieties south of it. Using the wrong type means you will get scallions, not bulbs.
Troubleshooting onion
Onion maggots tunnel into bulbs from below causing rot. The adult fly lays eggs at the base of seedlings. Rotate allium family crops (onions, garlic, leeks) on a 3-year cycle to break the pest cycle.
Growing onion in containers
Grow 4-6 bulbs per 5-gallon bucket, spaced evenly. Green onions (scallions) are even easierāplant sets 1 inch apart in any 6-inch deep container and harvest continuously.
Can you plant onion twice a year?
Generally noāonions are a single long-season crop taking 100-120 days. However, you can interplant fast crops like radishes or lettuce between young onion rows before they fill out.
Frequently asked questions
When should I plant onion in zone 10b?
Transplant around Dec 18. If starting from seed indoors, sow around Nov 6. Because onion is a cool-season crop, it tolerates light frost and can go out around Dec 18, before zone 10b's average last frost on Jan 15.
How long does onion take to grow in zone 10b?
About 100 days from transplanting to first harvest. A Dec 18 planting in zone 10b is typically ready to pick from around Mar 28. Zone 10b has about 350 frost-free days ā plenty of time for a second succession planting.
How far apart should I space onion?
Space onion about 4 inches apart in full sun. This gives each plant enough room for roots to spread and leaves to catch sunlight without competing.
What is the best tip for growing onion?
Day-length determines bulb formationāplant long-day varieties north of 37Ā°N latitude and short-day varieties south of it. Using the wrong type means you will get scallions, not bulbs.
What are common problems with onion and how do I fix them?
Onion maggots tunnel into bulbs from below causing rot. The adult fly lays eggs at the base of seedlings. Rotate allium family crops (onions, garlic, leeks) on a 3-year cycle to break the pest cycle.
Can I grow onion in a container?
Grow 4-6 bulbs per 5-gallon bucket, spaced evenly. Green onions (scallions) are even easierāplant sets 1 inch apart in any 6-inch deep container and harvest continuously.
Can I plant a second crop of onion for fall harvest?
Generally noāonions are a single long-season crop taking 100-120 days. However, you can interplant fast crops like radishes or lettuce between young onion rows before they fill out.
What grows well next to onion?
Good companion plants for onion are carrot, lettuce, beets. Avoid planting near warm-season crops that compete for space and nutrients. Onion also benefits from crop rotation ā do not plant it in the same spot year after year.