š§ When to Plant Onion in USDA Zone 10a (2026 Guide)
Long season. Start from seed indoors very early or use sets. Here is the cool-season planting schedule for onion in zone 10a (covering areas like Miami FL, Phoenix AZ). All dates are based on 2026 frost data for your zone.
Start Seeds Indoors
Nov 22
Transplant
Jan 3
Harvest From
Apr 13
Because onion is a cool-season crop, it tolerates light frost and can go out around Jan 3, before zone 10a's average last frost on Jan 31.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Zone 10a frost: Jan 31 ā Dec 30
- Season length: ~333 days
- Companions: carrot, lettuce, beets
Growing onion in zone 10a
Zone 10a has an average last spring frost around Jan 31 and a first fall frost around Dec 30, giving roughly 333 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 22, roughly 10 weeks before your last frost. Move the seedlings outside around Jan 3. Give each plant about 4 inches of space in full sun.
Zone 10a specific growing notes
In zone 10a (Miami FL, Phoenix AZ), your 333-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 30 means your last possible harvest is roughly late Dec. For a continuous supply, consider succession planting every 2-3 weeks from Jan 3 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 10a?
Transplant around Jan 3. If starting from seed indoors, sow around Nov 22. Because onion is a cool-season crop, it tolerates light frost and can go out around Jan 3, before zone 10a's average last frost on Jan 31.
How long does onion take to grow in zone 10a?
About 100 days from transplanting to first harvest. A Jan 3 planting in zone 10a is typically ready to pick from around Apr 13. Zone 10a has about 333 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.