🧅When to Plant Onion (2026)
Long season. Start from seed indoors very early or use sets.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Companions: carrot, lettuce, beets
Is Onion right for your USDA zone?
Onion is a cool-season crop, which means it thrives in the mild temperatures of spring and fall. Gardeners in zones 3a through 7b get the longest onion season — you can plant in early spring and again in late summer for a fall harvest. In warmer zones (8a-10b), grow onion as a winter crop from October through February when temperatures are coolest.
Onion needs about 100 days from transplant to harvest, so any zone with at least 114frost-free days can grow it successfully. Check your zone's specific dates in the table below.
How to grow onion
Start onion seeds indoors about 10 weeks before your last spring frost, then harden off and transplant the seedlings once frost danger has passed. Give each plant 4 inches of room in full sun, and keep the soil consistently moist while seeds germinate and plants establish.
Expect roughly 100 days to your first harvest. Watch for onion maggot, thrips, the most common pests of onion, and scout plants weekly so you can act early. Day-length determines bulb formation—plant long-day varieties north of 37°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 easier—plant sets 1 inch apart in any 6-inch deep container and harvest continuously.
Onion planting dates by zone (2026)
| Zone | Start Indoors | Sow / Transplant | Harvest From |
|---|---|---|---|
| Zone 3a | Mar 6 | Apr 17 | Jul 26 |
| Zone 3b | Mar 6 | Apr 17 | Jul 26 |
| Zone 4a | Mar 1 | Apr 12 | Jul 21 |
| Zone 4b | Feb 24 | Apr 7 | Jul 16 |
| Zone 5a | Feb 19 | Apr 2 | Jul 11 |
| Zone 5b | Feb 14 | Mar 28 | Jul 6 |
| Zone 6a | Feb 9 | Mar 23 | Jul 1 |
| Zone 6b | Feb 4 | Mar 18 | Jun 26 |
| Zone 7a | Jan 30 | Mar 13 | Jun 21 |
| Zone 7b | Jan 25 | Mar 8 | Jun 16 |
| Zone 8a | Jan 14 | Feb 25 | Jun 5 |
| Zone 8b | Jan 4 | Feb 15 | May 26 |
| Zone 9a | Dec 17 | Jan 28 | May 8 |
| Zone 9b | Dec 2 | Jan 13 | Apr 23 |
| Zone 10a | Nov 22 | Jan 3 | Apr 13 |
| Zone 10b | Nov 6 | Dec 18 | Mar 28 |
Onion planting FAQ
When is the best time to plant onion?
Onion is a cool-season crop. Plant it in early spring as soon as the soil can be worked, and again in late summer for a fall harvest. It tolerates light frost. Find the exact date for your USDA zone in the table below.
How long does onion take to grow?
Onion takes about 100 days from transplant to first harvest. Starting seeds indoors 10 weeks before your last frost gives the plants a head start.
How far apart should I space onion?
Space onion about 4 inches apart in full sun. Proper spacing gives roots room to spread and improves airflow, which reduces disease.
What is the best growing tip for onion?
Day-length determines bulb formation—plant long-day varieties north of 37°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?
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 easier—plant sets 1 inch apart in any 6-inch deep container and harvest continuously.
Can I plant onion for a second harvest?
Generally no—onions 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 with onion?
Good companion plants for onion are carrot, lettuce, beets. Companion planting can deter pests and make better use of garden space. Rotate where you plant onion each year to keep soil healthy.