š§ When to Plant Onion in USDA Zone 8a (2026 Guide)
Long season. Start from seed indoors very early or use sets. Here is the cool-season planting schedule for onion in zone 8a (covering areas like Dallas TX, Charleston SC). All dates are based on 2026 frost data for your zone.
Start Seeds Indoors
Jan 14
Transplant
Feb 25
Harvest From
Jun 5
Because onion is a cool-season crop, it tolerates light frost and can go out around Feb 25, before zone 8a's average last frost on Mar 25.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Zone 8a frost: Mar 25 ā Nov 10
- Season length: ~230 days
- Companions: carrot, lettuce, beets
Growing onion in zone 8a
Zone 8a has an average last spring frost around Mar 25 and a first fall frost around Nov 10, giving roughly 230 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 Jan 14, roughly 10 weeks before your last frost. Move the seedlings outside around Feb 25. Give each plant about 4 inches of space in full sun.
Zone 8a specific growing notes
In zone 8a (Dallas TX, Charleston SC), your 230-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 Nov 10 means your last possible harvest is roughly late Nov. For a continuous supply, consider succession planting every 2-3 weeks from Feb 25 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 8a?
Transplant around Feb 25. If starting from seed indoors, sow around Jan 14. Because onion is a cool-season crop, it tolerates light frost and can go out around Feb 25, before zone 8a's average last frost on Mar 25.
How long does onion take to grow in zone 8a?
About 100 days from transplanting to first harvest. A Feb 25 planting in zone 8a is typically ready to pick from around Jun 5. Zone 8a has about 230 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.