š§ When to Plant Onion in USDA Zone 6a (2026 Guide)
Long season. Start from seed indoors very early or use sets. Here is the cool-season planting schedule for onion in zone 6a (covering areas like St. Louis MO, Philadelphia PA). All dates are based on 2026 frost data for your zone.
Start Seeds Indoors
Feb 9
Transplant
Mar 23
Harvest From
Jul 1
Because onion is a cool-season crop, it tolerates light frost and can go out around Mar 23, before zone 6a's average last frost on Apr 20.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Zone 6a frost: Apr 20 ā Oct 15
- Season length: ~178 days
- Companions: carrot, lettuce, beets
Growing onion in zone 6a
Zone 6a has an average last spring frost around Apr 20 and a first fall frost around Oct 15, giving roughly 178 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 Feb 9, roughly 10 weeks before your last frost. Move the seedlings outside around Mar 23. Give each plant about 4 inches of space in full sun.
Zone 6a specific growing notes
In zone 6a (St. Louis MO, Philadelphia PA), your 178-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 Oct 15 means your last possible harvest is roughly late Oct. For a continuous supply, consider succession planting every 2-3 weeks from Mar 23 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 6a?
Transplant around Mar 23. If starting from seed indoors, sow around Feb 9. Because onion is a cool-season crop, it tolerates light frost and can go out around Mar 23, before zone 6a's average last frost on Apr 20.
How long does onion take to grow in zone 6a?
About 100 days from transplanting to first harvest. A Mar 23 planting in zone 6a is typically ready to pick from around Jul 1. Zone 6a has about 178 frost-free days ā plenty of time if you plant on schedule.
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.