š§ When to Plant Onion in USDA Zone 4a (2026 Guide)
Long season. Start from seed indoors very early or use sets. Here is the cool-season planting schedule for onion in zone 4a (covering areas like northern Wisconsin, Fargo ND). All dates are based on 2026 frost data for your zone.
Start Seeds Indoors
Mar 1
Transplant
Apr 12
Harvest From
Jul 21
Because onion is a cool-season crop, it tolerates light frost and can go out around Apr 12, before zone 4a's average last frost on May 10.
- Type: Cool-season
- Method: Transplant
- Days to harvest: 100
- Spacing: 4" apart
- Sun: Full sun
- Zone 4a frost: May 10 ā Sep 25
- Season length: ~138 days
- Companions: carrot, lettuce, beets
Growing onion in zone 4a
Zone 4a has an average last spring frost around May 10 and a first fall frost around Sep 25, giving roughly 138 frost-free growing days. That window comfortably fits onion's 100-day cycle, so plant on time to make the most of the season.
Get a head start by sowing seeds indoors around Mar 1, roughly 10 weeks before your last frost. Move the seedlings outside around Apr 12. Give each plant about 4 inches of space in full sun.
Zone 4a specific growing notes
In zone 4a (northern Wisconsin, Fargo ND), your 138-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 Sep 25 means your last possible harvest is roughly late Sep. For a continuous supply, consider planting early and using season extension techniques like row covers.
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 4a?
Transplant around Apr 12. If starting from seed indoors, sow around Mar 1. Because onion is a cool-season crop, it tolerates light frost and can go out around Apr 12, before zone 4a's average last frost on May 10.
How long does onion take to grow in zone 4a?
About 100 days from transplanting to first harvest. A Apr 12 planting in zone 4a is typically ready to pick from around Jul 21. Zone 4a has about 138 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.