Plant Onions in Zone 4b
Seed-starting, transplant & harvest dates for USDA Zone 4b
Zone 4b averages 146 frost-free days, running from a last spring frost around May 5 to a first fall frost around September 28. Onions needs roughly 100 days from planting out to first harvest, so it uses about 68 percent of that window. Onions is a hardy crop, which is what decides where in the window it belongs.
Because onions needs a head start, sow seed indoors around Feb 24, about 10 weeks ahead of the Zone 4b last frost. Transplants go out around Apr 21. Starting earlier than that in Zone 4b usually backfires: plants outgrow their cells and stall when they finally hit cold soil.
Planting onions for a fall harvest in Zone 4b
Yes. Plant onions between August 17 and September 7 in Zone 4b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Short-day onion sets planted in fall overwinter in zone 7 and warmer and bulb up months ahead of a spring planting.
Fall dates are counted backward from the Zone 4b average first frost, not forward from a sowing date. See the fall planting calendar by zone for the full method.
Spring vs fall onions in Zone 4b
| Planting | Sow or start | In the ground | Harvest |
|---|---|---|---|
| Spring | Feb 24 | Apr 21 | Jul 30 |
| Fall | by Sep 7 | Sep 7 | Next spring |
Onions in nearby zones
Further reading on onions
- When to plant garlic by zone
The fall planting window for garlic and other alliums in every zone.
- When to start seeds indoors by zone
Indoor start dates for onions, counted back from your last frost.
- Hardening off seedlings: a schedule
A day by day plan for moving onions seedlings outdoors.
Other crops in Zone 4b
Frequently asked questions
When should I start onions in Zone 4b?
Start onions seeds indoors around Feb 24 (Zone 4b).
Can I direct sow onions in Zone 4b?
Onions is best started indoors and transplanted in Zone 4b rather than direct sown.
Can I plant onions in the fall in Zone 4b?
Yes. Plant onions between August 17 and September 7 in Zone 4b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Will onions survive frost in Zone 4b?
Onions is a hardy crop and keeps going roughly 21 days past the first light frost in Zone 4b. A row cover or a straw mulch buys more.