Plant Onions in Zone 3b
Seed-starting, transplant & harvest dates for USDA Zone 3b
Zone 3b averages 128 frost-free days, running from a last spring frost around May 13 to a first fall frost around September 18. Onions needs roughly 100 days from planting out to first harvest, so it uses about 78 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 Mar 4, about 10 weeks ahead of the Zone 3b last frost. Transplants go out around Apr 29. Starting earlier than that in Zone 3b usually backfires: plants outgrow their cells and stall when they finally hit cold soil.
Planting onions for a fall harvest in Zone 3b
Yes. Plant onions between August 7 and August 28 in Zone 3b, 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 3b 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 3b
| Planting | Sow or start | In the ground | Harvest |
|---|---|---|---|
| Spring | Mar 4 | Apr 29 | Aug 7 |
| Fall | by Aug 28 | Aug 28 | 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 3b
Frequently asked questions
When should I start onions in Zone 3b?
Start onions seeds indoors around Mar 4 (Zone 3b).
Can I direct sow onions in Zone 3b?
Onions is best started indoors and transplanted in Zone 3b rather than direct sown.
Can I plant onions in the fall in Zone 3b?
Yes. Plant onions between August 7 and August 28 in Zone 3b, roughly 6 to 3 weeks before the average first frost, so it roots before the ground freezes.
Will onions survive frost in Zone 3b?
Onions is a hardy crop and keeps going roughly 21 days past the first light frost in Zone 3b. A row cover or a straw mulch buys more.