Onion in Zone 3a & 3b
Allium cepa
Yes, onion grows in zones 3a and 3b. Start seeds indoors Apr 4–Apr 11 and plant out May 9–May 16.
When to plant onion in zone 3b
Zones 3a and 3b share the same average frost dates, so this page covers both.
Method: Start indoors: Start indoors from seed, then transplant outside. It needs a head start most zones can't give it by direct-sowing alone.
| Start indoors | Apr 4–Apr 11 |
|---|---|
| Direct sow / transplant outside | May 9–May 16 |
| Estimated harvest | Aug 17–Aug 24 (~100 days after transplanting) |
Spacing: 16 per square foot (square-foot-gardening spacing). Mature height: 1-1.5 ft.
Plant alongside it in zone 3b
These go outside within two weeks of onion in zone 3b, so one planting day can cover them.
- RadishMay 2–May 9
- CabbageMay 16–May 23
- LettuceMay 16–May 23
- BeetMay 9–May 16
- Broccoli Rabe (Raab)May 9–May 16
- MizunaMay 9–May 16
Onions are one of the slower crops in the garden, around 100 days from transplant to harvest, and they start life indoors well before most of the garden is even being planned, which catches a lot of first-time growers off guard. There's no shortcut here: get the timing right up front and the rest of the crop mostly takes care of itself.
How onion fits zone 3's season
Zone 3b's frost-free window runs about 98 days, from May 30 to September 5. Onion needs about 100 days from transplant to first harvest.
Onion planting windows in every zone
Zone 3b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Harvest |
|---|---|---|---|
| 3a & 3b (your zone) | Apr 4–Apr 11 | May 9–May 16 | Aug 17–Aug 24 |
| 4a & 4b | Mar 20–Mar 27 | Apr 24–May 1 | Aug 2–Aug 9 |
| 5a & 5b | Mar 15–Mar 22 | Apr 19–Apr 26 | Jul 28–Aug 4 |
| 6a & 6b | Feb 28–Mar 7 | Apr 4–Apr 11 | Jul 13–Jul 20 |
| 7a & 7b | Feb 13–Feb 20 | Mar 20–Mar 27 | Jun 28–Jul 5 |
| 8a & 8b | Jan 28–Feb 4 | Mar 4–Mar 11 | Jun 12–Jun 19 |
| 9a & 9b | Dec 21–Dec 28 | Jan 25–Feb 1 | May 5–May 12 |
| 10a & 10bFall window | Aug–Nov | Oct–Jan | ~100 days after transplanting |
Harvest dates are about 100 days after the sow or plant-out date, not after starting seeds indoors.
Good companions
- Carrot: A real, peer-reviewed field study (Uvah & Coaker 1984) found intercropping onions with carrots reduced carrot rust fly damage, on the theory that mixed scents mask the host plant from pests hunting by smell.
- Tomato: Onion scent can deter tomato pests.
Where to buy onion seeds
Botanical Interests · True Leaf Market · Burpee · High Mowing Organic Seeds (each supplier's onion seeds)
Varieties we recommend, and where to get them:
- Ailsa Craig Exhibition: Also highly rated in Cornell's database at 4.5 stars, a large, slightly oval long-day type prized for sweetness over long storage life. Buy: High Mowing Organic Seeds Full notes
- 'Walla Walla Sweet' (best for fresh eating, not storage): Johnny's lists this long-day Northwest classic as ultra-mild and juicy, best fall-planted where winters stay above -10°F, but explicitly not a storage onion the way Copra is. Buy: True Leaf Market · High Mowing Organic Seeds Full notes
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing onion" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 3b calendar · Plan your beds in the Garden Designer →
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Growing Onion in USDA zone 3b: when to start seeds indoors, when to direct-sow, and when to expect a harvest.
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