When to Plant Onions by Zone (Plus Companion Planting)
By Ada Jones · Updated October 2026 · 8 min read
Short answer: plant onions 4-6 weeks before your last spring frost date in most of the US and Canada, or in fall (Sept-Nov) in mild-winter zones 7-10 for an overwintered crop. The exact window depends on your USDA hardiness zone and whether you’re growing from seed, sets, or transplants — here’s the breakdown by zone.
Onion Planting Calendar by USDA Zone
| USDA Zone | Planting Window | Recommended Day-Length Type |
|---|---|---|
| Zones 3-5 (cold climates) | Early spring, as soon as soil is workable (April-May); start seed indoors 8-10 weeks earlier | Long-day varieties only |
| Zones 6-7 (transitional) | Early spring (March-April), or fall planting of sets for overwintering in zone 7 | Intermediate-day varieties |
| Zones 8-9 (mild winter) | Fall (September-November) for overwintering, or late winter (Jan-Feb) | Short-day to intermediate-day varieties |
| Zone 10+ (frost-free) | Fall through winter (October-January) | Short-day varieties only |
Day-length type matters more than the calendar date: a long-day variety planted in zone 9 often won’t bulb properly because the days never get long enough there, regardless of when it’s planted.
Companion Planting: What Grows Well With Onions
| Good Companions | Why |
|---|---|
| Carrots | Onion scent is reported to help deter carrot rust fly; different root depths reduce competition |
| Tomatoes | Onions’ sulfur compounds may help repel some tomato pests; classic companion pairing |
| Brassicas (cabbage, broccoli) | Onion scent can mask brassicas from cabbage moths |
| Strawberries | Shallow-rooted, non-competing, and some growers report fewer pest issues when interplanted |
What to Avoid Planting Near Onions
- Beans and peas — onions can stunt legume growth, likely by interfering with their nitrogen-fixing bacteria
- Asparagus — generally considered a poor pairing with alliums by most growers
Companion planting benefits are based mostly on long-standing grower observation rather than rigorous controlled studies, so treat the “avoid” list as the firmer guidance and the “good companion” list as reasonable, low-risk choices rather than guaranteed pest control.
FAQ
Can I plant onions too early in spring?
Onions tolerate light frost well, but soil that’s too cold and wet can rot sets or seedlings before they establish. Wait until the soil is workable, not frozen or waterlogged, even if that’s a bit later than the calendar suggests.
What if I don’t know my USDA zone?
Search “USDA hardiness zone” plus your zip code (US) or check your local agricultural extension office, which will also confirm your area’s typical last-frost date.
Final Thoughts
Match your variety’s day-length type to your zone first, then plant within the window for your region. Ready to plant? See our seed-starting guide or sets guide for the next steps.
How This Guide Was Checked
Based on USDA hardiness zone data and extension-published planting calendars. Not sure which zone you’re in? Contact us and we’ll help.
Related Onion-Growing Guides
- Short-Day vs Long-Day Onions: Find Your Type by Zone
- Onion Harvest Date Calculator: Estimate by Planting Date
- How Long Does It Take to Grow Onions? A Timeline by Method
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.
