When can I plant onion outside?

Direct sow or transplant 4 weeks before your last spring frost.

How long until onion is ready to harvest?

90-120 days. Tops fall over and begin to dry; stop watering and let cure in ground for a week, then harvest and dry

When to Plant Onion

Allium cepa

vegetableZones 3–10intermediate

Essential cooking staple available in red, yellow, and white varieties. Choose long-day or short-day types based on your latitude. Storage varieties keep for months.

Planting calendar

Start seeds indoors

10 weeks before your last spring frost.

Direct sow / transplant

4 weeks before your last spring frost.

Choose long-day varieties (north of 36°N) or short-day varieties (south of 36°N)

Spacing & harvest

Plant spacing: 4-6 inches
Row spacing: 12-18 inches
Days to harvest: 90-120 days
Ready when: Tops fall over and begin to dry; stop watering and let cure in ground for a week, then harvest and dry

Companion planting

Plant near

Cabbage — Strong onion scent deters cabbage moths
Broccoli — Deters cabbage moths and aphids
Bell Pepper — Deters many common pests with its scent
Cauliflower — Deters pests with strong scent
Lacinato Kale — Deters cabbage moths
Kohlrabi — Deters common brassica pests
Brussels Sprouts — Deters common brassica pests
Carrot — Onions repel carrot flies, carrots repel onion flies
Lettuce — Good space companions

Keep away from

Asparagus — Alliums may stunt asparagus growth
Green Bean — Onions stunt bean growth
Snap Pea — Alliums inhibit legume growth

Get onion planting dates for your exact zone

Lawn Guardian turns this guidance into a schedule timed to your USDA zone and this week's actual weather — and adapts every time you log what you've done.

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Sources: Purdue University Turfgrass Science: Use Growing Degree Days to Better Time Your Applications · University of Missouri Extension: Cool-Season Grasses: Lawn Maintenance Calendar · University of Minnesota Extension: Pre-emergent Herbicides for Crabgrass Control in Lawns