How to grow onion
Family Amaryllidaceae. Hardy to USDA hardiness zone 3a+ (U.S. Department of Agriculture). Full sun. Ready in 90-120 days.
Sun
Full sun
Spacing
4-6 in
Days to harvest
90-120 days
Hardiness
Zone 3a+
What growers actually recorded
Loading zone 7b results…
When to plant onion
- Start indoors: ~10 weeks before last frost.
- Transplant out: ~-2 weeks after last frost.
- Direct sow: ~-3 weeks after last frost.
Companion plants for onion
Plant with: Carrot, Beet, Lettuce, Tomato.
Avoid near: Bean, Pea.
Common pests & problems
Thrips.
Feeding & fertilizer
N-P-K ratio: Moderate N + high P (5-10-10)
Organic options: Compost + bone meal at planting · Fish emulsion monthly through spring · Stop feeding as bulbs swell
When to feed: Feed through spring leaf growth, then stop once tops start to yellow so bulbs cure hard.
Watch for: Thick, soft necks = too much late nitrogen; bulbs won't store.
N = nitrogen (leaves), P = phosphorus (roots and flowers), K = potassium (fruit and disease resistance). How to read a fertilizer label →
Grower's tip
Match day-length to your latitude. Long-day north of 37°, short-day south.
When to plant onion in your zone
Free month-by-month sowing and transplanting lists, built from frost dates for your USDA (United States Department of Agriculture) hardiness zone.
Soil & water for onion
Soil & pH
6.0-7.0
Loose, well-drained loam.
Compost + bone meal. Cut nitrogen once bulbing begins.
Watering
1 in / week during bulbing; taper before harvest
Drip; keep foliage dry to prevent purple blotch.
Falling, yellowing tops = pull water and cure.
Knowing when to harvest
- 50%+ of tops flopped and yellowing. That's the plant sending sugars to the bulb.
- Loosen with a fork; leave in the field 1-2 dry days.
- Cure 2-3 weeks in a warm, dry, airy spot until necks are papery.
Storing & keeping
Braided or netted at 32-40°F, 65% humidity. Long-storage varieties keep 8-10 months; sweet onions 4-8 weeks.
What to expect
Per plant: 1 bulb (obviously)
Per sq ft: 9-16 per sq ft with tight spacing
Troubleshooting onion
Thick necks that won't cure
Likely cause: Too much nitrogen late
What to do: Stop feeding once bulbs start to swell (visible at soil line).
Small bulbs
Likely cause: Wrong day-length variety for your latitude
What to do: North of 37° use long-day; south of 37° use short-day; middle latitudes use day-neutral.
Varieties worth growing
Copra (long-day storage)
The gold-standard storage onion for the North.
Walla Walla (long-day sweet)
Mild, juicy, poor keeper: eat first.
Texas Grano (short-day)
Zone 8+ sweet onion.
Growing onion in containers
6+ in deep pot; use bunching (green) onion varieties for tight spaces.
Nutrition
Allicin, quercetin, prebiotic fiber.
Common questions
Sets, transplants, or seed?
Seed = biggest bulbs but slow; transplants = best balance; sets = fastest but higher bolt risk.
Where these numbers come from
- USDA Plant Hardiness Zone Map (2023 revision). Zone assignment and the 30-year normals behind every cold-hardiness figure.
- NOAA NCEI 1991-2020 Climate Normals. Frost dates, growing-season length, and the probability bands on both.
- Cornell, UMN, and UC Davis Cooperative Extension crop guides. Spacing, days to maturity, soil pH targets, and per-plant yield ranges.
- USDA FSIS / FDA Produce Safety guidance. Wash, cure, and cold-storage handling numbers.
Ranges are regional guidance, not a promise for your yard. Your own logged results always override the published range.